Performance of Equation of Motion-Coupled Cluster Methods for Computing Anharmonic Vibrational Frequencies of...

Equation of motion (EOM)-based coupled cluster (CC) methods compute radical anharmonic vibrational frequencies and rotational constants to within an average of 1.5% of experimental values for ground electronic states and 4.4% for the first electronically excited states via hybrid quartic force fields (QFFs). For the open-shell HOO, HNF, HSO, and HCF⁺ benchmarks, EOM-ionization potential (IP) and EOM-electron affinity (EA) in conjunction with the CCSD(T)(a) approximate triples and CCSDT full triples CC methods can predict anharmonic vibrational frequencies via QFFs with absolute errors as small as 0.0% in some cases. The hybrid EOM-IP-CCSD(T)(a)/CcCR+TZ QFF, where the cubic and quartic terms are computed at a lower level, performs exceptionally well for the ground state frequencies with a mean abslute percent error of 1.3%. Unfortunately, full triples are needed to generate accurate data for the electronically excited states; EOM-IP-CCSDT/CcCR+QZ produces anharmonic frequencies to within an average of 4.4% of experimental data at the expense of taking ∼70 times longer to compute than EOM-IP-CCSD(T)(a)/CcCR+TZ. Additionally, the two best benchmarked methods are applied to the X²A' and A²A″ states of SiOH to provide a full rovibronic characterization for the ground and first excited electronic states. Additionally, the previous Ar-matrix assignment of the C-F stretch in HCF⁺ at 1368 cm^-1 is also questioned.