Search

Nanophotonic concepts can improve measurement techniques by enhancing and tailoring the light–matter interaction. However, the optical response of devices that implement such techniques can be intricate, depending on the sample under investigation. Nanophotonics is therefore a ripe field for applying the concept of a digital twin: a digital representation of an entire real‐world device. In this work, the concept of a digital twin is detailed with the example of a nanophotonically enhanced chiral sensing platform. In that platform, helicity‐preserving cavities enhance the interaction between chiral molecules and light, allowing faster measurement of the circular dichroism of the molecules. The sheer presence of the molecules affects the cavity's functionality, demanding a holistic treatment to understand the device's performance. In the digital twin, optical and quantum chemistry simulations are fused to provide a comprehensive description of the system and predict the circular dichroism spectrum. Performing simulations in lockstep with the experiment will allow a clear interpretation of measurement results. This work also demonstrates how to design a cavity‐enhanced circular dichroism spectrometer by utilizing the digital twin. The digital twin can be used to guide experiments and analyze results, and its underlying concept can be translated to many other optical experiments. [ABSTRACT FROM AUTHOR]

Herein we present the results of a blind challenge to quantum chemical methods in the calculation of dimerization preferences in the low temperature gas phase. The target of study was the first step of the microsolvation of furan, 2-methylfuran and 2,5-dimethylfuran with methanol. The dimers were investigated through IR spectroscopy of a supersonic jet expansion. From the measured bands, it was possible to identify a persistent hydrogen bonding OH-O motif in the predominant species. From the presence of another band, which can be attributed to an OH-л interaction, we were able to assert that the energy gap between the two types of dimers should be less than or close to 1 kJ/mol across the series. These values served as a first evaluation ruler for the 12 entries featured in the challenge. A tentative stricter evaluation of the challenge results is also carried out, combining theoretical and experimental results in order to define a smaller error bar. The process was carried out in a double-blind fashion, with both theory and experimental groups unaware of the results on the other side, with the exception of the 2,5-dimethylfuran system which was featured in an earlier publication. [ABSTRACT FROM AUTHOR]

We herein report on new synthetic strategies for the preparation of pyridine and imidazole substituted 2,2'‐dihalo biphenyls. These structures are pre‐ligands suitable for the preparation of respective stannoles. The latter can successfully be transmetalated to K[AuCl4] forming non‐palindromic [(C^C^D)AuIII] pincer complexes featuring a lateral pyridine (D=N) or N‐heterocyclic carbene (NHC, D=C') donor. The latter is the first report on a pincer complex with two formally anionic sp2 and one carbenic carbon donor. The [(C^C^D)AuIII] complexes show intense phosphorescence in solution at room temperature. We discuss the developed multistep strategy and touch upon synthetic challenges. The prepared complexes have been fully characterized including X‐ray diffraction analysis. The gold(III) complexes' photophysical properties have been investigated by absorption and emission spectroscopy as well as quantum chemical calculations on the quasi‐relativistic two‐component TD‐DFT and GW/Bethe–Salpeter level including spin–orbit coupling. Thus, we shed light on the electronic influence of the non‐palindromic pincer ligand and reveal non‐radiative relaxation pathways of the different ligands employed. [ABSTRACT FROM AUTHOR]

From the 30th of June until the 5th of July 2019, the 9th Molecular Quantum Mechanics conference (MQM 2019) took place in the 'Kongresshaus Stadthalle' in Heidelberg, Germany. The conference had been organised to celebrate the 'German School' of quantum chemistry by honouring four quantum chemists that had pioneered the field in Germany. In particular, the 9th MQM conference was held to the memory of Reinhart Ahlrichs and in honour of Werner Kutzelnigg, Wilfried Meyer, and Sigrid Peyerimhoff. The authors of this article are very thankful to have had the opportunity to chair the historical session, respectively, to deliver the laudation for Werner Kutzelnigg. In what follows, we would like to give a few biographical notes on the life of Werner, say a few words about his personality, and highlight his most important and pioneering contributions to the field of quantum chemistry. [ABSTRACT FROM AUTHOR]

