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Accurate barrier heights using diffusion Monte Carlo.

Kittithat Krongchon¹, Brian Busemeyer¹, Lucas K Wagner¹

¹Department of Physics, University of Illinois at Urbana-Champaign, Urbana, Illinois 61801, USA.

The Journal of Chemical Physics

|April 8, 2017

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Fixed node diffusion Monte Carlo (DMC) accurately calculates reaction barrier heights. The nodal error in DMC calculations is robust to nodal surface variations, showing minimal impact on results.

Area of Science:

Computational chemistry
Quantum mechanics

Background:

Diffusion Monte Carlo (DMC) is a quantum mechanical method for calculating molecular energies.
The fixed-node approximation is commonly used in DMC to simplify calculations but introduces nodal error.
Accurate calculation of reaction barrier heights is crucial for understanding chemical reaction rates.

Purpose of the Study:

To assess the nodal error in fixed-node diffusion Monte Carlo (DMC) calculations for reaction barrier heights.
To investigate the robustness of DMC results to variations in the nodal surface.
To identify descriptors that correlate with the fixed-node error in DMC calculations.

Main Methods:

Performed fixed-node diffusion Monte Carlo (DMC) calculations on 19 non-hydrogen-transfer reactions.

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Assessed nodal error by comparing DMC results obtained with different mean-field techniques for generating nodal surfaces.

Analyzed correlations between DMC energies and various descriptors, including density changes and orbital energy gaps.

Main Results:

DMC results for barrier heights were robust to changes in the nodal surface, with errors of 1.5(1) kcal/mol when using single determinant nodal surfaces.
No significant correlation was found between the fixed-node error and descriptors such as changes in electron density.
A moderate correlation was observed between the fixed-node error and the energy gap between the highest occupied and lowest unoccupied molecular orbitals (HOMO-LUMO gap) from mean-field calculations.

Conclusions:

Fixed-node DMC provides reliable barrier height calculations for non-hydrogen-transfer reactions.
The choice of mean-field method for generating nodal surfaces has a limited impact on DMC accuracy.
The HOMO-LUMO gap in mean-field calculations may serve as a useful, albeit imperfect, descriptor for predicting fixed-node error in DMC.