Accelerating the Convergence of Higher-Order Coupled Cluster Methods II: Coupled Cluster $Λ$ Equations and Dynamic Damping

Abstract: The method of sub-iteration, which was previously applied to the higher-order coupled cluster amplitude equations, is extended to the case of the coupled cluster $\Lambda$ equations. The sub-iteration procedure for the $\Lambda$ equations is found to be highly similar to that for the amplitude equations, and to exhibit a similar improvement in rate of convergence relative to extrapolation of all $\hat{T}$ or $\hat{\Lambda}$ amplitudes using DIIS. A method of dynamic damping is also presented which is found to effectively recover rapid convergence in the case of oscillatory behavior in the amplitude or $\Lambda$ equations. Together, these techniques allow for the convergence of both the amplitude and $\Lambda$ equations necessary for the calculation of analytic gradients and properties of higher-order coupled cluster methods without the high memory or disk I/O cost of full DIIS extrapolation.