S. Hirata et al., Perturbative corrections to coupled-cluster and equation-of-motion coupled-cluster energies: A determinantal analysis, J CHEM PHYS, 114(9), 2001, pp. 3919-3928
We develop a combined coupled-cluster (CC) or equation-of-motion coupled-cl
uster (EOM-CC) theory and Rayleigh-Schrodinger perturbation theory on the b
asis of a perturbation expansion of the similarity-transformed Hamiltonian
(H) over bar =exp(-T)H exp(T). This theory generates a series of perturbati
ve corrections to any of the complete CC or EOM-CC models and hence a hiera
rchy of the methods designated by CC(m)PT(n) or EOM-CC(m)PT(n). These metho
ds systematically approach full configuration interaction (FCI) as the pert
urbation order (n) increases and/or as the cluster and linear excitation op
erators become closer to complete (m increases), while maintaining the orbi
tal-invariance property and size extensivity of CC at any perturbation orde
r, but not the size intensivity of EOM-CC. We implement the entire hierarch
y of CC(m)PT(n) and EOM-CC(m)PT(n) into a determinantal program capable of
computing their energies and wave functions for any given pair of m and n.
With this program, we perform CC(m)PT(n) and EOM-CC(m)PT(n) calculations of
the ground-state energies and vertical excitation energies of selected sma
ll molecules for all possible values of m and 0 less than or equal ton less
than or equal to5. When the Hartree-Fock determinant is dominant in the FC
I wave function, the second-order correction to CCSD [CC(2)PT(2)] reduces t
he differences in the ground-state energy between CCSD and FCI by more than
a factor of 10, and thereby significantly outperforms CCSD(T) or even CCSD
T. The third-order correction to CCSD [CC(2)PT(3)] further diminishes the e
nergy difference between CC(2)PT(2) and FCI and its performance parallels t
hat of some CCSD(TQ) models. CC(m)PT(n) for the ground state with some mult
ideterminantal character and EOM-CC(m)PT(n) for the excitation energies, ho
wever, appear to be rather slowly convergent with respect to n. (C) 2001 Am
erican Institute of Physics.