UGA-SSMRPT2 - A Multireference Perturbation Theory Predicting Accurate Electronic Excitation Energies in Diverse...

UGA-SSMRPT2, the spin-free perturbative analogue of Mukerjee's State-Specific Multireference Coupled Cluster Theory (MkMRCC), is known to be successful for size-extensive and intruder-free construction of dissociation curves. This work demonstrates that UGA-SSMRPT2 is also an accurate and computationally inexpensive framework for computing the excitation energies. The method achieves near-chemical accuracy for the vast majority of π → π*, n → π*, charge-transfer, valence-Rydberg, and Rydberg excited states commonly used for benchmarking electronic structure theories for excited states. Our results demonstrate that UGA-SSMRPT2 excitation energies lie within 0.20 eV of EOM-CCSD and/or well-established theoretical best estimates, often surpassing the popular MRPT2 approaches like NEVPT2, CASPT2, and MCQDPT while typically requiring smaller active spaces. Its state-specific formulation circumvents the well-known intruder-state problem and eliminates the need for empirical parameters, such as IPEA shifts in CASPT2. This work proposes UGA-SSMRPT2 as a robust and scalable approach for modeling challenging electronically excited states.