Convergence improvement for coupled-cluster calculations

Convergence problems in coupled-cluster iterations are discussed, and a new iteration scheme is proposed. Whereas the Jacobi method inverts only the d iagonal part of the large matrix of equation coefficients, we invert a matr ix which also includes a relatively small number of off-diagonal coefficien ts, selected according to the excitation amplitudes undergoing the largest change in the coupled-cluster iteration. A test case shows that the new inv ersion of partial matrix (IPM) method gives much better convergence than th e straightforward Jacobi-type scheme or such well known convergence aids as the reduced linear equations or direct inversion in iterative subspace met hods.