Foundations of variational discrete action theory

A new variational theory is proposed, based on the sequential product density matrix (SPD) ansatz, which is characterized by an integer and monotonically approaches the exact solution. The authors introduce the discrete action theory to evaluate the SPD, yielding the variational discrete action theory (VDAT). VDAT can be exactly evaluated for the multiband Anderson impurity model and the multiband Hubbard model in infinite dimensions. This provides an unprecedented combination of efficiency and precision within a single framework.