An automated implementation of the incremental scheme for the computation of MP2-F12 and CCSD(F12) energies is presented. The numerical accuracy of the approach is explored for a set of 15 chemical reactions using the limiting case of single orbital one-site domains as a worst case scenario. The results are analyzed by the maximum absolute deviation, the mean absolute error, and the root mean square error, with respect to the standard MP2-F12 and CCSD(F12) results. It is found that the MP2 reaction energies are within 1 kcal/mol accuracy at third order of the expansion, whereas the F12 corrections are already sufficiently accurate at second order. For the CCSD(F12) method 1 kcal/mol accuracy is obtained at fourth order. [ABSTRACT FROM AUTHOR]

Recent advances have seen the convergence of the R12 and Gaussian geminal explicitly correlated methods, such that the principal remaining distinction is the way in which the many-electron integrals are handled. Here we examine the weak orthogonality functional and the resolution of the identity and find that the first, although exact in the limit of infinite basis, introduces a conflict between the physical description of the electronic cusp and the satisfaction of the strong orthogonality constraint. This leads us to propose an improved weak orthogonality functional where the explicitly correlated pair functions are almost orthogonal to the occupied orbitals by construction. For applications where 95%-98% accuracy in the total correlation energy is sufficient, we recommend use of the strong orthogonality functional in combination with the resolution of the identity for three- and four-electron integral evaluations. [ABSTRACT FROM AUTHOR]

As known since about two decades, R12 methods, which include terms linear in the interelectronic distance r12 in the wave function, improve substantially the basis set convergence of the ground state correlation energy. In a previous study, however, it was found that the same approach does not give a similar systematic improvement if applied to excited states in the framework of coupled cluster response theory. In the present work, we examine the reason for this behavior and show that the inclusion of additional orbitals in the construction of the r12 pair functions leads to an enhanced basis set convergence (and thus a balanced description) also for the excited states. [ABSTRACT FROM AUTHOR]

A formalism is presented for the calculation of relativistic corrections to molecular electronic energies and properties. After a discussion of the Dirac and Breit equations and their first-order Foldy-Wouthuysen [Phys. Rev. 78, 29 (1950)] transformation, we construct a second-quantization electronic Hamiltonian, valid for all values of the fine-structure constant α. The resulting α-dependent Hamiltonian is then used to set up a perturbation theory in orders of α2, using the general framework of time-independent response theory, in the same manner as for geometrical and magnetic perturbations. Explicit expressions are given to second order in α2 for the Hartree-Fock model. However, since all relativistic considerations are contained in the α-dependent Hamiltonian operator rather than in the wave function, the same approach may be used for other wave-function models, following the general procedure of response theory. In particular, by constructing a variational Lagrangian using the α-dependent electronic Hamiltonian, relativistic corrections can be calculated for nonvariational methods as well. [ABSTRACT FROM AUTHOR]

The accuracy of atomization energies obtained from explicitly correlated coupled-cluster R12 calculations (CC-R12)—including single and double excitation operators (CCSD-R12) and a posteriori perturbative corrections for triple excitations [CCSD[T]-R12 and CCSD(T)-R12]—is studied for CH[sub 2]([sup 1]A[sub 1]), NH[sub 3], H[sub 2]O, HF, N[sub 2], CO, and F[sub 2]. The basis-set convergence with functions of high angular momentum is demonstrated. Unlike for conventional calculations, already the spdf saturation on nonhydrogen atoms and spd saturation on hydrogen are sufficient for CC-R12 calculations to provide results accurate to within 1 kJ/mol of the limit of a complete basis. Remaining small uncertainties at the CCSD[T]-R12 or CCSD(T)-R12 levels are attributed to the insufficient convergence within the coupled-cluster hierarchy towards the limit of full configuration interaction. It is shown that near the basis-set limit (as provided by CC-R12 calculations) the CCSD[T] variant of the triples correction gives, on average, results closer to the experimental data than its CCSD(T) counterpart. Approximate error bars are estimated by one single CC-R12 calculation from the difference between the CCSD[T] and CCSD(T) methods and from the second-order electronic cusp correction in standard approximation B. © 2001 American Institute of Physics. [ABSTRACT FROM AUTHOR]

