Energy-based truncation of multi-determinant wavefunctions in quantum Monte Carlo

We present a method for truncating large multi-determinant expansions for use in diffusion Monte Carlo calculations. Current approaches use wavefunction-based criteria to perform the truncation. Our method is more intuitively based on the contribution each determinant makes to the total energy. We show that this approach gives consistent behaviour across systems with varying correlation character, which leads to effective error cancellation in energy differences. This is demonstrated through accurate calculations of the electron affinity of oxygen and the atomisation energy of the carbon dimer. The approach is simple and easy to implement, requiring only quantities already accessible in standard configuration interaction calculations.