Benchmarking CASPT3 Vertical Excitation Energies

TL;DR

This study benchmarks the accuracy of CASPT3, a third-order multireference perturbation theory, in predicting vertical excitation energies across various excited states, comparing its performance to CASPT2 using a large reference dataset.

Contribution

It provides the first comprehensive assessment of CASPT3's accuracy for excited states, highlighting its similar performance to CASPT2 and insensitivity to the IPEA shift.

Findings

CASPT3 achieves a mean absolute error of 0.11 eV with IPEA shift.

CASPT3's accuracy is nearly unaffected by the IPEA shift.

CASPT3's error slightly reduces to 0.09 eV without IPEA shift.

Abstract

Based on 280 reference vertical transition energies of various natures (singlet, triplet, valence, Rydberg, $n\to\pi^*$, $\pi\to\pi^*$, and double excitations) extracted from the QUEST database, we assess the accuracy of third-order multireference perturbation theory, CASPT3, in the context of molecular excited states. When one applies the disputable ionization-potential-electron-affinity (IPEA) shift, we show that CASPT3 provides a similar accuracy as its second-order counterpart, CASPT2, with the same mean absolute error of $0.11$ eV. However, as already reported, we also observe that the accuracy of CASPT3 is almost insensitive to the IPEA shift, irrespective of the transition type and system size, with a small reduction of the mean absolute error to $0.09$ eV when the IPEA shift is switched off.