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Models of radiative quantum systems were constructed as analogs of multiphoton and Raman resonances of classical field quanta on an atomic system with the participation of cavity mode quanta. A distinctive feature of the models is the description of an atomic ensemble and quanta using either the generators of polynomial algebra or the two-mode Jordan–Schwinger representation of the su(2) algebra, so that one can talk about an atom–photon and/or photon clusters. The arising algebras are mathematically unsolvable, in contrast to the Heisenberg–Weyl algebra. This makes it possible to implement and observe fundamental effects of coherent nonlinear optics, such as photon echo, optical nutation, and superradiance, in ensembles of the described clusters. [ABSTRACT FROM AUTHOR]

Models based on sets of effective vibrational–rotational Hamiltonians, parameterized by a small number of spectroscopic reduced quartic, sextic, and octic constants, can conveniently describe highly complex vibrational–rotational spectra of free small-atomic molecules containing tens and hundreds of thousands of observable lines in the microwave and infrared range. These parameters are usually determined by solving inverse problems that combine interpreting a spectrum with step-by-step fitting of parameters. Quartic and sextic constants can be routinely calculated for equilibrium nuclear configurations using analytical formulas, while considerable theoretical and computational difficulties are encountered in determining vibrationally excited and octic constants. A theoretical method is proposed for calculating reduced parameters of effective Hamiltonians in the fourth and sixth orders of the operator perturbation theory, and computational algorithms are described. It is shown that in the fourth and sixth orders, the quartic and sextic constants are in better agreement with experiments, respectively. Octic constants obtained in the sixth order can be used for verification or refinement when solving the inverse problem. Theoretical results are illustrated by the example of calculating two isotopologues of the SO2 molecule. [ABSTRACT FROM AUTHOR]

Room temperature cavity ring-down spectra (CRDS) of hydrogen sulfide were recorded at three pressures of 10, 20, and 30 Torr in the 12 950–13 300 cm–1 range with the sensitivity on the order of 3 × 10–11 cm–1 in terms of absorption coefficient. The analysis of these spectra was performed. The line positions and intensities were retrieved from these spectra. The theoretical simulation of these spectra was performed within the method of effective operators. The comparison of the measured line positions and intensities to the values obtained within variational calculation was done. The considerable difference was found between calculated (Azzam A.A.A., Yurchenko S.N., Tennyson J., Naumenko O.V. Exomol line lists XVI: A hot line list for H2S // Mon. Not. R. Astron. Soc. 2016. V. 460. P. 4063–4074) and observed line positions and intensities. [ABSTRACT FROM AUTHOR]

Various approaches have been used in the literature for eliminating nonresonant levels in atomic systems and deriving effective Hamiltonians. Important among these are elimination techniques at the level of probability amplitudes, operator techniques to project the dynamics on to the subspace of resonant levels, Green’s function techniques, the James’ effective Hamiltonian approach, etc. None of the previous approaches is suitable for deriving effective Hamiltonians in intracavity situations. However, the James’ approach does work in the case of only two-photon transitions in a cavity. A generalization of the James’ approach works in the case of three-photon transitions in a cavity, but only under Raman-like resonant conditions. Another important approach for adiabatic elimination is based on an adaptation of the Markov approximation well-known in the theory of system–bath interactions. However, this approach has not been shown to work in intracavity situations. In this paper, we present a method of adiabatic elimination for atoms inside cavities in the presence of multiphoton transitions. We work in the Heisenberg picture, and our approach has the advantage that it allows one to derive effective Hamiltonians even when Raman-like resonance conditions do not hold. [ABSTRACT FROM AUTHOR]

Since Mayer and Jensen employed the single-particle shell model to interpret the magic numbers, various microscopic nuclear models have been developed to study the nuclear force and structure. The configuration–interaction shell model (CISM), performed in truncated model space with the inclusion of the residual interaction, is one widely-used nuclear structure model. In the last decade, CISM has progressed in investigating the cross-shell excitation in exotic light nuclei, the similarity and difference in mirror nuclei, and the isomerism and seniority conservation in medium and heavy nuclei. Additionally, researchers have attempted to construct effective Hamiltonians for nuclei near 1 3 2 Sn and 2 0 8 Pb through a unified way in the CISM framework. In parallel, related models, including the nucleon-pair approximation (NPA) approach, the Monte Carlo shell model (MCSM), the projected shell model (PSM), the Gamow shell model (GSM), etc., have also been extensively developed and validated in the last decade. This paper reviews the recent progress in CISM and some related models. [ABSTRACT FROM AUTHOR]

