A BENCHMARK COUPLED-CLUSTER SINGLE, DOUBLE, AND TRIPLE EXCITATION (CCSDT) STUDY OF THE STRUCTURE AND HARMONIC VIBRATIONAL FREQUENCIES OF THE OZONE MOLECULE

We report a benchmark calculation of the structure and harmonic vibrational frequencies of the ozone molecule using the full coupled-cluster single, double, and triple excitation model (CCSDT) with a DZP basis set. The results offer a definitive reference for assessing the accuracy of methods which approximate the effects of T3. For the bending and symmetric stretching frequencies, CCSD(T), CCSDT-2, and CCSDT-3 all give results close to CCSDT. For the asymmetric stretching frequency, however, the values predicted by the CCSD(T) and CCSDT-2 models are respectively about 100 cm-1 lower and higher than the CCSDT value, while the CCSDT-3 value exceeds the CCSDT value by 41 cm-1. It is suggested that effects of connected quadruple excitations (T4) and higher cluster operators may have a significant effect on the asymmetric stretching frequency.