Density-matrix theory versus coupled-cluster theory

Abstract

Coupled-cluster doubles theory (CCD) consists of the equation for two-particle-two-hole amplitudes. Time-dependent density-matrix theory (TDDM) consists of the equations of motion for one-body and two-body density matrices. Similarities and differences between CCD and TDDM are investigated. They are also applied to model Hamiltonians, and a comparison with the results of an exact diagonalization approach is made. It is pointed out that the effect of the fractional occupation of the single-particle states, which is included in TDDM, plays a role in suppressing two-body correlations in strongly interacting regimes.