Method of intermediate hamiltonians via eigenvalue-independent partitioning: application to theoretical spectroscopy

Abstract

We develop and apply in this paper a coupled cluster (CC)-based intermediate hamiltonian method that is suitable for describing both the lower- and higher-lying excited/ionized states relative to a closed-shell ground state. Generation of the main roots corresponding to the lower-lying states is attempted via an open-shell CC expansion. This expansion dresses the hamiltonian appropriately to incorporate the effect of the virtual space Q in a size-extensive manner. We have shown that by recasting the CC equations in a pseudo-eigenvalue equation form, we may also generate the higher-lying states approximately. The space in which the dressed matrix works is the union of the starting model space (which now becomes the "main" model space, P_m) and the space reached by the action of the first power of the cluster operator on the main model space functions (they span the intermediate space P_i). Using the wave operator in the Fock space, the same kind of dressing is maintained for both the main and the intermediate functions via the use of the same transformed hamiltonian for both these types. This dressing thus incorporates the same decoupling of the Q space from both the P_m and P_i spaces. The P_i, roots, however, miss the "extra" dressing which should arise because of extra hole-particle vacancies in the P_i-space functions; to this extent, they are distorted. The pseudo-eigenvalue form of our working equations bypasses the difficult problem of intruder states in a straightforward and obvious way. Applications to compute the main and satellite Auger spectra of H₂O produce encouraging results, indicating the viability of the method.