Method to Compute the Interaction Energy of a Molecule in Ground and Excited States with a Discrete Environment: The Case of Uracil in Water

In this work, we present a method that is able to compute the interaction energy of a system of interest, in the ground or excited state, with an arbitrary number of water molecules representing the environment. As a test case, we take uracil. We considered five clusters containing 1, 12, 24, 26, and 37 water molecules. The method is a first step toward a more general approach to determining the interaction energy between a molecule, treated at a high level of theory, and a complex molecular environment that can be described as an explicit solvent model. Ground and excited electronic states of uracil were optimized in free space at the variational quantum Monte Carlo (QMC) level. In this way, we sampled electronic configurations that are used to compute all the contributions to the interaction energy with the environment. Excitation energies from the ground state were computed at the diffusion Monte Carlo (DMC) level. Numerical results are in agreement with available literature data on the solvatochromic effect on the n → π* and π → π* vertical transitions of uracil in water. Our method provides specific contributions arising from Pauli repulsion, electrostatic, polarization, and dispersion interactions.