Multicomponent CASSCF Revisited: Large Active Spaces Are Needed for Qualitatively Accurate Protonic Densities

Multicomponent methods seek to treat select nuclei, typically protons, fully quantum mechanically and equivalent the electrons of a chemical system. In such methods, it is well-known that due neglect electron–proton correlation, Hartree–Fock (HF) description interaction catastrophically fails leading qualitatively incorrect protonic properties. single-component chemistry, qualitative failure HF normally indicative need for multireference as complete active space self-consistent field (CASSCF). While multicomponent CASSCF method was implemented nearly 20 years ago, only able perform calculations with very small spaces (∼105 configurations). Therefore, in order extend realm applicability method, this study derives implements new two-step uses heat-bath configuration step, enabling large (up 16 48 orbitals). We find electronic are needed obtain accurate densities HCN FHF– molecules. Additionally, here should have further applications double-well potentials systems inherently electronically multireference.