Analytic First-Order Derivatives of CASPT2 Combined with the Polarizable Continuum Model

The complete active space second-order perturbation theory (CASPT2) is valuable for accurately predicting electronic structures and transition energies. However, optimizing molecular geometries in the solution phase has proven challenging. In this study, we develop analytic first-order derivatives of CASPT2 using an implicit solvation model, specifically the polarizable continuum model (PCM), within the open-source package OpenMolcas. Analytic gradients and non-adiabatic coupling vectors are computed by solving a modified Z-vector equation. Comparisons with existing theoretical and experimental results demonstrate that the solvent effects can be qualitatively captured using the developed method.