Analytic UHF-CCSD(T) second derivatives: implementation and application tothe calculation of the vibration-rotation interaction constants of NCO andNCS

An implementation of analytic open-shell UHF-CCSD(T) second derivatives is presented. To demonstrate applicability and test the accuracy of the UHF-CC SD(T) approach for the determination of spectroscopical parameters, vibrati on-rotation interaction constants were calculated for the ground (1(2)Pi) a nd first electronically excited (1(2)Sigma) states of the NCO and NCS radic als. In addition, harmonic vibrational frequencies for both states, the Ren ner-Teller parameter for the ground state, as well as the 1(2)Pi --> 1(2)Si gma excitation energy are reported. While the computed values are in good a greement with reliable experimental information for NCO, most of the data p resented for NCS are predictions of quantities not well known from experime nt.