Development of spin-orbit coupling for stochastic configuration interaction techniques.

Area of Science:

• Quantum chemistry
• Atomic and molecular physics
• Computational chemistry

Background:

• Accurate zeroth-order wavefunctions are essential for spin-orbit coupling (SOC) calculations.
• Stochastic methods offer a promising avenue for constructing complex wavefunctions.

Purpose of the Study:

• Develop and validate the Monte Carlo Configuration Interaction (MCCI) method for SOC calculations.
• Assess MCCI's efficacy in predicting energy level splitting in atoms and small molecules.
• Lay the groundwork for MCCI applications in larger, multireference systems.

Main Methods:

• Iterative construction of multireference wavefunctions using a stochastic Monte Carlo Configuration Interaction (MCCI) approach.
• Incorporation of spin-orbit coupling effects via first-order degenerate perturbation theory with the Breit-Pauli Hamiltonian.
• Application to atoms (B, C, O, F, Si, S, Cl) and diatomic radicals (OH, CN, NO, C2).

Main Results:

• MCCI yields highly accurate results with compact wavefunctions, outperforming other methods.
• Calculated spin-orbit coupling constants and energy level splittings for various atomic and molecular species.
• Demonstrated convergence of MCCI to full configuration interaction (FCI) for stretched OH.
• Investigated singlet-triplet interactions in O2.

Conclusions:

• The MCCI method is a powerful and efficient tool for calculating spin-orbit coupling.
• MCCI provides accurate predictions of energy level splittings and interactions, even with compact wavefunctions.
• This work establishes MCCI as a viable method for future studies of complex electronic structures and relativistic effects.