  Full configuration interaction quantum Monte Carlo (FCIQMC) is a stochastic
approach for finding the ground state of a quantum many-body Hamiltonian. It is
based on the dynamical evolution of a walker population in Hilbert space, which
samples the ground state configuration vector over many iterations. Here we
present a modification of the original protocol for walker population control
of Booth et al. JCP 131, 054106 (2009) in order to achieve equilibration at a
pre-defined average walker number and to avoid walker number overshoots. The
dynamics of the walker population is described by a noisy damped harmonic
oscillator and controlled by two parameters responsible for damping and
forcing, respectively, for which reasonable values are suggested. We further
introduce a population growth witness that can be used to detect annihilation
plateaus related to overcoming the FCIQMC sign problem. Features of the new
population control procedure such as precise walker number control and fast
equilibration are demonstrated. The standard error of the shift estimator for
the ground state energy as well as the population control bias are found to be
unaffected by the population control procedure or its parameters. The improved
control of the walker number, and thereby memory consumption, is a desirable
feature required for automating FCIQMC calculations and requires minimal
modifications to existing code.
