Introducing QMC/MMpol: Quantum Monte Carlo in Polarizable Force Fields for Excited States

Area of Science:

• Computational Chemistry
• Quantum Mechanics
• Molecular Mechanics

Background:

• Accurately modeling solvation effects is crucial for understanding chemical processes.
• Traditional methods often simplify environmental interactions, limiting predictive power.

Purpose of the Study:

• To develop and validate a novel hybrid quantum mechanics/molecular mechanics (QMC/MMpol) scheme.
• To assess the performance of QMC/MMpol in calculating vertical excitation energies of solvated molecules.

Main Methods:

• Combining quantum Monte Carlo (QMC) with a classical polarizable dipole reaction field.
• Self-consistently generating optimal dipoles at the variational Monte Carlo level.
• Applying the generated dipoles to include environmental effects in diffusion Monte Carlo.

Main Results:

• The QMC/MMpol model accurately describes vertical excitation energies for methylenecyclopropene and s-trans acrolein in water.
• The 'polSS' polarization regime, accounting for environment response to solute excitation, yields superior results.
• QMC/MMpol outperforms methods using frozen environments (point charges or ground-state polarization).

Conclusions:

• QMC/MMpol offers a robust approach for incorporating environmental effects beyond static point charges.
• The method combines the accuracy of QMC with the computational efficiency of classical models.
• This scheme is particularly effective for systems where electrostatic solute-solvent interactions dominate.