Locating Conical Intersections Using the Quasidegenerate Partially and Strongly Contracted NEVPT2 Methods

Conical intersections (CIs) play an important role in photochemistry because they sometimes govern the non-radiative decay process. However, accurate characterizations of CIs are not always straightforward. In this study, analytic gradients and interstate coupling vectors for the quasidegenerate partially and strongly contracted n-electron valence state second-order perturbation theory (QD-PC-NEVPT2 and QD-SC-NEVPT2) were developed and applied to locating CIs of benzene. The pilot application demonstrates that the results of both the methods are similar to the extended multistate complete active space second-order perturbation theory (XMS-CASPT2), while the lack of dynamic electron correlation resulted in a deviation of 0.7 eV.