An assessment of the low-lying excitation energies and triplet instabilities of organic molecules with an ab initio Bethe-Salpeter equation approach

摘要

The accurate prediction of singlet and triplet excitation energies is ofsignificant fundamental interest and is critical for many applications. An areaof intense research, most calculations of singlet and triplet energies usetime-dependent density functional theory (TDDFT) in conjunction with anapproximate exchange-correlation functional. In this work, we examine andcritically assess an alternative method for predicting low-lying neutralexcitations with similar computational cost, the ab initio Bethe-Salpeterequation (BSE) approach, and compare results against high-accuracywavefunction-based methods. We consider singlet and triplet excitations of 27prototypical organic molecules, including members of Thiel's set, the aceneseries, and several aromatic hydrocarbons exhibiting charge-transfer-likeexcitations. Analogous to its impact in TDDFT, we find that the Tamm-Dancoffapproximation (TDA) overcomes triplet instabilities in the BSE approach,improving both triplet and singlet energetics relatively to higher leveltheories. Finally, we find that BSE-TDA calculations built on good DFT startingpoints, such as those utilizing optimally-tuned range-separated hybridfunctionals, can yield accurate singlet and triplet excitation energies forgas-phase organic molecules.