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