An Accurate and Linear Scaling Method to Calculate Charge-Transfer Excitation Energies and Diabatic Couplings

TL;DR

This paper introduces a linear-scaling, accurate quantum-chemical method for calculating charge-transfer excitation energies and diabatic couplings in large molecular systems, using subsystem DFT and a post-SCF approach.

Contribution

The authors develop a novel linear-scaling method combining subsystem DFT and a post-SCF calculation to efficiently compute charge-transfer properties in large non-covalent molecular systems.

Findings

Achieves chemical accuracy compared to Coupled-Cluster calculations.

Scales linearly with the number of molecules in the system.

Successfully applied to systems with up to 56 molecules.

Abstract

Quantum--Mechanical methods that are both computationally fast and accurate are not yet available for electronic excitations having charge transfer character. In this work, we present a significant step forward towards this goal for those charge transfer excitations that take place between non-covalently bound molecules. In particular, we present a method that scales linearly with the number of non-covalently bound molecules in the system and is based on a two-pronged approach: The molecular electronic structure of broken-symmetry charge-localized states is obtained with the Frozen Density Embedding formulation of subsystem Density-Functional Theory; subsequently, in a post-SCF calculation, the full-electron Hamiltonian and overlap matrix elements among the charge-localized states are evaluated with an algorithm which takes full advantage of the subsystem DFT density partitioning technique. The method is benchmarked against Coupled-Cluster calculations and achieves chemical accuracy for the systems considered for intermolecular separations ranging from hydrogen-bond distances to tents of {\AA}ngstroms. Numerical examples are provided for molecular clusters comprised of up to 56 non-covalently bound molecules.