Quantum Monte Carlo with variable spins.

Area of Science:

• Computational Chemistry
• Quantum Mechanics
• Electronic Structure Theory

Background:

• Quantum Monte Carlo (QMC) methods are powerful tools for electronic structure calculations.
• Incorporating spin-dependent interactions, particularly spin-orbit coupling, is crucial for accurate modeling of heavy elements and molecules.
• Previous methods often simplified or omitted variable spin treatments, limiting their applicability.

Purpose of the Study:

• To develop and validate a diffusion Monte Carlo (DMC) method that includes variable spins and spin-orbit interactions.
• To address technical challenges in implementing variable spins within the DMC framework.
• To demonstrate the method's accuracy through applications to atomic and molecular systems.

Main Methods:

• Extension of fixed-phase spin-orbit diffusion Monte Carlo to handle variable spins.
• Development of a proof for an upper-bound property of complex nonlocal operators to ensure variational properties via T-moves.
• Analysis of time step biases related to the chosen spin representation.

Main Results:

• Successful implementation of a variable spin-inclusive DMC method.
• Demonstration of the variational property using T-moves for complex operators.
• Accurate calculations of binding energies and geometries for PbH and Sn2 molecules.
• Precise prediction of electron affinities for 6p row elements.

Conclusions:

• The developed variable spin-orbit DMC method provides a significant advancement in electronic structure calculations.
• The method achieves high accuracy, comparable to experimental results, for challenging atomic and molecular systems.
• This approach offers a robust framework for future investigations involving spin-dependent phenomena in quantum chemistry.