A quantum Monte Carlo study of systems with effective core potentials and node nonlinearities.

We consider the real-space, fixed-node diffusion Monte Carlo (DMC) method that involves Hamiltonians with nonlocal operators, which could break the fixed-node constraint, and therefore proper algorithmic adjustments are necessary, for example, localization approximation. 

In addition, significant biases, even instabilities could occur due to larger nodal curvatures that increase energy fluctuations depending on the amplitude of nonlocal terms. We illustrate these issues using recently introduced correlation consistent effective core potentials (ccECPs) for high accuracy valence-only electronic structure calculations. They exhibit deeper nonlocal potential functions due to higher fidelity to all-electron settings and appropriate molecular systems examples. This provides an excellent testing ground for studying these effects. We find out that the issues can be addressed by straightforward adjustments such as upgrades of basis sets and the use of T-moves for nonlocal terms. The resulting accuracy corresponds to the ccECP target accuracy and is consistent with independent correlated calculations.  Further possibilities for upgrading the reliability of the DMC algorithm and considerations for better adapted and robust Jastrow factors for correlated calculations are discussed as well.