Accurate nonrelativistic ground-state energies of 3d transition metal atoms

TL;DR

This paper reports highly accurate nonrelativistic ground-state energies for 3d transition metal atoms using FN-DMC with CIPSI-derived trial wavefunctions, significantly reducing fixed-node errors and providing new benchmark values.

Contribution

It introduces a method combining FN-DMC with CIPSI-selected multi-determinantal wavefunctions to achieve unprecedented accuracy in atomic energy calculations.

Findings

Reduced fixed-node errors for some atoms compared to Hartree-Fock nodes.

Most accurate ground-state energies for 3d transition metals to date.

Provided lower bounds for all-electron correlation energies.

Abstract

We present accurate nonrelativistic ground-state energies of the transition metal atoms of the 3d series calculated with Fixed-Node Diffusion Monte Carlo (FN-DMC). Selected multi-determinantal expansions obtained with the CIPSI method (Configuration Interaction using a Perturbative Selection made Iteratively) and including the most prominent determinants of the full CI expansion are used as trial wavefunctions. Using a maximum of a few tens of thousands determinants, fixed-node errors on total DMC energies are found to be greatly reduced for some atoms with respect to those obtained with Hartree-Fock nodes. The FN-DMC/(CIPSI nodes) ground-state energies presented here are, to the best of our knowledge, the most accurate values reported so far. Thanks to the variational property of FN-DMC total energies, the results also provide lower bounds for the absolute value of all-electron correlation energies, $|E_c|$.