Coupled cluster study of the triple bond

Several small molecules containing triple bonds have been studied using cou pled cluster (CC) methods with inclusion of connected clusters through pent uple excitations. The calculations were performed with a recently developed CC approach that adds noniterative contributions due to the T-5 operator t o the connected triples and quadruples evaluated in standard CC methods. Th e results indicate that both for the bond lengths and for the harmonic freq uency we observe a slow convergence of the studied property with respect to the cluster expansion. For N-2, NO+ and CN- the frequency corrections due to the T-4 and T-5 clusters at the DZP level are: 20, 5; 19, 7, 10 and 3 cm (-1), respectively. (C) 2001 Elsevier Science B.V. All rights reserved.