Density dependence of fixed-node errors in diffusion quantum Monte Carlo: Triplet pair correlations

We analyze trial wave function fixed-node errors for a three-electron fully spin-polarized atomic systems with varying atomic number Z . The fully spin-polarized state 4 S ( 1 s 2 s 3 s ) is the lowest quadruplet with S -symmetry and even parity and it is nearly-degenerate with the 4 S ( 1 s 2 p 3 p ) state. We find significant fixed-node errors proportional to Z at the Hartree–Fock level while the two-configuration wave function leads to nearly exact energies. This broadly agrees with our analogous study on Be-like atoms so that the dependence of fixed-node errors on density appears to be similar in both spin-polarized and unpolarized atomic systems.