A principal source of error in electronic structure calculations is the inability of conventional CI (configuration interaction) expansions to describe the electron–electron cusp. This manifests itself in the slow convergence of correlation treatments with finite basis sets which are commonly applied in traditional ab initio quantum chemistry. In this paper we describe results obtained by adding special n-particle functions, which have terms linear in the interelectronic coordinate r12, to the usual trial wave function, which is an expansion in terms of Slater determinants. A vectorized and efficient computer program has been written for putting into practice second-order Mo\ller–Plesset perturbation theory with linear r12 terms (MP2-R12): the sore program. It exploits both direct integral evaluation strategies and techniques that permit the full (also nonabelian) use of molecular point group symmetry. These two ingredients to the program allow for the use of very large Gaussian basis sets in conjunction with the linear r12 terms. As a result we are now able to press into new territories of accuracy. Calculations on Be and Mg clusters illustrate applications of the program. Binding energies are discussed with regard to basis set saturation and with some emphasis on the basis set superposition error (BSSE). The combination of our MP2 basis set limits on one hand with results from CCSD(T) and MRCI calculations with standard basis sets on the other leads to reliable estimates of the binding energies of Be3 (27 kcal/mol), Be4 (88 kcal/mol), Mg3 (8 kcal/mol), and Mg4 (28 kcal/mol). The most extensive MP2-R12 calculations have been performed with very large uncontracted Cartesian Gaussian basis sets. Also, core–core and core–valence correlation effects have been accounted for. In this work we present the results of the first real large-scale calculations employing Hylleraas-type coordinates which have been performed so far on many-electron... [ABSTRACT FROM AUTHOR]

The potential energy surface (PES) of the H+3 ground state is computed by means of the single and double excitation configuration interaction with an explicit linear r12 term in the wave function (CISD-R12) developed recently by the present authors, with a nearly saturated basis set. The points of the PES suggested by Meyer, Botschwina, and Burton (MBB) were chosen and the fitting procedure of the same authors was followed. The present PES has both on an absolute and a relative scale (i.e., relative to the minimum) an error of a few microhartrees (μEh) in the relevant region, an accuracy that has never before been achieved in a quantum chemical calculation for a triatomic molecule. From the fit the vibrational term values for the fundamental bands and some overtones of H+3, H2D+, HD+2, and D+3 were computed by means of the TRIATOM package of Tennyson and Miller. The computed frequencies are in better agreement with experiment (maximum error ∼0.5 cm-1) than those of all previous ab initio calculations (without empirical adjustment). To achieve this accuracy, it is necessary to go beyond the Born–Oppenheimer approximation and to take care of the finite mass ratio between nuclei and electrons. [ABSTRACT FROM AUTHOR]

We present a new method for the evaluation of four-centre electron-repulsion integrals that combines density fitting (DF) with numerical integration by quadrature (QD). In this composite DF-QD approach, the orbital product density is fitted by a truncated spectral set of auxiliary functions and the fitting residual is expanded in a discrete space of functions localised at the eigenvalues of the coordinate operator. The new DF-QD approach has the advantage of DF of reducing computational effort and storage requirements of conventional quantum chemical algorithms, and allows for high accuracy and error control by means of QD. Systematic improvement of the accuracy is achieved for a chosen DF basis set by enlarging the number of grid points used for QD. Test calculations on a set of small molecules reveal that errors in absolute energy of the DF-QD approximation with less than 1000 and 3000 grid points per atom do not exceed one milli- and one microhartree, respectively, even if standard (non-optimised) AO basis sets are used for DF. [ABSTRACT FROM AUTHOR]

Data intense processes such as the establishment of quantitative structure–property relationships, the design of novel compounds and also the validation of new quantum chemical methods call for a structured approach for the processing of the results. Whereas in quantum chemistry there are established de facto standards for the computational methods, community standards for [exchange-] data formats are still under development. In this article we present a benchmark study of the (auxiliary-) basis set dependence of the binding energies of an array of conformers of the ethene dimer using the RI-MP2-F12 method. The study was performed using a version of the TURBOMOLE program package modified to provide output in an extended CML format to be imported in an eXist database. This infrastructure for the generation, archival, analysis, and exchange of quantum chemical data is briefly introduced in this article. [ABSTRACT FROM AUTHOR]

