Analytic gradients for restricted active space second-order perturbation theory (RASPT2)

The computational cost of analytic derivatives in multireference perturbation theory is strongly affected by the size active space employed reference self-consistent field calculation. To overcome previous limits on size, gradients single-state restricted second-order (RASPT2) and its complete (CASPT2) have been developed implemented a local version OpenMolcas. Similar to implementations CASPT2, RASPT2 implementation employs Lagrangian or Z-vector method. numerical results show that spaces with up 20 electrons orbitals can now be for geometry optimizations.