Coupled Cluster Simulations of Vibrational Circular Dichroism Spectra

We report the first simulations of vibrational circular dichroism (VCD) spectra at the coupled-cluster level of theory. Using a generalization of Stephens's pioneering magnetic-field perturbation theory framework for the magnetic-dipole vibrational transition moment to biorthonormal wave functions, we have derived generalized expressions for the corresponding atomic axial tensors (AATs) and velocity-gauge atomic polar tensors that collapse to Stephens's expressions for real, Hermitian states. Adapting these exact-state expressions to coupled-cluster left- and right-hand biorthonormal wave functions and using our recently developed density-based formulation yields compact equations that involve only perturbed ground-state amplitudes, as well as unperturbed and perturbed, unrelaxed one-electron densities; two-electron-density contributions vanish exactly. We have validated the resulting AATs by comparison with our previous configuration interaction formulation for a two-electron system, size-intensivity tests, a translational sum rule, and other tests. Simulated VCD spectra for (S)-3-chloro-1-butyne at the coupled-cluster singles and doubles level with the correlation-consistent triple-zeta basis set reveal significant discrepancies between length and velocity gauges. We find that the addition of large-exponent functions via a core-valence basis set for the Hartree--Fock reference components yields substantially improved gauge invariance without the need for gauge-including atomic orbitals.