Abstract: Five [2]pseudorotaxanes built from the interaction of a dibenzylammonium cation with five different crown ethers have been investigated by means of high-level ab initio quantum chemical computations. A genetic algorithm has been applied to determine a series of energetically low-lying equilibrium structures for each [2]pseudorotaxane. Different sources for binding interactions such as [N+–H⋯O] and aromatic interactions (π⋯π stacking) are displayed by the optimised structures. In the framework of the genetic algorithm, the equilibrium structures were determined at the level of density functional theory using the meta-GGA (meta generalised gradient approximation) functional TPSS. Subsequent single-point-energy calculations at the level of second-order Møller–Plesset perturbation theory (MP2) in a large basis set (def2-TZVPP) as well as subsequent spin-component-scaled MP2 geometry optimisations were carried out. Binding energies are reported including counterpoise corrections for the basis set superposition error (BSSE) and fragment deformation energies. For two of the five systems studied theoretically, 1:1 complexes (that is, [2]pseudorotaxanes) were identified experimentally by means of 1H NMR spectroscopy, albeit with a very weak association constant for the [2]pseudorotaxane built from dibenzylammonium and the macrocyclic 12,14-diketo-benzo-25-crown-8 molecule. [ABSTRACT FROM AUTHOR]

Explicitly correlated coupled-cluster theory has developed into a valuable computational tool for the calculation of electronic energies close to the limit of a complete basis set of atomic orbitals. In particular at the level of coupled-cluster theory with single and double excitations (CCSD), the space of double excitations is quickly extended towards a complete basis when Slater-type geminals are added to the wave function expansion. The purpose of the present article is to demonstrate the accuracy and efficiency that can be obtained in computational thermochemistry by a CCSD model that uses such Slater-type geminals. This model is denoted as CCSD(F12), where the acronym F12 highlights the fact that the Slater-type geminals are functions f( r12) of the interelectronic distances r12 in the system. The performance of explicitly correlated CCSD(F12) coupled-cluster theory is demonstrated by computing the atomization energies of 73 molecules (containing H, C, N, O, and F) with an estimated root-mean-square deviation from the values compiled in the Active Thermochemical Tables of σ = 0.10 kJ/mol per valence electron. To reach this accuracy, not only the frozen-core CCSD basis-set limit but also high-order excitations (connected triple and quadruple excitations), core–valence correlation effects, anharmonic vibrational zero-point energies, and scalar and spin–orbit relativistic effects must be taken into account. [ABSTRACT FROM AUTHOR]

Abstract: The thermochemistry of the radical is of crucial importance for the modeling of the atmospheric sulfur-oxidation cycle, in which is formed in the reaction . We performed high-level quantum chemical calculations to characterize the energetics of reaction and obtained the following values for the enthalpy of reaction: =−71.4±2.0 and =−72.9±3.0kJ/mol. For the enthalpy of formation of , it follows: and . These new thermochemical data can serve as a basis for kinetic studies of reactions in the atmosphere. [Copyright &y& Elsevier]

We review the current status of quantum chemistry as a predictive tool of chemistry and molecular physics, capable of providing highly accurate, quantitative data about molecular systems. We begin by reviewing wave-function based electronic-structure theory, emphasizing the N-electron hierarchy of coupled-cluster theory and the one-electron hierarchy of correlation-consistent basis sets. Following a discussion of the slow basis-set convergence of dynamical correlation and basis-set extrapolations, we consider the methods of explicit correlation, from the early work of Hylleraas in the 1920s to the latest developments in such methods, capable of yielding high-accuracy results in medium-sized basis sets. Next, we consider the small corrections to the electronic energy (high-order virtual excitations, vibrational, relativistic, and diagonal Born-Oppenheimer corrections) needed for high accuracy and conclude with a review of the composite methods and computational protocols of electronic-structure theory. [ABSTRACT FROM AUTHOR]

The introduction of the resolution-of-the-identity (RI) approximation for electron repulsion integrals in quantum chemical calculations requires in addition to the orbital basis so-called auxiliary or fitting basis sets. We report here such auxiliary basis sets optimized for second-order Møller–Plesset perturbation theory for the recently published (Weigend and Ahlrichs Phys Chem Chem Phys, 2005, 7, 3297–3305) segmented contracted Gaussian basis sets of split, triple-ζ and quadruple-ζ valence quality for the atoms Rb–Rn (except lanthanides). These basis sets are designed for use in connection with small-core effective core potentials including scalar relativistic corrections. Hereby accurate resolution-of-the-identity calculations with second-order Møller–Plesset perturbation theory (MP2) and related methods can now be performed for molecules containing elements from H to Rn. The error of the RI approximation has been evaluated for a test set of 385 small and medium sized molecules, which represent the common oxidation states of each element, and is compared with the one-electron basis set error, estimated based on highly accurate explicitly correlated MP2–R12 calculations. With the reported auxiliary basis sets the RI error for MP2 correlation energies is typically two orders of magnitude smaller than the one-electron basis set error, independent on the position of the atoms in the periodic table. [ABSTRACT FROM AUTHOR]

