Analytical Nuclear Gradients for the Multiconfigurational Self-Consistent Field Method Coupled with the Polarizable Fluctuating Charges Model

TL;DR

This paper develops and implements analytical nuclear gradients for the combined MCSCF and polarizable Fluctuating Charges model, enabling accurate simulation of solvated aromatic molecules' spectra.

Contribution

It extends the MCSCF/FQ multiscale model to include analytical gradients, validated against references, and applied to simulate solvent effects on aromatic molecules.

Findings

Accurate vibronic spectra of benzene and phenol in water

Successful integration with molecular dynamics for solvation effects

Reproduction of experimental spectral profiles

Abstract

The multiscale model combining the multiconfigurational self-consistent field (MCSCF) method with the fully atomistic polarizable Fluctuating Charges (FQ) force field (J. Chem. Theory Comput. 2024, 20, 9954-9967) is here extended to the calculation of analytical nuclear gradients. The gradients are derived from first principles, implemented in the OpenMolcas package, and validated against numerical references. The resulting MCSCF/FQ nuclear gradients are employed to simulate vibronic absorption spectra of aromatic molecules in aqueous solution, namely benzene and phenol. By integrating this approach with molecular dynamics simulations, both solute conformational flexibility and the dynamical aspects of solvation are properly captured. The computed spectra reproduce experimental profiles and relative band intensities with remarkable accuracy, demonstrating the capability of the MCSCF/FQ model to simultaneously describe the multireference character of the solute and its interaction with the solvent environment.