Connection of an elementary class of parametric twoelectron reduceddensitymatrix methods to the coupled electronpair approximations
Abstract
Parametric twoelectron reduceddensitymatrix (p2RDM) methods have recently been shown to exhibit accuracies greater than coupled cluster with single and double substitutions (CCSD) at a lower computational cost. In this paper we derive an elementary class of parametric 2RDM methods with connections to the coupled electron pair approximations (CEPA). Three parametric 2RDM methods p2RDM/n are presented that correspond to the CEPA/n family where n = 1, 2, 3. We isolate the function distinguishing the stationary condition of the parametric 2RDM methods from the nonlinear equations of CEPA. Calculations of energies, geometries, and harmonic frequencies show that p2RDM/n and CEPA/n are very similar for a variety of closedshell systems. Finally, each of the p2RDM/n methods is extended to satisfy particlehole symmetry by the removal of exclusion principle violating terms in the virtual space. These extensions denoted p2RDM′/n are shown to be essential for the proper dissociation of CH radical. Both p2RDM/n and p2RDM′/n form an elementary class of parametric 2RDM methods with accuracy like CCSD; more general parametric 2RDM methods [D.A. Mazziotti, Phys. Rev. Lett. 101, 253002 (2008)] have an accuracy approaching CCSD with full triple excitations.
 Publication:

Molecular Physics
 Pub Date:
 August 2012
 DOI:
 10.1080/00268976.2012.695027
 Bibcode:
 2012MolPh.110.1917D
 Keywords:

 electron correlation;
 reduced density matrices;
 Nrepresentability conditions;
 particlehole duality;
 variational principle