We study the effective fronts of first order front propagations in two dimensions (n = 2) in the periodic setting. Using PDE-based approaches, we show that for every a G (0,1), the class of centrally symmetric polygons with rational vertices (i.e., vectors in Uagr AZ2) and nonempty interior is admissible as effective fronts for front speeds in C1,a(T2, (0, oo)). This result can also be formulated in the language of stable norms corresponding to periodic metrics in T2. Similar results were known long ago when n > 3 for front speeds in Cco(Tn, (0, oo)). The two-dimensional case is much more subtle due to topological restrictions. In fact, for given C1,1(T2, (0, oo)) front speeds, the effective front is C1 and hence cannot be a polygon. Our regularity requirements on front speeds are hence optimal. To the best of our knowledge, this is the first time that polygonal effective fronts have been constructed in two dimensions. [ABSTRACT FROM AUTHOR]

The performance of embedding methods is directly tied to the quality of the bath orbital construction. In this paper, we develop a versatile framework, enabling the investigation of the optimal construction of the orbitals of the bath. As of today, in state-of-the-art embedding methods, the orbitals of the bath are constructed by performing a Singular Value Decomposition (SVD) on the impurity-environment part of the one-body reduced density matrix, as originally presented in Density Matrix Embedding Theory. Recently, the equivalence between the SVD protocol and the use of unitary transformation, the so-called Block-Householder transformation, has been established. We present a generalization of the Block-Householder transformation by introducing additional flexible parameters. The additional parameters are optimized such that the bath-orbitals fulfill physically motivated constraints. The efficiency of the approach is discussed and exemplified in the context of the half-filled Hubbard model in one-dimension. [ABSTRACT FROM AUTHOR]

The performance parameters for characterizing the electrocaloric effect are isothermal entropy change and the adiabatic temperature change, respectively. This paper reviews the electrocaloric effect of ferroelectric materials based on different theoretical models. First, it provides four different calculation scales (the first-principle-based effective Hamiltonian, the Landau-Devonshire thermodynamic theory, phase-field simulation, and finite element analysis) to explain the basic theory of calculating the electrocaloric effect. Then, it comprehensively reviews the recent progress of these methods in regulating the electrocaloric effect and the generation mechanism of the electrocaloric effect. Finally, it summarizes and anticipates the exploration of more novel electrocaloric materials based on the framework constructed by the different computational methods. [ABSTRACT FROM AUTHOR]

• 1382 transitions are assigned in the lower energy part of the triacontad of 12CH 4 near 1.28 µm. • Experimental line positions were modeled to an rms of 0.006 cm−1. • 33 new vibrational band origins were determined for the first time. Quantum assignments are reported for 1382 transitions towards 33 vibrational sub-levels belonging to three weak Triacontad bands 6ν 4 , ν 2 +5ν 4 , ν 1 +4ν 4 of 12CH 4 1.28µm. Empirical values of the origins of the corresponded sub-bands were determined for the first time from high resolution spectra analysis. The positions of all assigned lines were fitted to a model adopting an effective Hamiltonian whose initial values are derived from the molecular potential energy surface by using the contact transformation method. An empirical optimization of this effective model resulted in the root-means-square deviation of 0.006 cm−1 for newly assigned transitions. A comparison with variational ab initio calculations is discussed. [ABSTRACT FROM AUTHOR]

The performance of embedding methods is directly tied to the quality of the bath orbital construction. In this paper, we develop a versatile framework, enabling the investigation of the optimal construction of the orbitals of the bath. As of today, in state-of-the-art embedding methods, the orbitals of the bath are constructed by performing a Singular Value Decomposition (SVD) on the impurity-environment part of the one-body reduced density matrix, as originally presented in Density Matrix Embedding Theory. Recently, the equivalence between the SVD protocol and the use of unitary transformation, the so-called Block-Householder transformation, has been established. We present a generalization of the Block-Householder transformation by introducing additional flexible parameters. The additional parameters are optimized such that the bath-orbitals fulfill physically motivated constraints. The efficiency of the approach is discussed and exemplified in the context of the half-filled Hubbard model in one-dimension.

Aimed at fundamental understanding and design of advanced magnetic resonance experiments on basis of Hamiltonians, we describe highly convergent and exact effective Hamiltonian methods which alleviate important deficits of current less accurate methods. This involves single-spin vector effective Hamiltonian theory (SSV-EHT) to first order in the interaction frame of rf and chemical shift offsets as well as exact effective Hamiltonian theory (EEHT) being an exact approach to average Hamiltonian theory not relying on interaction frame transformations. Bringing these methods together, we present tools to analyze challenging experiments in need of considering large static components in Hamiltonian (e.g., offsets) while economizing with radiofrequency irradiation power. It is demonstrated how the two complementary tools may provide important new insight into the detailed effective Hamiltonians of advanced NMR experiments, noting that the methods are by no means restricted to NMR. This is demonstrated for isotropic mixing in liquid-state NMR and dipolar recoupling in solid-state NMR where insight into the delicate interplay between bilinear two-spin and linear single-spin terms in the effective Hamiltonian may increase understanding of determinants for broadband excitation and the formation of recoupling resonances. Furthermore, we demonstrate how simple products single-spin effective Hamiltonians may be used as generators of multiple-spin effective Hamiltonians and though this a new approach to density operator calculations for large multiple-spin systems.

