Accelerating the convergence of higher-order coupled cluster methods

The problem of the generally inferior convergence behavior of higher-order coupled cluster methods, such as CCSDT and CCSDTQ, compared to CCSD is analyzed in terms of Moller-Plesset perturbation theory. A new structure for the CCSDT and CCSDTQ equations (and various approximations of these) is proposed which reorders contributions between the various cluster amplitudes and emphasizes lower-order corrections to the energy at each iteration. Numerical testing of the proposed method compared to the widely used direct inversion in the iterative subspace convergence acceleration technique shows significant improvement in the rate of convergence and total time-to-solution, especially for methods including quadruple excitations. (C) 2015 AIP Publishing LLC.