Stochastic determination of effective Hamiltonian for the full configuration interaction solution of quasi-degenerate electronic states

We propose a novel quantum Monte Carlo method in configuration space, which stochastically samples the contribution from a large secondary space to the effective Hamiltonian in the energy dependent partitioning of Löwdin. The method treats quasi-degenerate electronic states on a target energy with bond dissociations and electronic excitations avoiding significant amount of the negative sign problem. The performance is tested with small model systems of H4 and N2 at various configurations with quasi-degeneracy.