The recent bottom‐up synthesis of atomically precise nanoporous graphene (NPG) offers a way of tuning graphene's properties by forming NPG/graphene (Grp) bilayers. Depending on the size, shape, and periodicity of the nanopores in NPG, the heterobilayers can exhibit various functionalities. This theoretical work presents an inverse design of NPG/Grp bilayers with electric‐field‐tunable bandgaps as a target property. The interlayer interaction in such heterobilayers can induce a bandgap in graphene either by breaking inversion symmetry (type I) or by moving and merging Dirac points of graphene (type II). The bandgap opening also requires electron–hole symmetry breaking induced by an applied perpendicular electric field, leading to two distinct, linear versus nonlinear, field dependences of the bandgap for the type‐I and type‐II cases, respectively. To translate the underlying physics of the bandgap opening in graphene into real atomic structures, the authors develop an inverse design method and find NPG/Grp bilayers with the target functionality. The field‐tunable bandgap in graphene, supported by first‐principles calculations for the inverse‐designed systems, holds promise for new types of graphene transistors. [ABSTRACT FROM AUTHOR]

The paper describes methods and fast computational algorithm for building effective Hamiltonians in molecular physics using perturbative approach. Separations of fast and slow variables are considered in the framework of contact transformations (CT). The particular focus is on a systematic derivation of effective models for rovibrational spectroscopy from ab initio-based potential energy surfaces with an exhaustive review of previous studies in this field. We consider applications to several types of polyads coupled by Fermi, Coriolis, Darling-Dennison and other types of resonance interactions with examples for asymmetric top, symmetric top and spherical top molecules. A flexible choice of the modelling operator accounts for strong couplings of various types of nuclear motion in molecules among closely lying levels including vibrational resonance schemes (2:1:2...), (2:1:2:1), (4:2:6:3), (3:2:1:2:1:1), etc. that occur for C2v, C3v and Td molecules and their isotopic species. The method is implemented in the MOL_CT programme suite, which offers a complementary tool to variational methods in terms of convergence and computational time. The range of applications is also different. The goal of the CT method is providing mathematical models for analyses of molecular spectra with the high-resolution accuracy using physically meaningful parameters derived from ab initio functions. [ABSTRACT FROM AUTHOR]

We introduce the effective Gibbs state for the observables averaged with respect to fast free dynamics. We prove that the information loss due to the restriction of our measurement capabilities to such averaged observables is non-negative and discuss a thermodynamic role of it. We show that there are a lot of similarities between this effective Hamiltonian and the mean force Hamiltonian, which suggests a generalization of quantum thermodynamics including both cases. We also perturbatively calculate the effective Hamiltonian and correspondent corrections to the thermodynamic quantities and illustrate it with several examples. [ABSTRACT FROM AUTHOR]

In this paper, we study the problem of wave scattering from finite heterogeneities (in 1- and 2-D) by using the Atomistic Green's Function (AGF) technique. The application of AGF to classical wave scattering problems is novel and it allows us to compute the Green's function of the scatterers, which is central to understanding the dynamics of the problem and is, in general, difficult to obtain. The AGF method also allows us to efficiently compute the numerically exact transmission and reflection coefficients without the need for any artificial truncating boundaries such as perfectly matched layers or Dirichlet to Neumann (DtN) maps. The technique generates the effective Hamiltonian of the wave scatterer and uses it to compute the numerically exact Green's function of the scatterer. The formalism presented here is especially suited to scattering problems involving waveguides, phononic crystals, metamaterials, and metasurfaces. To illustrate the utility of the technique, we demonstrate the application of the method to three scattering problems: scattering from a slab (1D), scattering from a finite phononic crystal (1D), and scattering from defects in a waveguide (2D). [Display omitted] • The Atomistic Green's Function (AGF) method adopts an open system perspective. • AGF method eliminates the necessity for non-reflecting boundary conditions. • Scattering problems in an infinite domain are addressed by formulating an effective Hamiltonian. • The method remains unconcerned about the complexity of the scatterer. [ABSTRACT FROM AUTHOR]

Recently, the AlB2-type compounds such as AlB2 and MgB2 have attracted immense interest due to the Dirac Nodal Line (DNL). This unique electronic structure is very important for the design and discovery of topological superconductivity, but the DNL of AlB2-type compounds has not been unambiguously comparative studied. Here, we systematically investigated the electronic topological properties of AlB2 and MgB2 by tight-binding model analysis and first-principles calculations. The Slater–Koster method fitted band structure results showed that the slope of σ-bond in Γ-A direction was mainly controlled by the absolute value of Vppπ. Moreover, the band structure fitted by two pz Symmetry-Adapted Wannier Function (SAWF) was consistent with the band structure obtained by the effective Hamiltonian of two orbital. In the constructed effective Hamiltonian, the external on-site energies in MgB2 and AlB2 were 0.075 and 2.47 eV, respectively, which were mainly due to the distance between Fermi level and Dirac point at the high-symmetry K and H points. More strikingly, it was found that the degree of bending of the topological surface state was related to the number of the selected orbits, and the larger the number of orbits, the stronger the degree of bending. This important finding might be due to the asymmetry of the crystal structure and band structure. This work provides a meaningful reference for exploration of the topological property of DNL in AlB2-type topological superconductivity materials. [ABSTRACT FROM AUTHOR]

