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Determining the properties of molecules and materials is one of the premier applications of quantum computing. A major question in the field is how to use imperfect near-term quantum computers to solve problems of practical value. Inspired by the recently developed variants of the quantum counterpart of the equation-of-motion (qEOM) approach and the orbital-optimized variational quantum eigensolver (oo-VQE), we present a quantum algorithm (oo-VQE-qEOM) for the calculation of molecular properties by computing expectation values on a quantum computer. We perform noise-free quantum simulations of BeH2 in the series of STO-3G/6-31G/6-31G* basis sets, H4 and H2O in 6-31G using an active space of four electrons and four spatial orbitals (8 qubits) to evaluate excitation energies, electronic absorption, and for twisted H4, circular dichroism spectra. We demonstrate that the proposed algorithm can reproduce the results of conventional classical CASSCF calculations for these molecular systems.

We explore Davidson methods for obtaining excitation energies and other linear response properties within quantum self-consistent linear response (q-sc-LR) theory. Davidson-type methods allow for obtaining only a few selected excitation energies without explicitly constructing the electronic Hessian since they only require the ability to perform Hessian-vector multiplications. We apply the Davidson method to calculate the excitation energies of hydrogen chains (up to H10) and analyze aspects of statistical noise for computing excitation energies on quantum simulators. Additionally, we apply Davidson methods for computing linear response properties such as static polarizabilities for H2, LiH, H2O, OH−, and NH3, and show that unitary coupled cluster outperforms classical projected coupled cluster for molecular systems with strong correlation. Finally, we formulate the Davidson method for damped (complex) linear response, with application to the nitrogen K-edge X-ray absorption of ammonia, and the C6 coefficients of H2, LiH, H2O, OH−, and NH3.

We explore Davidson methods for obtaining excitation energies and other linear response properties within quantum self-consistent linear response (q-sc-LR) theory. Davidson-type methods allow for obtaining only a few selected excitation energies without explicitly constructing the electronic Hessian since they only require the ability to perform Hessian-vector multiplications. We apply the Davidson method to calculate the excitation energies of hydrogen chains (up to H10) and analyze aspects of statistical noise for computing excitation energies on quantum simulators. Additionally, we apply Davidson methods for computing linear response properties such as static polarizabilities for H2, LiH, H2O, OH−, and NH3, and show that unitary coupled cluster outperforms classical projected coupled cluster for molecular systems with strong correlation. Finally, we formulate the Davidson method for damped (complex) linear response, with application to the nitrogen K-edge X-ray absorption of ammonia, and the C6 coefficients of H2, LiH, H2O, OH−, and NH3.

Determining the properties of molecules and materials is one of the premier applications of quantum computing. A major question in the field is how to use imperfect near-term quantum computers to solve problems of practical value. Inspired by the recently developed variants of the quantum counterpart of the equation-of-motion (qEOM) approach and the orbital-optimized variational quantum eigensolver (oo-VQE), we present a quantum algorithm (oo-VQE-qEOM) for the calculation of molecular properties by computing expectation values on a quantum computer. We perform noise-free quantum simulations of BeH2 in the series of STO-3G/6-31G/6-31G* basis sets, H4 and H2O in 6-31G using an active space of four electrons and four spatial orbitals (8 qubits) to evaluate excitation energies, electronic absorption, and for twisted H4, circular dichroism spectra. We demonstrate that the proposed algorithm can reproduce the results of conventional classical CASSCF calculations for these molecular systems.

We explore Davidson methods for obtaining excitation energies and other linear response properties within quantum self-consistent linear response (q-sc-LR) theory. Davidson-type methods allow for obtaining only a few selected excitation energies without explicitly constructing the electronic Hessian since they only require the ability to perform Hessian-vector multiplications. We apply the Davidson method to calculate the excitation energies of hydrogen chains (up to H10) and analyze aspects of statistical noise for computing excitation energies on quantum simulators. Additionally, we apply Davidson methods for computing linear response properties such as static polarizabilities for H2, LiH, H2O, OH−, and NH3, and show that unitary coupled cluster outperforms classical projected coupled cluster for molecular systems with strong correlation. Finally, we formulate the Davidson method for damped (complex) linear response, with application to the nitrogen K-edge X-ray absorption of ammonia, and the C6 coefficients of H2, LiH, H2O, OH−, and NH3.

