Assessing Excited State Energy Gaps with Time-Dependent Density Functional Theory on Ru(II) Complexes

TL;DR

This study evaluates various density functionals for calculating excited state energy gaps in Ru(II) complexes, emphasizing the importance of state character and ordering, and finds pure functionals outperform hybrids in certain aspects.

Contribution

It introduces a systematic approach using wavefunction overlaps to assess the impact of different density functionals on excited state properties, focusing on energy gaps and state character.

Findings

Pure functionals better reproduce energy gaps than hybrid functionals.

Hybrid functionals give more accurate vertical excitation energies.

Pure functionals align more closely with MS-CASPT2 in state character and ordering.

Abstract

A set of density functionals coming from different rungs on Jacob's ladder are employed to evaluate the electronic excited states of three Ru(II) complexes. While most studies on the performance of density functionals compare the vertical excitation energies, in this work we focus on the energy gaps between the electronic excited states, of the same and different multiplicity. Excited state energy gaps are important for example to determine radiationless transition probabilities. Besides energies, a functional should deliver the correct state character and state ordering. Therefore, wavefunction overlaps are introduced to systematically evaluate the effect of different functionals on the character of the excited states. As a reference, the energies and state characters from multi-state second-order perturbation theory complete active space (MS-CASPT2) are used. In comparison to MS-CASPT2, it is found that while hybrid functionals provide better vertical excitation energies, pure functionals typically give more accurate excited state energy gaps. Pure functionals are also found to reproduce the state character and ordering in closer agreement to MS-CASPT2 than the hybrid functionals.