Proton transfer via tunneling is a fundamental quantum‐mechanical phenomenon. We report rotational spectroscopy measurements of this process in the complex of the formic acid dimer with fluorobenzene. The assignment of the spectrum indicates that this complex exists in the form of a π–π stacked structure. Each rotational transition of the parent isotopologue exhibits splitting. Isotopic substitution experiments show that the spectral splitting results from double‐proton transfer tunneling in the formic acid dimer. Presence of fluorobenzene as a neighboring molecule does not quench the double proton transfer in the formic acid dimer but decreases its tunneling splitting from 341(3) MHz to 267.608(1) MHz. Calculations suggest that the presence of the weakly bounded fluorobenzene does not influence the activation energy of the proton transfer. The fluorobenzene is reoriented with respect to the formic acid dimer during the course of the reaction, slowing down the proton transfer motion. [ABSTRACT FROM AUTHOR]

The hyperfine structure of the transitions in the (X , v ′ ′ = 0 → (B , c) , v ′ = 2) band system in 39 K 85 Rb and 39 K 87 Rb is studied by using several selective Doppler-free spectroscopy techniques. Significant splitting of the line profiles is observable for the B 1 Π state levels coupled to the c 3 Σ + levels through spin–orbit interactions. The splitting is the result from an interplay between spin–spin, spin–rotation, spin–orbit interactions and the Fermi-contact interaction between the electron's spin and the nuclear spins. Energy shifts of the perturbed levels were analyzed and modeled within the effective Hamiltonian approach. Molecular and coupling constants were determined, which successfully model the experimental observations. Although the highest experimental resolution of 50 MHz was not sufficient to resolve entirely the hyperfine structure, the model successfully explains the line broadening in 39 K 85 Rb and the line splitting in 39 K 87 Rb. • High resolution spectroscopy. • Diatomic molecules. • Hyperfine structure. [ABSTRACT FROM AUTHOR]

We introduce the effective Gibbs state for the observables averaged with respect to fast free dynamics. We prove that the information loss due to the restriction of our measurement capabilities to such averaged observables is non-negative and discuss a thermodynamic role of it. We show that there are a lot of similarities between this effective Hamiltonian and the mean force Hamiltonian, which suggests a generalization of quantum thermodynamics including both cases. We also perturbatively calculate the effective Hamiltonian and correspondent corrections to the thermodynamic quantities and illustrate it with several examples.

Using a separable many-body variational wavefunction, we formulate a self-consistent effective Hamiltonian theory for fermionic many-body system. The theory is applied to the two-dimensional (2D) Hubbard model as an example to demonstrate its capability and computational effectiveness. Most remarkably for the Hubbard model in 2D, a highly unconventional quadruple-fermion non-Cooper pair order parameter is discovered. [ABSTRACT FROM AUTHOR]

We study the eigenvalues of the two-dimensional Schrödinger operator with a large constant magnetic field perturbed by a decaying scalar potential. For each Landau level, we give the precise asymptotic distribution of eigenvalues created by the minimum, maximum and the closed energy curve of the potential. Normal form reduction, WKB construction and pseudodifferential calculus are applied to the effective Hamiltonian. [ABSTRACT FROM AUTHOR]

In this paper, the crystal field parameters (CFPs) have been calculated in the framework of the density functional theory using a novel theoretical approach proposed by Pavel Novák et al. and extracting the WANNIER functions from the Bloch eigenstates for the CeCl3 compound. Then, the calculated CFPs have been used in an effective atomic-like Hamiltonian, including the crystal field, 4f-4f correlation and spin-orbit coupling, and the splitted energy levels of Ce3+ ion by crystal field have been derived by diagonalization of the Hamiltonian. A hybridization parameter, , has been used to improve the results. The results are found to be in agreement with the experimental data

In this paper, we consider some second-order effective Hamiltonians describing the interaction of the quantum electromagnetic field with atoms or molecules in the nonrelativistic limit. Our procedure is valid only for off-energy-shell processes, specifically virtual processes such as those relevant for ground-state energy shifts and dispersion van der Waals and Casimir-Polder interactions, while on-energy-shell processes are excluded. These effective Hamiltonians allow for a considerable simplification of the calculation of radiative energy shifts, dispersion, and Casimir-Polder interactions, including in the presence of boundary conditions. They can also provide clear physical insights into the processes involved. We clarify that the form of the effective Hamiltonian depends on the field states considered, and consequently different expressions can be obtained, each of them with a well-defined range of validity and possible applications. We also apply our results to some specific cases, mainly the Lamb shift, the Casimir-Polder atom-surface interaction, and the dispersion interactions between atoms, molecules, or, in general, polarizable bodies.

