Separability properties of reduced and effective density matrices in the equationhyphen;ofhyphen;motion coupled cluster method

摘要

Certain aspects of final state descriptions provided by the equationhyphen;ofhyphen;motion coupled cluster (EOMhyphen;CC) method are analyzed, particularly the asymptotic separability of density matrices. Specific attention is focused on a supermolecule system that consists of two fragments, one of which is in an excited electronic state. For this example, the reducednhyphen;particle density matrix lsqb;rgr;(n)rsqb; associated with EOMhyphen;CC theory exhibits unphysical long range behavior due to a lack of bra state multiplicative separability. More satisfactory is the effective density lsqb;D(n)rsqb; which includes the first order response of ground state correlation. In particular, local components ofD(n)reduce to the corresponding monomer quantities in the limit of infinite separation. This feature guarantees that observables calculated by contractingD(n)with local operators are size consistent. However, the situation with respect to physical phenomena associated with nonlocal operators is less clear, as multicenter matrix elements of both rgr;(n)andD(n)behave incorrectly fornge;2 in the separated limit. The latter consideration should not be misconstrued as an inherent failure of the EOMhyphen;CC approximation, as problems associated with nonlocal parts of the density are already present in the normal coupled cluster treatment of the ground state.