Controlling errors in fixed node diffusion Monte Carlo calculations for spin transitions in Co³⁺ clusters

Previously, our investigations revealed that diffusion Monte Carlo (DMC) computations suggested an antiferromagnetic arrangement for LaCoO₃ [1], in contrast to experimental observations that characterize the ground state of LaCoO₃ as non-magnetic. This discrepancy is intriguing, considering that DMC calculations employing similar configurations have proven successful in predicting the stable polymorphs and formation energies of transition metal oxides [ref]. In this context, we propose a systematic examination of the LaCoO₃ issue by specifically addressing the fixed node and locality approximations within the DMC framework. We will study the [CoHe₆]³⁺ cluster as a representative model to understand DMC’s accuracy in calculating the spin states of Co-octahedra in LaCoO₃.We address the locality approximation error through our recently developed L2 pseudopotentials. Our model calculation results will be evaluated with respect to ab-initio DMC and configuration interaction (CI) calculations hence providing an explanation for the contradiction between the theory and experiment.