We explore Davidson methods for obtaining excitation energies and other linear response properties within quantum self-consistent linear response (q-sc-LR) theory. Davidson-type methods allow for obtaining only a few selected excitation energies without explicitly constructing the electronic Hessian since they only require the ability to perform Hessian-vector multiplications. We apply the Davidson method to calculate the excitation energies of hydrogen chains (up to H10) and analyze aspects of statistical noise for computing excitation energies on quantum simulators. Additionally, we apply Davidson methods for computing linear response properties such as static polarizabilities for H2, LiH, H2O, OH−, and NH3, and show that unitary coupled cluster outperforms classical projected coupled cluster for molecular systems with strong correlation. Finally, we formulate the Davidson method for damped (complex) linear response, with application to the nitrogen K-edge X-ray absorption of ammonia, and the C6 coefficients of H2, LiH, H2O, OH−, and NH3.

The excited state hydrogen transfer (ESHT) reaction in pyrrole-ammonia clusters (PyH·(NH3)n, n = 2–5) at excitation wavelengths below 218 nm down to 199 nm, has been studied using a combination of velocity map imaging and non-resonant detection of the NH4(NH3)n−1 products. Special care has been taken to avoid evaporation of solvent molecules from the excited clusters by controlling the intensity of both the excitation and probing lasers. The high resolution translational energy distributions obtained are analyzed on the base of an impulsive mechanism for the hydrogen transfer, which mimics the direct N−H bond dissociation of the bare pyrrole. In spite of the low dissociation wavelengths attained (∼200 nm) no evidence of hydrogen-loss statistical dynamics has been observed. The effects of clustering of pyrrole with ammonia molecules on the possible statistical decomposition channels of the bare pyrrole are discussed, MICINN, MICINN, Depto. de Química Física, Fac. de Ciencias Químicas, TRUE, pub

This study examines the computational challenges in elucidating intricate chemical systems, particularly through ab-initio methodologies. This work highlights the Divide-Expand-Consolidate (DEC) approach for coupled cluster (CC) theory—a linear-scaling, massively parallel framework—as a viable solution. Detailed scrutiny of the DEC framework reveals its extensive applicability for large chemical systems, yet it also acknowledges inherent limitations. To mitigate these constraints, the cluster perturbation theory is presented as an effective remedy. Attention is then directed towards the CPS (D-3) model, explicitly derived from a CC singles parent and a doubles auxiliary excitation space, for computing excitation energies. The reviewed new algorithms for the CPS (D-3) method efficiently capitalize on multiple nodes and graphical processing units, expediting heavy tensor contractions. As a result, CPS (D-3) emerges as a scalable, rapid, and precise solution for computing molecular properties in large molecular systems, marking it an efficient contender to conventional CC models.

Determining the properties of molecules and materials is one of the premier applications of quantum computing. A major question in the field is how to use imperfect near-term quantum computers to solve problems of practical value. Inspired by the recently developed variants of the quantum counterpart of the equation-of-motion (qEOM) approach and the orbital-optimized variational quantum eigensolver (oo-VQE), we present a quantum algorithm (oo-VQE-qEOM) for the calculation of molecular properties by computing expectation values on a quantum computer. We perform noise-free quantum simulations of BeH2 in the series of STO-3G/6-31G/6-31G* basis sets, H4 and H2O in 6-31G using an active space of four electrons and four spatial orbitals (8 qubits) to evaluate excitation energies, electronic absorption, and for twisted H4, circular dichroism spectra. We demonstrate that the proposed algorithm can reproduce the results of conventional classical CASSCF calculations for these molecular systems.

This study examines the computational challenges in elucidating intricate chemical systems, particularly through ab-initio methodologies. This work highlights the Divide-Expand-Consolidate (DEC) approach for coupled cluster (CC) theory—a linear-scaling, massively parallel framework—as a viable solution. Detailed scrutiny of the DEC framework reveals its extensive applicability for large chemical systems, yet it also acknowledges inherent limitations. To mitigate these constraints, the cluster perturbation theory is presented as an effective remedy. Attention is then directed towards the CPS (D-3) model, explicitly derived from a CC singles parent and a doubles auxiliary excitation space, for computing excitation energies. The reviewed new algorithms for the CPS (D-3) method efficiently capitalize on multiple nodes and graphical processing units, expediting heavy tensor contractions. As a result, CPS (D-3) emerges as a scalable, rapid, and precise solution for computing molecular properties in large molecular systems, marking it an efficient contender to conventional CC models.

