REDUCED MULTIREFERENCE CCSD METHOD - AN EFFECTIVE APPROACH TO QUASI-DEGENERATE STATES

Standard multireference (MR) coupled cluster (CC) approaches are based on the effective Hamiltonian formalism and generalized Bloch equation . Their implementation, relying on the valence universal or state univ ersal cluster Ansatz, is very demanding and their practical exploitati on is often plagued with intruder state and multiple solution problems . These problems are avoided in the so-called state selective or state specific (SS) MR approaches that concentrate on one state at a time. To preserve as much as possible the flexibility and generality offered by the general MR CC approaches, yet obtaining a reliable and managea ble algorithm, we propose a novel SS strategy providing a size-extensi ve CC formalism, while exploiting the MR model space and the correspon ding excited state manifold. This strategy involves three steps: (i) T he construction of a variational configuration interaction (CI) wave f unction within the singly (S) and doubly (D) excited state manifold, ( ii) the cluster analysis of this CI wave function providing the inform ation about the higher than pair cluster amplitudes, and (iii) the exp loitation of these amplitudes in the so-called externally corrected CC SD procedure. This approach is referred to as the reduced MR (RMR) SS CCSD method and is implemented at the ab initio level and applied to s everal model systems for which the exact full CI results are available . These include two four electron H-4 systems (usually referred to as the H4 and S4 models), an eight electron H-8 model and the singlet-tri plet separation problem in CH2. It is shown that the RMR CCSD approach produces highly accurate results, is free from intruder state problem s, is very general and effective and applicable to both closed and ope n shell systems. (C) 1997 American Institute of Physics.