General Approach for Multireference Ground and Excited States Using Nonorthogonal Configuration Interaction

Area of Science:

• Quantum Chemistry
• Computational Chemistry
• Theoretical Chemistry

Background:

• Accurate description of ground and excited states is crucial for understanding chemical processes.
• Existing methods struggle with static correlation and computational scaling.
• Multiple Hartree-Fock (HF) solutions offer a basis for multireference calculations but face challenges with solution coalescence.

Purpose of the Study:

• To develop a general protocol for computing nonorthogonal configuration interaction (NOCI) ground and excited-state energies using multiple HF solutions.
• To enable reliable calculations across diverse molecular geometries, overcoming limitations of previous methods.
• To demonstrate the applicability of the proposed method for multireference systems.

Main Methods:

• Utilizing an active space variation of self-consistent field (SCF) metadynamics to identify relevant HF states.
• Employing holomorphic HF (h-HF) theory to analytically continue HF solutions beyond vanishing points.
• Exploiting the topology of h-HF solutions in the complex plane to trace states across geometries.

Main Results:

• A general protocol for computing NOCI energies using multiple HF solutions has been established.
• The method successfully identifies and traces chemically relevant HF states across various molecular geometries.
• Demonstrated applicability for the dissociation of fluorine dimer and pseudo-Jahn-Teller distortion of cyclobutadiene.

Conclusions:

• The proposed approach provides a robust framework for multireference ground and excited-state calculations.
• Holomorphic HF theory combined with SCF metadynamics overcomes limitations in tracing HF solutions.
• This work advances computational chemistry for systems with significant static correlation.