Determining the properties of molecules and materials is one of the premier applications of quantum computing. A major question in the field is how to use imperfect near-term quantum computers to solve problems of practical value. Inspired by the recently developed variants of the quantum counterpart of the equation-of-motion (qEOM) approach and the orbital-optimized variational quantum eigensolver (oo-VQE), we present a quantum algorithm (oo-VQE-qEOM) for the calculation of molecular properties by computing expectation values on a quantum computer. We perform noise-free quantum simulations of BeH2 in the series of STO-3G/6-31G/6-31G* basis sets, H4 and H2O in 6-31G using an active space of four electrons and four spatial orbitals (8 qubits) to evaluate excitation energies, electronic absorption, and for twisted H4, circular dichroism spectra. We demonstrate that the proposed algorithm can reproduce the results of conventional classical CASSCF calculations for these molecular systems.

This thesis presents the implementation, assessment, and applicability of the currently most accurate density functional approximations (DFAs) for the calculation of vertical excitation energies by means of linear-response time-dependent density functional theory (TD-DFT). Firstly, we demonstrate how the inclusion of a single parameter, which controls the interplay between interlectronic short- and long-regimes, improves global double-hybrid density functional approximations (DHDFAs) when it comes to long-range excitations; such as Rydberg states and charge-transfer (CT) transitions, without much loss of accuracy for local-valence ones. In this context, we define the two first long-range corrected (LC) DHDFAs optimised for excitation energies, namely, wB2PLYP and wB2GP-PLYP, being the best TD-DFT methods until then. Furthermore, we include an additional analysis of the CT problem after noticing some misconceptions by some in the developer community regarding the application of global DHDFAs as an alternative to the LC ones. In the second part, we investigate the performance of the best DHDFAs for singlet-singlet transitions in the Tamm-Dancoff Approximation (TDA) but, more importantly, we revive the applicability of DHDFAs for singlet-triplet excitations after 12 years of slumber. Herein, we demonstrate once again that our LC-DHDFAs are the best-performing methods not only for singlet but also for triplet excitations, being the most accurate and robust methods until then. Finally, inspired by the precursory work done by Schwabe and Goerigk in 2017, we extend our two previous studies by combining our methods with the Spin-Component Scaling (SCS) and Spin-Opposite Scaling (SOS) techniques, with the main difference being the inclusion of LC-DHDFAs and singlet-triplet excitations in the context of DHDFAs. We also defined the currently two best LC-DHDFAs optimised for excitation energies on the market, namely, wB88PP86 and wPBEPP86. We particularly note that the SCS and

Large configuration interaction (CI) calculations can be performed if part of the interaction is treated perturbatively. To evaluate the combined CI and perturbative method, we compute excitation energies for the states in Mg-like iron. Starting from a CI calculation including valence and core-valence correlation effects, it is found that the perturbative inclusion of core-core electron correlation halves the mean relative differences between calculated and observed excitation energies. The effect of the core-core electron correlation is largest for the more excited states. The final relative differences between calculated and observed excitation energies is 0.023%, which is small enough for the calculated energies to be of direct use in line identifications in astrophysical and laboratory spectra.

Large configuration interaction (CI) calculations can be performed if part of the interaction is treated perturbatively. To evaluate the combined CI and perturbative method, we compute excitation energies for the states in Mg-like iron. Starting from a CI calculation including valence and core-valence correlation effects, it is found that the perturbative inclusion of core-core electron correlation halves the mean relative differences between calculated and observed excitation energies. The effect of the core-core electron correlation is largest for the more excited states. The final relative differences between calculated and observed excitation energies is 0.023%, which is small enough for the calculated energies to be of direct use in line identifications in astrophysical and laboratory spectra.

Light-matter interactions lead to a variety of interesting phenomena, for example photosynthesis which is a process fundamental to life on earth. There exists many different spectroscopic methods to measure light-matter interactions, for example UV/Vis spectroscopy, that can provide information about electronically excited states. However, numerical methods and theory are important to model and gain understanding of these experiments. Quantum chemistry provides that understanding, giving the possibility to numerically calculate molecular properties like excitation energies. The aim of this thesis was to implement a reduced-space algorithm in Dalton, to solve an eigenvalue equation obtained by response theory, for the calculation of excitation energies of molecular systems. There already was a similar algorithm in Dalton, that was able to perform these calculations. However, in a different module of Dalton used mainly for complex response theory, an algorithm to obtain eigenvalues was missing. The new implementation was similar to the existing one, except for the division of the reduced space into even and odd parts used in the complex response module. The thesis starts with a quick introduction of light-matter interactions and proceeds with a description of many-body theory, including numerical methods used in that field. In the end of the theoretical part, the eigenvalue equation, used to calculate excitation energies, is derived. In the following section, the reduced-space algorithm is described. In the end of the thesis, numerical results obtained with the algorithm are presented, including a small basis set and method study. The comparison with the existing implementation of the similar algorithm verified the successful implementation of the algorithm presented in this thesis.

