Electronic Excitations in Complex Molecular Environments: Many-Body Green's Functions Theory in VOTCA-XTP.

Area of Science:

• Computational Chemistry
• Quantum Mechanics
• Spectroscopy

Background:

• Accurate calculation of electronically excited states is vital for understanding molecular properties and reactions.
• Existing methods often struggle with complex molecular environments and specific excitation types like charge-transfer.
• The GW approximation and Bethe-Salpeter Equation (GW-BSE) are powerful theoretical tools for excited-state calculations.

Purpose of the Study:

• To implement and validate a many-body Green's functions (GW-BSE) approach within the VOTCA-XTP software for complex molecular systems.
• To investigate the influence of molecular environment and polarization on electronic excitations, particularly charge-transfer (CT) excitations.
• To apply the GW-BSE/MM method to aqueous DNA as a benchmark system.

Main Methods:

• Implementation of GW-BSE theory using Gaussian-type atomic orbitals and resolution of identity techniques for nonperiodic systems.
• Coupling GW-BSE calculations with molecular dynamics (MD) simulations for atomistically resolved environments (GW-BSE/MM).
• Validation against a small molecule reference set and application to aqueous DNA, focusing on adenine base pairs.

Main Results:

• Successful validation of the GW-BSE methodology for various excitation types in single molecules (2-8 eV range).
• Demonstration of environmental effects on CT excitations in adenine base pairs, showing a significant redshift (~1 eV) due to polarization.
• Predicted lowest CT excitation energies in DNA are below single nucleobase excitations, suggesting rapid decay pathways.

Conclusions:

• The open-source VOTCA-XTP software with its GW-BSE/MM implementation is a robust tool for studying electronic excitations in complex environments.
• Environmental polarization and electrostatic coupling play a critical role in determining the energies of charge-transfer excitations.
• The findings provide insights into UV excited state dynamics in DNA, with implications for photochemistry and photophysics.