Diffusion Monte Carlo in internal coordinates.

Area of Science:

• Quantum chemistry
• Computational physics
• Molecular dynamics

Background:

• Diffusion Monte Carlo (DMC) is a powerful quantum mechanical simulation method.
• Accurately modeling molecules with large-amplitude vibrational motions, like H(3)(+), remains challenging.
• Reduced-dimensional approaches can simplify complex molecular simulations.

Purpose of the Study:

• To develop an internal coordinate extension of DMC for generalized reduced-dimensional calculations.
• To apply the method to H(3)(+) and its isotopologues to assess its capabilities.
• To investigate the suitability of different internal coordinates for fixed-node DMC.

Main Methods:

• An internal coordinate formulation of DMC was developed without constraints on coordinate independence.
• The method was tested on H(3)(+) and its isotopologues to compute ground state properties.
• The fixed-node approximation was combined with the internal coordinate DMC for excited states.

Main Results:

• The internal coordinate DMC successfully described the ground state properties of H(3)(+) and its isotopologues.
• Analysis of probability distributions identified less coupled internal coordinates.
• Curvilinear normal mode coordinates proved effective for nodal surfaces in fluxional molecules like H(2)D(+) and D(2)H(+).

Conclusions:

• The developed internal coordinate DMC is a promising step towards generalized reduced-dimensional calculations.
• The method accurately captures large-amplitude zero-point vibrational motions.
• Insights into coordinate coupling aid in optimizing fixed-node DMC calculations for complex molecular systems.