We demonstrate theoretically that laser-induced coherent quantum interference control of asymptotic states of dissociating molecules is possible, starting from a single vibrational eigenstate, after the interaction with two laser pulses-at a fixed time delay-both operating in the weak-field limit. Thus, phase dependence in the interaction with the second fixed-energy phase-modulated pulse persists after the pulse is over. This is illustrated for the nonadiabatic process: I + Br*←IBr → I + Br, where the relative yield of excited Br* can be changed by pure phase modulation. Furthermore, a strong frequency dependence of the branching ratio is observed and related to the re-crossing dynamics of the avoided crossing in the above-mentioned process.

This paper investigates the concept of piezoaeroelasticity for energy harvesting. The focus is placed on mathematical modeling and experimental validations of the problem of generating electricity at the flutter boundary of a piezoaeroelastic airfoil. An electrical power output of 10.7 mW is delivered to a 100 k load at the linear flutter speed of 9.30 m/s (which is 5.1% larger than the short-circuit flutter speed). The effect of piezoelectric power generation on the linear flutter speed is also discussed and a useful consequence of having nonlinearities in the system is addressed. (C) 2010 American Institute of Physics. [doi:10.1063/1.3427405]

We report one- (OPA) and two-photon absorption (TPA) excitation energies and cross-sections for a series of platinum acetylides using time-dependent density functional theory. Because of the facile rotations of the trimethylphosphinyl and phenylene groups, we apply a Boltzmann-weighted average over thermally accessible conformations to obtain the final spectra, resulting in better agreement with experimental observations. We examine various basis sets and functionals to evaluate their performance in the gas-phase and in solution. Effects of donor and acceptor groups on the OPA and TPA resonances and intensities are also discussed., Published in Journal of Physical Chemistry A, v113 n50 p13943-13952, Dec 2009.

A series of one-photon (OPA) absorption spectra for fluorine-based donor pi acceptor molecules is presented and spectroscopically assigned, based upon the results obtained from time-dependent density functional theory (TDDFT). The computed excitation energies were generally shown to be in good agreement with experiment, particularly when compared to results from measurements carried out in a nonpolar solvent, which were available for some molecules. The computed oscillator strengths may resolve discordant experimental values in some cases, for example, for AF-380, AF-270, and AF-295. However, a quantitative comparison between computed and observed oscillator strengths is complicated by band overlapping. Thus, the computed extinction coefficients obtained by summing over the Gaussian bands are useful in such cases., Submitted to the Journal of Physical Chemistry.

We report one- (OPA) and two-photon absorption (TPA) excitation energies and cross sections for a series of 7-aminocoumarins using time-dependent density functional theory with various basis sets and functionals, including exchange-correlation functionals using the Coulomb-attenuating method (CAMB3LYP and mCAMB3LYP), to evaluate their performance in gas-phase and in solvents. Except for the CAMB3LYP results , the computed OPA excitation energies and transition dipole moments are in good agreement with experiment. The range of errors obtained from various functionals is discussed in detail. The relationship of donor and acceptor groups with OPA and TPA resonances and intensities is also discussed., Prepared in collaboration with UES, Inc., Dayton, OH, and General Dynamics Information Technology, Dayton, OH.

The appearance of gap states in benzylic amide catenane thin films following potassium intercalation was investigated by electron energy loss spectroscopy and quantum chemical calculations. Both theory and experience find an excitation energy for transitions into these new states of ∼2 eV. The characteristics of these states are discussed. © 2002 Elsevier Science B.V. All rights reserved.

Vertical excitation energies for the lowest eleven singlet states of T-d N-4 were calculated using the TD-DFT method with the B3LYP functional, and at the EOM-CCSD level of theory. The vertical excitation energies for the five lowest-lying excited states were also obtained using the state-averaged CASSCF, CASPT2, CASPT3, and MRCI + Q methods. Our results show that the five lowest-lying states are of valence character. EOM-CCSD/d-aug-cc-pVTZ calculations predict that there are two weakly allowed optical transitions of T-2 symmetry at 10.44 and 10.82 eV. The transition to the third T-2 state, which is predicted to be at 10.89 eV, has an oscillator strength about one order of magnitude higher., QC 20100525

