Perturbation theory corrections to the two-particle reduced density matrix variational method

摘要

In the variational 2-particle-reduced-density-matrix (2-RDM) method,the ground-state energy is minimized with respect to the 2-particle reduced density matrix,constrained by N-representability conditions.Consider the N-electron Hamiltonian H() as a function of the parameter X where we recover the Fock Hamiltonian at X = 0 and we recover the fully correlated Hamiltonian at LAMBDA= 1.We explore using the accuracy of perturbation theory at small X to correct the 2-RDM variational energies at lambda=1 where the Hamiltonian represents correlated atoms and molecules.A key assumption in the correction is that the 2-RDM method will capture a fairly constant percentage of the correlation energy for lambdaepsilon(0,l] because the nonperturbative 2-RDM approach depends more significantly upon the nature rather than the strength of the two-body Hamiltonian interaction.For a variety of molecules we observe that this correction improves the 2-RDM energies in the equilibrium bonding region,while the 2-RDM energies at stretched or nearly dissociated geometries,already highly accurate,are not significantly changed.At equilibrium geometries the corrected 2-RDM energies are similar in accuracy to those from coupled-cluster singles and doubles (CCSD),but at nonequilibrium geometries the 2-RDM energies are often dramatically more accurate as shown in the bond stretching and dissociation data for water and nitrogen.