  We introduce a simple but efficient method for grand-canonical twist
averaging in quantum Monte Carlo calculations. By evaluating the thermodynamic
grand potential instead of the ground state total energy, we greatly reduce the
sampling errors caused by twist-dependent fluctuations in the particle number.
We apply this method to the electron gas and to metallic lithium, aluminum, and
solid atomic hydrogen. We show that, even when using a small number of twists,
grand-canonical twist averaging of the grand potential produces better
estimates of ground state energies than the widely used canonical
twist-averaging approach.
