Coupled-cluster calculations of structure and vibrational frequencies of ozone: Are triple excitations enough?

Coupled-cluster calculations with full inclusion of singles, doubles, and triples (CCSDT) with a double-zeta plus polarization and correlation-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 approximate CC methods, and the effects of the different terms in the triple excitation equation are analyzed. The effect of basis set extension on the CCSDT results has been estimated. In the limit of a large basis set, it appears that the complete CCSDT method will give smaller bond lengths than experiment and an asymmetric stretching frequency about $\text{50\hspace{0.17em}}{\mathrm{cm}}^{\mathrm{-1}}$ above the experimental value. That is, it would appear that connected quadruple excitations are needed for quantitative calculations of the structure and frequencies of ozone.