Coupled cluster study of the triple bond

Several small molecules containing triple bonds have been studied using coupled cluster (CC) methods with inclusion of connected clusters through pentuple excitations. The calculations were performed with a recently developed CC approach that adds noniterative contributions due to the T-5 operator to the connected triples and quadruples evaluated in standard CC methods. The results indicate that both for the bond lengths and for the harmonic frequency 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.