Method to Compute the Solute-Solvent Dispersion Contribution to the Electronic Excitation Energy in Solution

Area of Science:

• Computational Chemistry
• Quantum Mechanics
• Spectroscopy

Background:

• Understanding electronic excitation energies in solution is crucial for chemistry and biology.
• Solvent effects, particularly dispersion forces, significantly influence these energies.
• Accurate theoretical methods are needed to model these complex interactions.

Purpose of the Study:

• To develop and apply a novel quantum Monte Carlo (QMC) method for calculating solute-solvent dispersion contributions to electronic excitation energies.
• To extend existing QMC methods, previously used for ground states, to excited electronic states.
• To investigate the impact of dispersion forces on solvatochromic shifts.

Main Methods:

• Formulation of a method combining the polarizable continuum model (PCM) with QMC for electronic states.
• Utilizing variational QMC to compute London dispersion forces by measuring solute electronic electric field fluctuations.
• Introducing a parameter Ω for Casimir-Polder integration, with specific conditions for excited states.
• Performing benchmark calculations for n → π* and π → π* transitions in various molecules and solvents.

Main Results:

• The developed QMC method successfully calculates solute-solvent dispersion interactions for excited states.
• Benchmark calculations on formaldehyde, acrolein, and acetone demonstrated the method's applicability.
• Solvent properties like ionization potential and refractive index at frequency Ω were used for characterization.
• A consistent stabilization of excited solute states by dispersion forces was observed.

Conclusions:

• The study presents a robust QMC-based approach for modeling dispersion interactions in excited states.
• Dispersion forces were found to stabilize excited states, leading to red (negative) solvatochromic shifts.
• The method provides valuable insights into solvent effects on molecular electronic transitions.