Finally, VCD and ROA spectra were perfectly reproduced by the DFT/PCM calculations for the Boltzmann-averaged G and G- conformers. The behavior of the specific optical rotation values with the different solvents was correctly reproduced by time-dependent DFT calculations using the polarizable continuum model (PCM), except for the benzene for which explicit solvent model should be necessary. The population ratios of the two main conformers were modified for solvents exhibiting higher dielectric constants (G- form decreases whereas G form increases). Density functional theory (DFT) calculations at the B3LYP/aug-cc-pVTZ level revealed the presence of three conformers (G , G-, and cis) with Gibbs populations of 51, 44, and 5% for the isolated molecule, respectively. The two enantiomers of 2,2'-bioxirane were synthesized, and their chiroptical properties were thoroughly investigated in various solvents by polarimetry, vibrational circular dichroism (VCD), and Raman optical activity (ROA). Our results demonstrate that linear-response DCT is a promising theoretical approach for excited states of molecules. In a study of butadiene and hexatriene, LR-ODC-12 correctly describes the relative energies of the singly-excited $1^1\mathrm$ state by almost 1 eV. For small molecules, LR-ODC-12 shows smaller mean absolute errors in excitation energies than equation-of-motion coupled cluster theory with single and double excitations (EOM-CCSD), relative to the reference data from EOM-CCSDT. We discuss the derivation of linear-response DCT, present its implementation for the ODC-12 method (LR-ODC-12), and benchmark its performance for excitation energies in small molecules (N$_2$, CO, HCN, HNC, C$_2$H$_2$, and H$_2$CO), as well as challenging excited states in ethylene, butadiene, and hexatriene. In the original DCT formulation, only information about a single electronic state (usually, the ground state) is obtained. We present a linear-response formulation of density cumulant theory (DCT) that provides a balanced and accurate description of many electronic states simultaneously. For higher intensities, however, TD-OCEPA0 tends to overestimate the correlation effect, as occasionally observed for CEPA0 in the ground-state correlation energy calculations. The TD-OCEPA0 results show good agreement with TD-CASSCF ones for moderate laser intensities. The computed results, including high-harmonic generation spectra and ionization yields, are compared with those of various other methods ranging from uncorrelated time-dependent Hartree–Fock to fully correlated (within the active orbital space) time-dependent complete-active-space self-consistent field (TD-CASSCF). We employed this method to simulate the electron dynamics in Ne and Ar atoms exposed to intense near infrared laser pulses with various intensities. It is size extensive, gauge invariant, and computationally much more efficient than the TD-OCC method with double excitations. The method, designated as TD-OCEPA0, is a time-dependent extension of the simplest version of the coupled-electron pair approximation with optimized orbitals. We report the implementation of a cost-effective approximation method within the framework of the time-dependent optimized coupled-cluster (TD-OCC) method for real-time simulations of intense laser-driven multielectron dynamics. The potential repercussions of this strong geometry dependence on the magnitude of vibrational corrections are discussed. This effect is tested for methyloxirane in which the C–O bond associated with the stereogenic carbon is stretched and for a pyramidalized fluoroformaldehyde where the corresponding C=O bond is similarly extended. However, small distortions away from equilibrium affect the optimized-orbital coupled cluster approach to a much greater extent than its Hartree–Fock counterpart. For molecular structures near equilibrium, where the coupled cluster method is relatively insensitive to the choice of reference wave function, the two approaches yield similar results for test cases such as (S)-methyloxirane, (S)-methylthiirane, (S)-2-chloropropionitrile, and (1S, 4S)-norbornenone. The impact of variational optimization of the molecular orbitals used in coupled cluster response theory is compared to the use of conventional Hartree–Fock orbitals for the computation of optical rotations in a number of chiral compounds.
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