Importance of High Angular-Momentum Channels in Pseudopotentials for Quantum Monte Carlo

Quantum Monte Carlo methods provide in principle a highly accurate treatment of the many-body problem of the ground and excited states of condensed systems. In practice, however, uncontrolled errors such as those arising from the fixed-node and pseudopotential approximations can be problematic. We show that the accuracy of some quantum Monte Carlo calculations is limited by using available pseudopotentials. The use of pseudopotentials involves several approximations, and we will focus on one which is relatively simple to correct during the pseudopotential design phase. It is necessary to include angular momentum channels in the pseudopotential for excited angular momentum states and to choose the local channel appropriately to obtain accurate results. Variational and diffusion Monte Carlo calculations for Zn, O, and Si atoms and ions demonstrate these issues. Adding higher-angular momentum channels into the pseudopotential description reduces such errors without a significant increase in computational cost.