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

Area of Science:

• Quantum Chemistry
• Computational Chemistry
• Theoretical Chemistry

Background:

• Multicomponent methods treat nuclei quantum mechanically alongside electrons.
• Hartree-Fock (HF) fails for electron-proton interactions, yielding incorrect protonic properties.
• Existing multicomponent CASSCF methods are limited to small active spaces.

Purpose of the Study:

• To develop an improved multicomponent CASSCF method for larger systems.
• To enable accurate calculations of protonic properties by addressing limitations of previous methods.
• To extend the applicability of quantum mechanical treatment for nuclei.

Main Methods:

• Derivation and implementation of a two-step multicomponent CASSCF method.
• Utilizing multicomponent heat-bath configuration interaction for the configuration interaction step.
• Enabling calculations with significantly larger active spaces (up to 16 electrons in 48 orbitals).

Main Results:

• The new method allows for very large active space calculations.
• Accurate protonic densities were obtained for HCN and FHF- molecules.
• Demonstrated the necessity of large electronic active spaces for accurate results.

Conclusions:

• The enhanced multicomponent CASSCF method significantly expands the scope of quantum mechanical nuclear treatments.
• This approach is crucial for accurately describing systems with strong electron-proton correlation.
• Potential applications include double-well protonic potentials and electronically multireference systems.