H₄: A model system for assessing the performance of diffusion Monte Carlo calculations using a single Slater...

Area of Science:

• Computational Chemistry
• Quantum Mechanics
• Materials Science

Background:

• Diffusion Monte Carlo (DMC) is a quantum mechanical method for simulating many-body systems.
• Accurate nodal surfaces are crucial for the efficiency and accuracy of DMC calculations.
• The H4 system serves as a model to test computational methods due to its electronic structure.

Purpose of the Study:

• To investigate the accuracy of diffusion Monte Carlo (DMC) calculations for a model H4 system.
• To assess the impact of using a single Slater determinant (SD) trial function to fix the nodal surface in DMC.
• To understand how geometric distortions affect the description of diradical character in H4.

Main Methods:

• Utilized a model H4 system for computational analysis.
• Employed diffusion Monte Carlo (DMC) calculations with a single Slater determinant (SD) trial function.
• Investigated distortions from a square to rectangular geometry.

Main Results:

• The lowest energy singlet state of square H4 is a diradical, poorly described by SD-DMC.
• Distorting H4 to rectangular structures reduces diradical character and improves DMC accuracy.
• The error in SD-DMC energy decreases more rapidly with small distortions than large ones.
• Significant errors (∼0.1 eV) persist even when the dominant configuration coefficient is high (0.9).

Conclusions:

• Single Slater determinant trial functions are insufficient for accurately describing diradical systems like H4 with DMC.
• Geometric distortions can mitigate but not entirely eliminate errors in SD-DMC for diradical systems.
• Further development of trial functions is needed for accurate DMC simulations of strongly correlated systems.