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

## TL;DR

This paper introduces a new method combining quantum and classical models to accurately simulate how aromatic molecules interact with water.

## Contribution

The novel contribution is deriving analytical nuclear gradients for the MCSCF/FQ model and validating its use in simulating vibronic spectra.

## Key findings

- Analytical nuclear gradients were successfully derived and implemented in OpenMolcas.
- The MCSCF/FQ model accurately reproduced experimental vibronic spectra of benzene and phenol in water.
- The method captures both solute flexibility and solvent dynamics in simulations.

## Abstract

The multiscale model combining the multiconfigurational
self-consistent
field (MCSCF) method with the fully atomistic polarizable Fluctuating
Charges (FQ) force field (Sepali, C.; et al. 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.