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Anharmonic force field from coupled-cluster methods and accurate computation of infrared spectra

2021    Ermis, Betul; Unal, Asli; Soydas, Emine; Bozkaya, Ugur

In this study, an efficient implementation of the anharmonic force field based on an analytic gradients is presented for coupled-cluster (CC) methods, such as coupled cluster singles and doubles (CCSD) and the CCSD with perturbative triples [CCSD(T)]. A naturally parallel procedure, in which all second, third, and fourth energy derivatives are evaluated, has been used for the approach developed for this work. The vibrational second-order perturbation theory (VPT2) is exploited to consider the anharmonic effects in vibrational frequencies. The presented approach is implemented in a new computer code (denoted by INFRARED) and integrated with the DFOCC module as well as with the several quantum chemistry software, such as PSI4, GAUSSIAN, MOLPRO, and ORCA. The developed anharmonic force field method has been applied to harmonic and anharmonic vibrational frequencies of three molecular sets, denoted by S14, M14, and L15 sets. For all molecular sets considered, anharmonic frequencies of the CCSD(T) method are in very good agreement with the experimental values. Overall, we believe that the presented method may extend the applicability of the anharmonic force fields at the CCSD(T) level.