Ferroelectric materials are attractive candidates for potential solid-state refrigeration. Their useful broad applications require investigation of several properties. However, the magnitude corresponding to electrocaloric effect (ECE) is seemingly too small. Insight from a theoretical approach is needed to provide support through theoretical description, leading mainly to better understanding and prediction of the electrocaloric performance, as well as additional properties that might be useful. Here, we investigate the elastic, electronic, thermoelectric, and ferroelectric properties as well as ECE of S n T i O 3 by utilizing first-principles calculations and effective Hamiltonian. From the calculations toward a higher symmetry direction, the direct band gap is obtained. In addition, we have comprehensively investigated the effect of M n -doping on the electronic and thermoelectric properties of S n T i O 3 to achieve a better understanding of this material. The polarization, pyroelectric coefficient, changes in entropy Δ S and adiabatic temperature Δ T are examined in detail as a function of temperature. This work also reveals the contribution with particular attention of the applied electric field especially on the magnitude of ECE, resulting in slightly anomalous volume expansion. Typical hysteresis loops are observed to confirm the order-disorder phase transition. An enhancement route of the ECE is elucidated, which may open the door for more discussions and practical applications. [ABSTRACT FROM AUTHOR]

A bank of spectral line parameters of the principal isotopologue of the hydrogen sulfide molecule (H232S) is presented. The databank is based on the global simulation of line positions and intensities of this molecule within the method of effective operators. Parameters of the global effective Hamiltonian and effective dipole moment operator were derived from their fitting to the line positions and intensities observed, respectively, which were taken from the literature. The databank covers the 552.76–8424.32 cm−1 spectral range and includes the calculated values of the following spectral line parameters: line position and intensity, energies of lower and upper states, Einstein coefficient for the spontaneous emission, and the statistical weights of the lower and upper states. The intensity cutoff was chosen equal to 10−28 cm/molecule at T = 296 K. In total, the databank contains about 88 thousand lines. This databank is deployed on the website of V.E. Zuev Institute of Atmospheric Optics, Siberian Branch, Russian Academy of Sciences (ftp://ftp.iao.ru/pub/H2S/). [ABSTRACT FROM AUTHOR]

In this work the matrix elements of four photonic optical transitions accompanied between the subbands of the valence band of the cubic symmetry semiconductor were calculated, where the contribution of the simultaneous three photon interactions to the total matrix element of the four photonic optical transitions was taken into account and it was shown that this contribution depends on both the sign and the numerical value of the parameter in front of the cubic by the wave vector of holes, the term in the effective Hamiltonian. [ABSTRACT FROM AUTHOR]

In this work the matrix elements of three-photon optical transitions accompanied between the subbands of the valence band of the semiconductor of cubic symmetry was calculated. In this case, the contribution of the simultaneous three-photon interaction to the total matrix element of the three-photon optical transitions was taken into account and it was shown that this contribution depends both on the sign and the numerical value of the parameter in front of the effective wave Hamiltonian term that is cubic in terms of the wave vector of holes. [ABSTRACT FROM AUTHOR]

The matrix elements of the effective carrier Hamiltonian are calculated as in the Kane approximation, where the conduction band, the valence band consisting of light and heavy hole subbands, and the spin-split band, as well as in the Luttinger-Kohn model, are considered. [ABSTRACT FROM AUTHOR]

The matrix elements of the effective Hamiltonian of current carriers are calculated as in the Kane approximation, where the conduction band, the valence band consisting of light and heavy hole subbands, and the spin-split band, as well as in the Luttinger-Kohn model, are considered. [ABSTRACT FROM AUTHOR]

We propose a perturbative approximation to the vibrational coupled cluster method in bosonic representation to reduce the cost of calculating the cluster matrix elements by considering only the first order of S and σ for the construction of the effective Hamiltonian e σ e - S H e S e - σ . With the systematic analysis of the results of two molecules, H 2 O and 1,1-difluoroethylene, we find that the accuracy of the transition energies with such low order approximation is comparable to the fully converged VCCM. The lowest order perturbative approximation is used to calculate the cluster matrix elements of the vibrational coupled cluster method in bosonic representation. The computational scaling for cluster matric elements is reduced from iterative N6 to non-iterative N4. [ABSTRACT FROM AUTHOR]

