Heavy-atom systems in quantum Monte Carlo: pseudopotentials and beyond*

We study the electronic structure of selected systems with heavy atoms such as Ru, Ir, Pb, Bi and I, using quantum Monte Carlo (QMC). This is motivated by expanding real space QMC to systems with strong spin-orbit interactions and significant correlation effects. Such studies require accurate effective core potentials (ECPs), the ability to obtain accurate spinors and the eventual inclusion of multi-reference expansions of trial wave functions. We start by assessing the accuracy of ECPs and their impact on the most basic quantities such as the lowest energy excitations and binding in atomic and molecular systems. Moreover, we try to assess the errors caused by averaged vs. explicit spin-orbit interaction using the recently developed two-component spinor fixed-phase QMC method. We study also the corresponding biases that stem from the fixed-node vs fixed-phase approximations. Furthermore, we try to explore the cases where spin-orbit and correlation are of the same magnitude and can impact important quantities such as band gaps and magnetic states in periodic materials.