The UV photolysis of Ar-HCl is simulated by an exact wave packet calculation. Partial fragmentation of the cluster into H and Ar-Cl fragments is studied by projecting out the asymptotic wave packet onto the product states, at several excitation energies in the range of the Ar-HCl absorption spectrum. The partial fragmentation pathway is found to dominate the photolysis process at very low excitation energies, and to be intense also at high energies. At medium excitation energies the other competing fragmentation pathway, namely total fragmentation into H, Ar, and Cl, dominates almost completely the photodissociation dynamics. The relative intensity of the two fragmentation pathways depends on the extent to which the hydrogen is initially blocked by Ar and Cl. The Ar-Cl radicals are produced with high rotational and low vibrational excitation at most of the Ar-HCl energies studied. The internal energy distributions of Ar-Cl show remarkable differences in shape depending on the regions of the absorption spectrum which are excited. This effect can be exploited to control both the efficiency of Ar-Cl generation and the internal excitation of the radical prepared, by changing the excitation energy of the parent cluster. © 2000 American Institute of Physics.

The feasibility of exciting a localized X-wave pulse from a finite aperture is addressed. Also, the possibility of using a finite-time excitation of a dynamic aperture to generate a finite-energy approximation to an X-wave pulse is explored. The analysis is carried out by using a Gaussian time window to time limit the infinite X-wave initial excitation. Huygens' construction is used to calculate the amplitude of the radiated wave field away from the finite-time source. The decay rate of the peak of the X wave is compared to that of a quasi-monochromatic signal. It is shown that the finite-time X-wave propagates to much farther distances without significant decay. Furthermore, the decay pattern of the radiated X-wave pulse is derived for a source consisting of an array of concentric annular sections. The decay behavior of the radiated pulse is similar to that of an X-wave launched from a finite-time aperture. This confirms the fact that time windowing the infinite energy X-wave excitation is a viable scheme for constructing finite apertures. A discussion of the diffraction limit of the X-wave pulse is also provided.

The elements sulfur, selenium and tellurium have been extensively studied in the liquid state on account of their interesting electrical and physical properties. According to current understanding, these materials exist in the liquid in the form of chain- or ring-shaped molecules, depending on the material and temperature range. Their electrical properties are strongly influenced by bonding irregularities in the predominant structure, such as chain ends, branches or impurities. As temperature increases, the relative concentration of these effects in bonding also increases, with a concomitant increase in the conductivity. Physically, the viscosity of the liquids decreases rapidly with increasing temperature, suggesting that the liquids undergo marked structural changes (ring-chain transformation, and shortening of chain length with increasing temperature, for example). Although powerful tools such as NMR, neutron scattering, and viscosity measurements, among many others, have been used to study these materials, a complete understanding of the dynamical behavior of these liquids is still lacking. We describe here an application of time-differential perturbed angular correlation (TDPAC) to the study of molecular dynamics in liquid S and Se., Prepared in cooperation with Maryland Univ. Baltimore County, Catonsville.

A Görling–Levy (GL)-based perturbation theory along the range-separated adiabatic connection is assessed for the calculation of electronic excitation energies. In comparison with the Rayleigh–Schrödinger (RS)-based perturbation theory this GL-based perturbation theory keeps the ground-state density constant at each order and thus gives the correct ionisation energy at each order. Excitation energies up to first order in the perturbation have been calculated numerically for the helium and beryllium atoms and the hydrogen molecule without introducing any density-functional approximations. In comparison with the RS-based perturbation theory, the present GL-based perturbation theory gives much more accurate excitation energies for Rydberg states but similar excitation energies for valence states.

A Görling–Levy (GL)-based perturbation theory along the range-separated adiabatic connection is assessed for the calculation of electronic excitation energies. In comparison with the Rayleigh–Schrödinger (RS)-based perturbation theory this GL-based perturbation theory keeps the ground-state density constant at each order and thus gives the correct ionisation energy at each order. Excitation energies up to first order in the perturbation have been calculated numerically for the helium and beryllium atoms and the hydrogen molecule without introducing any density-functional approximations. In comparison with the RS-based perturbation theory, the present GL-based perturbation theory gives much more accurate excitation energies for Rydberg states but similar excitation energies for valence states.

A Görling–Levy (GL)-based perturbation theory along the range-separated adiabatic connection is assessed for the calculation of electronic excitation energies. In comparison with the Rayleigh–Schrödinger (RS)-based perturbation theory this GL-based perturbation theory keeps the ground-state density constant at each order and thus gives the correct ionisation energy at each order. Excitation energies up to first order in the perturbation have been calculated numerically for the helium and beryllium atoms and the hydrogen molecule without introducing any density-functional approximations. In comparison with the RS-based perturbation theory, the present GL-based perturbation theory gives much more accurate excitation energies for Rydberg states but similar excitation energies for valence states.