The non-equilibrium dynamics of mixing, oscillations and equilibration is studied in a field theory of flavored neutral mesons that effectively models two flavors of mixed neutrinos, in interaction with other mesons that represent a thermal bath of hadrons or quarks and charged leptons. This model describes the general features of neutrino mixing and relaxation via charged currents in a medium. The reduced density matrix and the non-equilibrium effective action that describes the propagation of neutrinos is obtained by integrating out the bath degrees of freedom. We obtain the dispersion relations, mixing angles and relaxation rates of ``neutrino'' quasiparticles. The dispersion relations and mixing angles are of the same form as those of neutrinos in the medium, and the relaxation rates are given by $\Gamma_1(k) = \Gamma_{ee}(k) \cos^2\theta_m(k)+\Gamma_{\mu\mu}(k)\sin^2\theta_m(k) ; \Gamma_2(k)= \Gamma_{\mu\mu}(k) \cos^2\theta_m(k)+\Gamma_{ee}(k)\sin^2\theta_m(k) $ where $\Gamma_{\alpha\alpha}(k)$ are the relaxation rates of the flavor fields in \emph{absence} of mixing, and $\theta_m(k)$ is the mixing angle in the medium. A Weisskopf-Wigner approximation that describes the asymptotic time evolution in terms of a non-hermitian Hamiltonian is derived. At long time $>>\Gamma^{-1}_{1,2}$ ``neutrinos'' equilibrate with the bath. The equilibrium density matrix is nearly diagonal in the basis of eigenstates of an \emph{effective Hamiltonian that includes self-energy corrections in the medium}. The equilibration of ``sterile neutrinos'' via active-sterile mixing is discussed.

Due to its major interest for the chemistry of planetary atmospheres and exobiology, accurate spectroscopy data of phosphine are required for the search of signatures of this molecule in astronomical observations. In this work, high resolution infrared laboratory spectra of phosphine were analyzed for the first time in the full Tetradecad region (3769–4763 cm−1) involving 26 rotationally resolved bands. Overall, 3242 lines were assigned in spectra previously recorded by Fourier transform spectroscopy at temperatures 200 K and 296 K, using a combined theoretical model based on ab initio calculations. The total nuclear motion Hamiltonian of PH 3 including ab initio potential energy surface, was reduced to an effective Hamiltonian using the high-order contact transformation method adapted to vibrational polyads of the AB 3 symmetric top molecules, followed by empirical optimization of the parameters. At this step, the experimental line positions were reproduced with a standard deviation of 0.0026 cm−1 that provided unambiguous identification of observed transitions. The effective dipole transition moments of the bands were obtained by fitting to the intensities obtained from variational calculations using the ab initio dipole moment surface. The assigned lines were used to newly determine 1609 experimental vibration-rotational levels up to J max = 18 with energy in the range 3896–6037 cm−1 that represents significant extension towards higher energies compared to previous works. Transitions for all 26 sublevels of the Tetradecad were identified but with noticeably fewer transitions for fourfold excited bands because of their weaker intensity. At the final step, pressure-broadened half widths were attached to each transition and a composite line list adopting ab initio intensities and empirical line positions corrected to the accuracy of about 0.001 cm−1 for strong and medium transitions was validated against experimental spectra available in the literature. [ABSTRACT FROM AUTHOR]

• High resolution 13CH 4 absorption spectrum at 80 K was analyzed in the 5471–5852 cm−1 region. • 3251 line positions were theoretically modeled with rms deviation of 0.0013 cm−1. • 1624 experimental line intensities were fitted with rms deviation of 9%. • Assigned experimental and theoretical line lists at 80 K are provided. This work continues our investigation on the high resolution infrared 13C-enriched spectrum of methane recorded on a Bruker IFS-125HR Fourier transform spectrometer at 80 K in the 4970–6200 cm−1 range. In this paper we report the results of assignment and modelling of the line positions and intensities of 13CH 4 in the range between 5471 and 5852 cm−1, corresponding to the middle part of Tetradecad. The analysis was performed using a non-empirical effective Hamiltonian (EH) derived from the ab initio potential energy surface (PES) by high-order Contact Transformations (CT). The EH parameters were empirically optimized during the treatment of experimental data. In total, 3297 lines belonging to nine cold bands of the Tetradecad of 13CH 4 were assigned in the 5471–5852 cm−1 range up to J max = 11. The 3297 lines included correspond to 91% of the total line number in the region. Among them, 3251 line positions were fitted with an rms deviation of 1.33×10−3 cm−1. Measured line intensities for 1624 transitions were modeled using the effective dipole transition moments approach with an rms deviation of about 9%. Finally, the observed transitions were incorporated to fit simultaneously the 13CH 4 Hamiltonian parameters for the {Ground state/Dyad/Pentad/Octad/Tetradecad} system and the dipole moment parameters for the {Ground state - Tetradecad} system. [ABSTRACT FROM AUTHOR]

