An efficient closed-shell singles and doubles coupled-cluster method

Abstract

A reformulated set of equations for the closed-shell singles and doubles coupled-cluster (CCSD) method is presented. A computational cost of 1 2 n v 4 n 0 2+7 n v 3 n 0 3+1 n v 2 n 0 4 for the n 6 steps is obtained, where n v is the number of virtual molecular orbitals included in the CCSD procedure, n 0 is the number of doubly occupied molecular orbitals and n= n 0+ n v. Test calculations for the cis and trans isomers of FNNF and planar and pyramidal CH 3 − are presented. Equilibrium structures determined with large Gaussian basis sets at the second-order Møller-Plesset (MP2) perturbation level of theory are reported and used for the other electron correlation methods. With the largest one-particle basis set (144 contracted Gaussian functions), the equilibrium geometries of cis- and trans-FNNF agree with experiment. Based on analyses of planar and pyramidal CH 3 − wavefunctions and the calculated inversion barrier, it is suggested that the molecular anion may not exist in a planar configuration but that autodetachment of an electron occurs before the transition state is reached. Comparisons of our new CCSD procedure demonstrate that coupled-cluster methods are not significantly more expensive than similar electron correlation techniques.