COUPLED-CLUSTER CALCULATIONS OF STRUCTURE AND VIBRATIONAL FREQUENCIESOF OZONE - ARE TRIPLE EXCITATIONS ENOUGH

Coupled-cluster calculations with full inclusion of singles, doubles, and triples (CCSDT) with a double-zeta plus polarization and correlati on-consistent polarized valence triple-zeta basis sets have been used to calculate the structure and harmonic vibrational frequencies of the ozone molecule. These results have been compared with those of more a pproximate CC methods, and the effects of the different terms in the t riple excitation equation are analyzed. The effect of basis set extens ion on the CCSDT results has been estimated. In the limit of a large b asis set, it appears that the complete CCSDT method will give smaller bond lengths than experiment and an asymmetric stretching frequency ab out 50 cm(-1) above the experimental value. That is, it would appear t hat connected quadruple excitations are needed for quantitative calcul ations of the structure and frequencies of ozone. (C) 1998 American In stitute of Physics.