• Detailed analysis of the ν 1 + 3ν 3 band of 14N16O 2 near 1.66 µm. • More than 3000 lines are assigned with N and K a up to 59 and 13, respectively. • Line positions are reproduced by an effective Hamiltonian with an rms of 0.0023 cm−1. • The (103) upper level is perturbed by the (122), (410) and (080) "dark" states. • The main effective dipole moment parameter is determined. The high resolution absorption spectrum of the NO 2 molecule is recorded between 5855 and 6006 cm−1 by high sensitivity cavity ring down spectroscopy (CRDS). Positions and intensities in the 5 × 10−27 − 4 × 10−23 cm/molecule range are derived from the profile fitting of more than 6900 lines. The absorption is dominated by the transitions of the (103)–(000) vibrational band centered at 5984.704 cm−1. The spectrum is assigned and modeled using an effective Hamiltonian (EH) which explicitly takes into account a spin–rotational interaction and interaction of the (103) bright state with three "dark" states: (122), (080) and (410). The ν 1 + 3ν 3 band was already analyzed by Fourier transform spectroscopy (FTS) in Miljanic S., Perrin A., Orphal J., Fellows C.E., Chélin, J. Mol. Spectrosc. 2008, V. 251, P. 9–15, where resonance interactions between the (103) state and the (122) and (080) "dark" states were taken into account. More than 3000 lines are presently assigned with rotational quantum numbers N and K a up to 59 and 13, respectively, while 1147 transitions with maximum N and K a values of 47 and 8, respectively, were previously identified by FTS. The measured line positions are reproduced with an (obs.-calc.) rms of 0.0023 cm−1 by variation of 41 EH parameters. About 80 transitions reaching the (080) highly excited bending upper state and borrowing their intensity from the resonance coupling with the (103)–(000) band were assigned for the first time. The main parameter in the transition moment series of the ν 1 + 3ν 3 band is determined from the fitting of the measured intensities. Image, graphical abstract [ABSTRACT FROM AUTHOR]

• Global fitting of 14N 2 16O line positions in the 0–14,917 cm−1 region is performed. • Non-polyad model of effective Hamiltonian is used. • Value of the weighted standard deviation of the fit is 1.71. • Comparison with HITRAN2016 is presented. The global modeling of 14N 2 16O line positions in the 0.8–14,917 cm−1 region has been performed using the non-polyad model of effective Hamiltonian. The effective Hamiltonian parameters were fitted to the measured line positions collected from the literature. The dimensionless weighted standard deviation of the fit is 1.71. The fitted set of 195 effective Hamiltonian parameters allowed reproduction 56,888 measured line positions with an RMS value of 0.006 cm−1. Comparisons of the calculated line positions based on the fitted model with HITRAN2016 and a recent line list by Bertin et al. (J Quant Spectrosc Radiat Transf 2019;229:40–9) are presented and discussed. Image, graphical abstract [ABSTRACT FROM AUTHOR]

For line intensity: • Systematic investigation on the isotopologue consistency of CO 2 and SO 2 IR/MW intensity. • Interesting TDM/Einstein-A patterns found for CO 2 vibrational polyads, SO 2 and CO 2 MW, fundamentals, hotbands, etc. For line positions: • First 2nd-round implementation of BTRHE strategy to minimize the isotope/mass dependent residuals. • –– Two orders of magnitude improvement for SO 2 MW prediction accuracy: 0.01-0.02 MHz (A/B/C), 0-5 MHz (J/Ka<20 lines). • This approach starts from and targets at higher accuracy, can easily extend to other bands or molecules. The semi-empirical molecular rovibrational IR line lists, such as ExoMol, TheoReTs, and Ames, combine the experimental accuracy and theoretical power to reach better than 0.1 cm−1 accuracy for line positions and better than 80–90% agreement for line intensities. The quality of these existing semi-empirical IR lists allows further improvements of intensity and line positions for those unobserved minor isotopologues. This paper presents our new BTRHE (Best Theory + Reliable High-resolution Experiment) strategy implementation. For line intensity, the isotopologue consistency and the patterns of mass dependence in the Ames-296 K SO 2 and CO 2 IR lists are quantitatively presented along the mass-inverse coordinates. The consistency and patterns are better than those in existing experimental data. The methodology proposed here can be used to identify inconsistencies, outliers, and mistakes in intensities, and help improve Effective Dipole Model (EDM) and molecular IR databases. We call for an experimental study on the 50006 and 60007 bands of CO 2 628. For line position predictions, a simple approach combining the variational IR line lists with Effective Hamiltonian (EH) model may refine the effective rotational constants A 0 /B 0 /C 0 and quartic centrifugal distortion constants of minor isotopologues. The prediction accuracy may be improved by two orders of magnitude, i.e. reaching 0–5 MHz prediction accuracy in the range of J < 20–30, K a < 10–20, and 0.01–0.02 MHz accuracy for A 0 /B 0 /C 0. Several important factors have been systematically investigated and discussed, e.g. convergence, uncertainties, higher order terms, fixing EH parameters, mass coordinates, etc. A microwave (MW) line set consisting of 644,636 strong transitions (at 296K) for all 30 isotopologues and corresponding refined EH(Ames) parameters are reported in the supplementary material. This approach may be easily extended to rovibrational bands, hot bands, and other molecular systems. [ABSTRACT FROM AUTHOR]

Ewald summation is an important technique used to deal with long-range Coulomb interaction. While it is widely used in simulations of molecules and solid state materials, many important results are dispersed in literature and their implementations are often buried deep in large software packages. Since reliable and systematic calculation of Coulomb interaction is critical for the investigation of perovskites, here we start from the fundamentals of Ewald summation and derive clear expressions for long-range charge-charge, dipole-dipole, and charge-dipole interactions, which can be readily used for numerical computations. We also provide the interaction matrix for efficient Monte Carlo simulations involving charges and dipoles, implementing them in a Python software package. A new type of interaction matrix, which accounts for the electrostatic energy change when ions are displaced, is also derived and implemented. These results are the foundations for the investigation of ferroelectric perovskites. [ABSTRACT FROM AUTHOR]

