Analytic Gradients for Complete and Restricted Active Space Second-order Perturbation Theory within the Diagonal Approximation

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