Bivariational coupled-cluster approach for the study of static electronic ${\mathrm{properties}}^{/\mathrm{e}\mathrm{m}\mathrm{p}\mathrm{h}>\mathrm{}}$

A bivariational coupled-cluster method is advocated for the calculation of static electronic properties for closed-shell atomic and molecular systems. The proposed method uses a perturbed form of the Hamiltonian H(λ) which includes the field with which the system interacts, giving rise to the response in various orders. Higher-order properties can be calculated without the use of excited states. A novel linked energy functional ${\mathrm{scrJ}}_H^{\lambda }$[T,T’’] is used with two different types of cluster parameters T(λ) and T’’(λ), each of which is a power series in λ. Unlike the Euler energy functional proposed in one of our earlier works, this functional is a terminating series in cluster parameters. It is shown that the set of stationarity equations of ${\mathrm{scrJ}}_H^{\lambda }$ with respect to arbitrary variation of the cluster parameters of T(λ) and T’’(λ) corresponding to any power of λ is the same for certain conditions and furnishes all the relevant values of cluster parameters. The equations are derived for calculating static properties to any order with these stationary optimum values.