Graphical abstract Highlights • Spin-torsion dominated hyperfine splittings in vt = 1 torsional state of methanol. • Proton-spin magnetic hyperfine interactions in E torsional states of methanol. • Least-squares fit to effective spin-rotation, spin-torsion, & spin–spin Hamiltonian. • Lamb-dip sub-mm-wave measurements for methanol from three laboratories. Abstract Doublet, triplet and quartet hyperfine splitting patterns have been observed in the E-species component of the first excited (v t = 1) torsional state of CH 3 OH. Four series of lines dominate the available data: (i) a K = 6 ← 7, Q branch series of quartets, with 7 ≤ J ≤ 15; (ii) a K = 3 ← 2, Q branch series, with 3 ≤ J ≤ 18, which starts as quartets, changes to doublets at J = 7, and then finally to singlets at J = 17; (iii) a K = −2 ← −3, P branch series of doublets, with 8 ≤ J ≤ 13; and (iv) a K = 8 ← 7, Q branch series, with 8 ≤ J ≤ 24, which starts as triplets and becomes doublets at J = 12. There are also a few isolated doublets and quartets, which do not form long spectroscopic branches. We have modeled the hyperfine quartet and doublet splittings with empirically chosen symmetry-allowed spin-torsion, spin-rotation, and spin–spin interaction terms appropriate for the two I = ½ spin systems arising from the OH proton and from the CH 3 protons, respectively, and have achieved a least-squares fit of 144 hyperfine components (88 hyperfine intervals) to six hyperfine parameters with a standard deviation (0.75 kHz) near experimental measurement accuracy. The three spin-rotation parameters associated with one I = ½ spin system in the fit agree well with ab initio predictions for I OH parameters in the literature. The two spin-torsion parameters agree to some extent with other fitted values in the literature. The physical effects included in the present Hamiltonian differ from those included in the Hamiltonian for our earlier work on hyperfine splittings in v t = 0 E states, because the experimentally observed v t = 1 splittings are found to decrease approximately as 1/ J , whereas the v t = 0 splittings increase approximately as J. The position and relative intensities of the weak central features of the triplets are explained as three-level Λ-type crossover resonances. [ABSTRACT FROM AUTHOR]

Highlights • 9015 line positions and intensities were retrieved in the 3760–4100 cm−1 region. • More than 3500 new assignments were made in the region. • 5703 experimental line positions were modeled to 0.00179 cm−1. • 2659 experimental line intensities at 296 K were modeled to 7.46% rms. Abstract Eight spectra of normal samples of CH 4 in the 3760–4100 cm−1 region were recorded by using a Fourier transform spectrometer in Reims, France at long paths (202 m, 602 m, 1604 m and 1804 m) and different pressures. Additional spectra of 12CH 4 covering the same region were obtained at 100 K and very low sample pressures, which was used for low-J line position identification. Line positions and intensities were retrieved by least squares curve-fitting procedures and analyzed using the effective Hamiltonian and the effective dipole. A new measured line list contains positions and intensities for 9000 features. Quantum assignments were made for more than 3500 new transitions, which represent ∼95% of the integrated line intensity observed in this region. The resulting list of lines is much more accurate than the list of lines HITRAN 2016. All assigned 5703 line positions were fitted to RMS standard deviations of 0.0018 cm−1. The sum of observed intensities between 3760 and 4100 cm−1 fell within 2% of the predicted value from ab initio variational calculations reported in the TheoReTS database (http://theorets.univ-reims.fr ; http://theorets.tsu.ru). [ABSTRACT FROM AUTHOR]

A variant of coupled-cluster theory is described here, wherein the degrees of freedom are fluctuations of fragments between internally correlated states. The effects of intra-fragment correlation on the inter-fragment interaction are pre-computed and permanently folded into an effective Hamiltonian, thus avoiding redundant evaluations of local relaxations associated with coupled fluctuations. A companion article shows that a low-scaling step may be used to cast the electronic Hamiltonians of real systems into the form required. Two proof-of-principle demonstrations are presented here for non-covalent interactions. One uses harmonic oscillators, for which accuracy and algorithm structure can be carefully controlled in comparisons. The other uses small electronic systems (Be atoms) to demonstrate compelling accuracy and efficiency, also when inter-fragment electron exchange and charge transfer must be handled. Since the cost of the global calculation does not depend directly on the correlation models used for the fragments, this should provide a way to incorporate difficult electronic structure problems into large systems. This framework opens a promising path for building tunable, systematically improvable methods to capture properties of systems interacting with a large number of other systems. The extension to excited states is also straightforward. [ABSTRACT FROM AUTHOR]