# How Dispersion Interactions at the Excited State Can Tune Photochromism of Embedded Chromophores

## TL;DR

This paper introduces a new computational method to study how dispersion forces affect the behavior of chromophores in excited states.

## Contribution

The novel QM/MMPol-cLR3 framework incorporates state-specific dispersion for accurate excited-state simulations.

## Key findings

- Dispersion interactions control excited-state solvatochromism through two distinct mechanisms.
- In azulene, dispersion causes opposite shifts of La and Lb states due to polarizability changes.
- Application to the LH2 complex shows pigment-dependent dispersion shifts affecting energy transfer.

## Abstract

We present QM/MMPol-cLR3, a polarizable embedding
quantum
mechanics/molecular mechanics (QM/MM) framework that includes explicit,
state-specific dispersion terms. This method enables a rigorous treatment
of dispersion on top of electrostatic and induction effects in ground-
and excited-state calculations. Using QM/MMPol-cLR3, we
show that dispersion interactions control excited-state solvatochromism
through two distinct mechanisms. In azulene, opposite shifts of the
La and Lb states arise from state-specific dispersion
linked to changes in excited-state polarizability. In bacteriochlorophyll
a, dispersion instead stems from the interplay between polarizability
changes and transition-dipole-driven response, governing the Q

y
and Q

x
shifts. Finally, application to the LH2
complex reveals pigment-dependent dispersion shifts between the B800
and B850 rings, impacting the excitation-energy transfer. These results
establish dispersion as an essential, nonempirical component for predictive
excited-state simulations in complex environments.