Open shell coupled cluster methods: extensions and applications

Abstract

The extension of multi-reference Fock space coupled cluster theory to the (0,2) sector is implemented into the ACESII quantum chemistry computational package. This new method is applied to the diatomic molecule $He\sb2,$ and its singly and double ionized states. The results for different model spaces are presented and compared with single reference coupled cluster theoretical results. For neutral $He\sb2,$ a shallow minimum is found on the potential energy surface in line with previous results. $He\sbsp{2}{+}$ is found to have a substantial potential well able to support bound vibrational states, whereas $He\sbsp{2}{2+}$ is found to be meta-stable. Some of the limitations of the new model are discussed. A variety of open-shell coupled cluster methods were applied to the $SiC\sb2$ and $Si\sb2C\sb2$ clusters, as well as their ionic states. In addition, comparisons were made with the results from density functional calculations. The ground state for the $SiC\sbsp{2}{+}$ ion is identified as being a linear $SiCC\sp{+},\ \sp2\Sigma\sp{+}$ state, contrary to previous results which indicated that this cluster should be cyclic. Vibrational frequencies at the QRHF-CCSD/6-31G$\sp*$ level of theory are assigned; $\rm\omega\sb1(\sigma )=742\ cm\sp{-1},\ \omega\sb2(\pi )=177\ cm\sp{-1},\ \omega\sb3(\sigma )=2250\ cm\sp{-1},$ with intensities of 246 km/mol, 12 km/mol and 1072 km/mol respectively. Isotopic shifts are presented to aid in the experimental identification of this ionic cluster. In comparison between coupled cluster and density functional calculations, problems associated with both approaches are highlighted. Calculations performed on $Si\sb2C\sb2$ and its singly ionized state were in support of results published concurrent with the present investigation. These results predict a linear $SiCCSi\sp{+},\ \sp2\Pi\sb{g}$ ground state, for the $Si\sb2C\sbsp{2}{+}$ cluster. The calculation of vibrational frequencies indicates that this molecule will be extremely difficult to detect in the infrared, due to the very low intensity of the infrared active vibrational modes.