Molecular gradients for the second-order generalized Van Vleck variant of multireference perturbation theory

A study on molecular gradients for the second-order generalized Van Vleck variant of multireference perturbation theory (GVVPT2) is presented. The formulas for analytic derivatives of the GVVPT2 energy with respect to nuclear perturbations are presented. It is shown that the modification of the energy denominator, which addresses the intruder-state problem of MRPT is analytically differentiable with respect to nuclear perturbation and only requires the use of matrices available, or directly obtainable, from the underlying multiconfigurational self-consistent field calculation.