Analytic Computation of Vibrational Circular Dichroism Spectra Using Configuration Interaction Methods

In this work, we present the first derivation and implementation of analytic-gradient methods for the computation of the atomic axial tensors (AATs) required for simulations of vibrational circular dichroism (VCD) spectra using configuration interaction methods including double (CID) and single and double (CISD) excitations. Our new implementation includes the use of noncanonical perturbed orbitals to improve the numerical stability of the gradients in the presence of orbital near-degeneracies, as well as frozen-core capabilities. We validated our analytic CID and CISD formulations against two new finite-difference approaches. Using this new implementation, we investigated the significance of singly excited determinants and the role of CI-coefficient optimization in VCD simulations by comparisons among the Hartree-Fock (HF) theory, second-order Møller-Plesset perturbation (MP2) theory, CID, and CISD theories. For our molecular test set including (<i>P</i>)-hydrogen peroxide, (<i>S</i>)-methyloxirane, (<i>R</i>)-3-chloro-1-butene, (<i>R</i>)-4-methyl-2-oxetanone, and (<i>M</i>)-1,3-dimethylallene, we noted sign discrepancies between the HF and MP2 methods compared with that of the new CID and CISD methods for four of the five molecules as well as similar discrepancies between the CID and CISD methods for (<i>M</i>)-1,3-dimethylallene.