The recently developed singles-and-doubles linear-r12 corrected coupled-cluster model CCSD(R12) has been extended by inclusion of triples corrections in exactly the same manner as in the popular CCSD(T) model. The resulting model is denoted CCSD(T)(R12). An orthonormal auxiliary basis set is used for the resolution-of-the-identity approximation to calculate the three- and four-electron integrals needed in the linear-r12 Ansatz. CCSD(T)(R12) reaction energies are calculated for a test set of 15 chemical reactions involving 23 small molecules. The CCSD(T)(R12), CCSD(R12), and MP2-R12 results are compared with experimental data obtained by correcting experimental enthalpies of reaction for zero-point vibrational energies and finite temperature contributions. Conventional MP2, CCSD, and CCSD(T) results are presented for comparison. An estimate of the basis set limit is obtained by an aug-cc-pV(Q + d)Z/aug-cc-pV(5 + d)Z CCSD(T) extrapolation for a subset of 12 reactions. All CCSD(T) results are compared with the extrapolated values, and the errors are analyzed for the CCSD(T) and CCSD(T)(R12) models in various basis sets. © 2006 Wiley Periodicals, Inc. Int J Quantum Chem, 2006 [ABSTRACT FROM AUTHOR]

The second-order Møller-Plesset (MP2) correlation energies of the valence shells of C 2 H 4 and C 2 H 6 (with fixed nuclear coordinates) were computed as accurately as technically feasible. The pair energies for singlet and triplet pairs are reported for localized molecular orbitals. An accuracy of the order of 0.01m E h is achieved for all of the individual pair energies. The best estimates of the total valence-shell MP2 correlation energies of C 2 H 4 and C 2 H 6 in the chosen geometries are -373.6±0. and -410.0±0.1m E h , respectively. In the aug-cc-pVQZ, aug-cc-pV5Z and aug-cc-pV6Z basis sets, 96.2, 98.0 and 98.8% of the these values are recovered, respectively. The percentages are improved to 99.8% by means of spin-adapted two-point extrapolations from the aug-cc-pVQZ and aug-cc-pV5Z results. An extrapolation based on the aug-cc-pV5Z and aug-cc-pV6Z correlation energies yields 99.9%. [ABSTRACT FROM AUTHOR]

Abstract: In the complete-basis-set (CBS) model chemistries, an interference correction is added to the total energy. It is added because the CBS-type extrapolation to the basis-set limit of second-order Møller–Plesset (MP2) perturbation theory usually overestimates the magnitude of the basis-set truncation error at higher levels of electron-correlation treatment. In this Letter, the question is pursued whether the CBS-type interference correction could also improve the computational results when the MP2 basis set limit is approached by means of explicitly-correlated MP2 theory using Slater-type geminals. [Copyright &y& Elsevier]

Abstract: We present a universally applicable explicit electron correlation (F12) correction and apply it to Mukherjee’s multi-reference perturbation theory (Mk-MRPT2). Two different F12 corrections are proposed: one is a universal F12 correction which is added to the conventional orbital correction, which is referred to as Mk-MRPT2+F12. In the second type of F12 correction the individual F12 contributions are added to each matrix element of the effective Hamiltonian. Subsequent diagonalization yields the Mk-MRPT2-F12 correction. Thereby, we achieve for both F12 corrections the accuracy of a quadruple- basis set calculation when a triple- basis set is employed and the F12 correction is added. Both F12 corrections reduce to MP2-F12/1A (fixed) in the single-reference limit. [Copyright &y& Elsevier]

Abstract: The phenyl+phenyl reaction was studied in the temperature range 1230–1530K at pressures around 1bar in a shock tube with time-resolved mass-spectrometric detection. We found evidence for the displacement reaction to be important. To quantify the role of this reaction, the formation of ortho-, meta- and para-C6H4 in the singlet and triplet states was characterized by statistical rate theory based on high-level quantum-chemical calculations. Arrhenius parameters for use in kinetic modeling calculations are given. [Copyright &y& Elsevier]