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Solvent Effects on Excited-State Structures: A Quantum Monte Carlo and Density Functional Study.

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Quantum Monte Carlo (QMC) with the polarizable continuum model (PCM) now optimizes excited-state geometries in solution. This method shows good agreement with other correlated methods, unlike TDDFT which overestimates solvation effects.

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Area of Science:

  • Computational Chemistry
  • Quantum Chemistry
  • Theoretical Chemistry

Background:

  • Excited-state geometry optimization in solution is crucial for understanding photochemical processes.
  • Accurate theoretical methods are needed to model solvent effects on molecular structures.
  • Previous methods often struggle with the interplay of electron correlation and solvation.

Purpose of the Study:

  • To introduce the first application of variational Quantum Monte Carlo (QMC) combined with the polarizable continuum model (PCM) for excited-state geometry optimization in solution.
  • To assess the performance of this new QMC-PCM approach by comparing it with established methods.
  • To investigate the influence of solvation on the excited-state geometries of various molecules.

Main Methods:

  • Implementation of a self-consistent reaction field (volume and surface polarization charges) within the QMC framework.
  • Variational Quantum Monte Carlo (QMC) calculations for excited-state geometry optimization.
  • Comparison with second-order perturbation theory (CASPT2) and time-dependent density functional theory (TDDFT).

Main Results:

  • QMC-PCM accurately predicts excited-state geometries in solution for acrolein, acetone, methylenecyclopropene, and the propenoic acid anion.
  • QMC-PCM results generally agree well with CASPT2, except for the acrolein π → π* state where QMC shows robust geometry.
  • Time-dependent density functional theory (TDDFT) systematically overestimates geometrical changes upon solvation, sometimes predicting opposite trends compared to QMC.

Conclusions:

  • The QMC-PCM method provides a robust and accurate approach for excited-state geometry optimization in solution.
  • QMC-PCM offers a reliable alternative to CASPT2 and a significant improvement over TDDFT for modeling solvation effects on excited-state geometries.
  • This work paves the way for more accurate theoretical studies of photochemistry in condensed phases.