METHOD OF INTERMEDIATE HAMILTONIANS VIA EIGENVALUE-INDEPENDENT PARTITIONING - APPLICATION TO THEORETICAL SPECTROSCOPY

We develop and apply in this paper a coupled cluster (CC)-based interm ediate hamiltonian method that is suitable for describing both the low er- 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 expans ion 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 m ay also generate the higher-lying states approximately. The space in w hich the dressed matrix works is the union of the starting model space (which now becomes the ''main'' model space, P-m) and the space reach ed by the action of the first power of the cluster operator on the mai n model space functions (they span the intermediate space Pi). Using t he wave operator in the Fock space, the same kind of dressing is maint ained for both the main and the intermediate functions via the use of the same transformed hamiltonian for both these types, This dressing t hus 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 wh ich should arise because of extra hole-particle vacancies in the P-i-s pace functions; to this extent, they are distorted. The pseudo-eigenva lue form of our working equations bypasses the difficult problem of in truder states in a straightforward and obvious way. Applications to co mpute the main and satellite Auger spectra of H2O produce encouraging results, indicating the viability of the method.