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

TL;DR

This paper develops a new multicomponent CASSCF method capable of handling large active spaces, which is essential for accurately predicting protonic densities in molecules like HCN and FHF-, advancing quantum chemical modeling of proton-involving systems.

Contribution

The study introduces a two-step multicomponent CASSCF approach with heat-bath CI, enabling large active space calculations for improved protonic density predictions.

Findings

Large active spaces are necessary for accurate protonic densities.

The new method can handle up to 16 electrons in 48 orbitals.

Improved qualitative accuracy in protonic density calculations.

Abstract

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.