Computing coupled-cluster wave functions with arbitrary excitations

An algorithm is presented for solving coupled-cluster (CC) equations by suc cessive diagonalization of 2 x 2 matrices. It is more expensive than usual procedures, but it is capable of solving a CC problem where any arbitrary e xcitation is included in the cluster operator. Equation-of-motion coupled-c luster (EOMCC) excitation energies can also be determined by this method re gardless of the type of excitations in the cluster operator and the space w here the effective Hamiltonian is diagonalized. The algorithm is applied to the study of the convergence of CC and EOMCC series in some small bases. ( C) 2000 American Institute of Physics. [S0021-9606(00)30428-7].