Automated Active Space Selection with CASCI Dipole Moments

Abstract

Multireference calculations are well-suited for describing properties of highly correlated systems. In order to realize the full potential of multireference methods, it is necessary to judiciously choose the active space. It is possible to do this manually, but an automatic procedure for selecting the active space can benefit high-throughput applications and is not susceptible to human error. We previously developed two protocols, GDM-AS and EDM-AS, where the dipole moment calculated at the complete active space self-consistent field (CASSCF) level of theory is used to select the active space, and demonstrated the effectiveness of these protocols when the chosen active spaces are used to find low-lying excitation energies for molecules with nonzero dipole moments. In this work, we demonstrate that using complete active space configuration interaction (CASCI) dipole moments, with the orbitals from the second-order Møller–Plesset perturbation theory, instead of CASSCF dipole moments in our dipole moment active space selection (DM-AS) protocols can significantly improve the efficiency of active space selection compared to our previous methods while maintaining reasonable complete active space second-order perturbation theory (CASPT2) and complete active space pair-density functional theory (CAS-PDFT) vertical excitation energy. We also present new protocols, CASCI-D2DM-AS and its directional version, designed for charge transfer states. We discuss the performance of our new protocols in their ability to describe excitation energy of not only small organic molecules tested in our previous work, but also larger conjugated polar molecules, nonpolar molecules, and bimolecular complexes, to describe the ground-state spin state of transition metal oxides, and to describe bond dissociation curves of small molecules.