Orbital-optimized versus time-dependent density functional calculations of intramolecular charge transfer excited states

TL;DR

This study compares orbital-optimized and time-dependent density functional methods for calculating intramolecular charge transfer excited states, finding orbital optimization offers more accurate charge transfer distances and energies, especially for long-range transfer.

Contribution

It introduces a direct optimization method for excited states using local and GGA functionals, demonstrating improved accuracy over TD-DFT for charge transfer excitations.

Findings

Orbital-optimized methods yield more accurate charge transfer distances.

Orbital-optimized calculations outperform TD-DFT in energy accuracy for long-range charge transfer.

Including exact exchange improves excitation energies significantly.

Abstract

The performance of time-independent, orbital optimized calculations of excited states is assessed with respect to charge transfer excitations in organic molecules in comparison to the linear-response time-dependent density functional theory (TD-DFT) approach. A direct optimization method to converge on saddle points of the electronic energy surface is used to carry out calculations with the local density approximation (LDA) and the generalized gradient approximation (GGA) functionals PBE and BLYP for a set of 27 excitations in 15 molecules. The time-independent approach is fully variational and provides a relaxed excited state electron density from which the extent of charge transfer is quantified. The TD-DFT calculations are generally found to provide larger charge transfer distances compared to the orbital optimized calculations, even when including orbital relaxation effects with the Z-vector method. While the error on the excitation energy relative to theoretical best estimates is found to increase with the extent of charge transfer up to ca. $-2$ eV for TD-DFT, no correlation is observed for the orbital optimized approach. The orbital optimized calculations with the LDA and the GGA functionals provide a mean absolute error of $\sim$0.7 eV, outperforming TD-DFT with both local and global hybrid functionals for excitations with long-range charge transfer character. Orbital optimized calculations with the global hybrid functional B3LYP and the range-separated hybrid functional CAM-B3LYP on a selection of states with short- and long-range charge transfer indicate that inclusion of exact exchange has a small effect on the charge transfer distance, while it significantly improves the excitation energy, with the best performing functional CAM-B3LYP providing an absolute error typically around 0.15 eV.