Electronic structure quantum Monte Carlo fixed-node vs exact energies: benchmarks for valence-only calculations

We employ combination of methods including basis set quantum chemistry approaches such as Coupled Cluster and Configuration Interaction to calculate exact or nearly exact ground states and few excitations for selected sets of atoms, molecules and a few solids. Most of the calculations are valence-only with correlation consistent effective core potentials (ccECP) and include also transition elements. We use the results for analysis of fixed-node errors of single-reference and selected multi-reference diffusion Monte Carlo calculations. We quantify previously identified trends where the fixed-node biases that were shown to grow/decrease with increasing/decreasing localization of the electron density in a variety of systems. In general, such increase of the electron density implies a growth of the kinetic energy and related higher curvatures of the nodal surfaces. This applies also for all-electron atoms as shown recently up to Og with the atomic number Z=118. The calculations provide data for further analysis and explorations within approximate or other many-body methods as well as for validation of the constructed ccECPs. We also calculate the kinetic energies that offer another important characteristic that measures accuracy of the many-body wave functions.