Study of approximate coupled cluster methods for first-order static properties

In this paper, we analyse the algebraic structure of the equations for calculating the first order static properties using several approximate versions of Coupled Cluster (CC) methods. In particular, the non-variational and the variational method using a CC wavefunction corresponding to an appropriately defined perturbed Hamiltonian as well as the simple expectation value expression using a CC stationary state are studied under different approximations. Two different models are proposed: (a) use of maximum overlap orbitals where the pertinent approximations are TT₂, T⁽¹⁾ T₂⁽¹⁾, (b) use of Hartree-Fock orbitals and T(T₁+T₂), T⁽¹⁾(T₁⁽¹⁾+T₂⁽¹⁾) approximations. It is analytically shown that in both these models certain approximate versions of the methods under purview yield identical results for first order static properties.NCL Communication No. 3725