Automatic State Interaction with Large Localized Active Spaces for Multimetallic Systems.

Area of Science:

• Quantum Chemistry
• Computational Chemistry
• Electronic Structure Theory

Background:

• Localized Active Space Self-Consistent Field (LASSCF) method factorizes wave functions into localized fragments.
• Localized Active Space State Interaction (LASSI) reintroduces correlation via Hamiltonian diagonalization.
• Optimal procedures for defining LASSCF fragments and LASSI model spaces are currently unknown.

Purpose of the Study:

• To present an automated framework for exploring systematically convergent model spaces in LASSI.
• To introduce the LASSI[r, q] method, requiring only user-defined parameters r and q.
• To demonstrate efficient convergence to the Complete Active Space Configuration Interaction (CASCI) limit.

Main Methods:

• Developed an automated framework to systematically explore model spaces for LASSI.
• Introduced the LASSI[r, q] method, controlled by electron hops (r) and fragment basis functions (q).
• The method converges to CASCI as r and q approach infinity.

Main Results:

• Numerical tests on trimetal oxo-centered complexes show efficient convergence to the CASCI limit.
• LASSI[r, q] requires 4-10 orders of magnitude fewer states compared to traditional CASCI.
• The framework provides a systematic and efficient approach to defining LASSI model spaces.

Conclusions:

• The LASSI[r, q] method offers a computationally efficient alternative to CASCI for electronic structure calculations.
• This automated framework simplifies the process of defining model spaces for correlated systems.
• The approach significantly reduces the number of states needed for accurate calculations.