# Modeling the absorption spectrum of the permanganate ion in vacuum and in aqueous solution

## TL;DR

This paper demonstrates that combining polarizable embedding with CAS-srDFT and molecular dynamics effectively models the solvent and temperature effects on the MnO₄⁻ absorption spectrum, surpassing traditional implicit solvent approaches.

## Contribution

It introduces a novel computational approach integrating PE, CAS-srDFT, and MD simulations to accurately predict solvent and temperature effects on the MnO₄⁻ spectrum.

## Key findings

- Implicit solvent models are insufficient for accurate solvent shift prediction.
- The combined PE and CAS-srDFT approach successfully reproduces experimental solvent shifts.
- Finite-temperature averaging clarifies the spectral band assignments.

## Abstract

The absorption spectrum of the MnO$_{4}$$^{-}$ ion has been a test-bed for quantum-chemical methods over the last decades. Its correct description requires highly-correlated multiconfigurational methods, which are incompatible with the inclusion of finite-temperature and solvent effects due to their high computational demands. Therefore, implicit solvent models are usually employed. Here we show that implicit solvent models are not sufficiently accurate to model the solvent shift of MnO$_{4}$$^{-}$, and we analyze the origins of their failure. We obtain the correct solvent shift for MnO$_{4}$$^{-}$ in aqueous solution by employing the polarizable embedding (PE) model combined with a range-separated complete active space short-range density functional theory method (CAS-srDFT). Finite-temperature effects are taken into account by averaging over structures obtained from ab initio molecular dynamics simulations. The explicit treatment of finite-temperature and solvent effects facilitates the interpretation of the bands in the low-energy region of the MnO$_{4}$$^{-}$ absorption spectrum, whose assignment has been elusive.