Approximate evaluation of the effect of threebody cluster operators in the valenceuniversal coupledcluster excitation energy calculations for Be and Mg
Abstract
The atomically oriented valenceuniversal coupledcluster (VUCC/R) method in the intermediate Hamiltonian formulation is used to calculate excitation energies for the beryllium and magnesium atoms. Having the basic VUCC/R scheme which includes one and twoparticle cluster operators (VUCCSD/R), the effect of approximate inclusion of the threebody terms in the cluster operator is investigated. The evaluation is based on the perturbative analysis of equations for the threebody part of the cluster operator at one and twovalence levels. In this way, an explicit consideration of the threebody terms is avoided since they are expressed as functions of the twobody clusters. Our results show that the effect of the threebody clusters seems rather small for the systems under consideration which suggests that discrepancies between calculated and experimental values cannot be exclusively attributed to the truncated character of the cluster operator in the VUCCSD/R calculation.
 Publication:

Journal of Physics B Atomic Molecular Physics
 Pub Date:
 June 2004
 DOI:
 10.1088/09534075/37/11/015
 Bibcode:
 2004JPhB...37.2387M