DIATOMIC CHROMIUM (CR-2) - APPLICATION OF THE COUPLED CLUSTER METHOD INCLUDING ALL SINGLE AND DOUBLE EXCITATION (CCSD)

The Cr2 molecule has been investigated using ab initio quantum mechanical methods in conjunction with basis sets as large as (14s11p6d2flg/10s8p3d2flg), i.e. 144 contracted Gaussian functions. The CCSD method, which incorporates all single and double excitations and in addition disconnected higher excitations (i.e. unlinked clusters), was applied in addition to ordinary self-consistent-field (SCF) theory. Although CCSD represents a significant improvement over SCF, it is clear that connected triple excitations (excluded in the CCSD model) will play an important role in theoretical prediction of the Cr2 bond distance and vibrational frequency. © 1990.