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

A method formulated within the polarizable continuum model of the solvent and a quantum Monte Carlo treatment of the electronic states of the solute molecule is presented for the calculation of the solute-solvent dispersion contribution to the electronic excitation energy in solution. Variational quantum Monte Carlo is exploited to measure the fluctuations of the electronic electric field of the solute molecule to compute the London's dispersion forces with the solvent. The method previously applied to the ground state of the solute is here extended to excited states. To perform the Casimir-Polder integration, we introduce a positive parameter omega whose value is properly chosen for this purpose. We derive a general expression that for omega = 0 reduces to that already proposed for the ground state. For an excited state, omega must be less than the first transition electronic energy of the solvent molecule but greater than the transition energy from the ground to excited electronic state of the solute molecule. Benchmark calculations were performed on the n-* pi* transition for formaldehyde, acrolein, and acetone in six solvents, including water, ethanol, cyclohexane, chloroform, carbon tetrachloride, and toluene, and the pi-* pi* transition of acrolein in cyclohexane. Solvents are characterized by their ionization potential and the refractive index at frequency omega. In all cases, we found that the solute-solvent interaction stabilizes the excited state of the solutes to red solvatochromic shifts.

Automated summary

This paper presents a method for calculating the solute-solvent dispersion contribution to the electronic excitation energy in solution and extends the method to excited states. Utilizing variational quantum Monte Carlo and Casimir-Polder integration, it concludes that the solute-solvent interaction stabilizes the excited state of the solutes.