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 nv4n02+7nv3n03+1nv2n04 for the n6 steps is obtained, where nv is the number of virtual molecular orbitals included in the CCSD procedure, n0 is the number of doubly occupied molecular orbitals and n=n0+nv. Test calculations for the cis and trans isomers of FNNF and planar and pyramidal CH3− are presented. Equilibrium structures determined with large Gaussian basis sets at the second-order Moller-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 CH3− 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.

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