Recently, there has been renewed interest in selected configuration interaction approaches for the approximate solution of the complete active space configuration interaction (CASCI) problem in active spaces larger than (18,18). Here, we demonstrate an extremely simple "FlowCI" approach to selected CI, based on the interpretation of the Davidson-Liu iterative eigensolver as a flow generator which purifies mixed guess states into converged eigenfunctions. FlowCI makes no distinction between ground and excited-state wavefunctions. Together with an efficient C++ implementation of the required Hamiltonian-vector product, FlowCI is routinely capable of converging wavefunctions with well over 10<sup>8</sup> significant determinants, in active spaces of up to ~(60,60). Using this method, we probe the convergence of a wide variety of properties with respect to the number of retained determinants in a selected CI computation. Our results indicate that coarse selected CI computations may be acceptable for certain properties involving relative energies, but that other properties are difficult to converge even with many millions of retained parameters in the selected CI wavefunction.

Large-scale selected configuration interaction based on a Davidson-Liu flow

253rd National Meeting (2017)

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