Downfolded Configuration Interaction for Chemically Accurate Electron Correlation

A model subspace configuration interaction method is developed to obtain chemically accurate electron correlations by diagonalising a very compact effective Hamiltonian of realistic molecule. The construction of the effective Hamiltonian is deterministic and implemented by iteratively building a sufficiently small model subspace comprising local clusters of a small number of determinants. Through the low-rank reciprocal of interaction Hamiltonian, important determinants in the outer subspace are identified to couple with the subspace of pairwise clusters. This method avoids direct ordering and selection of the configurations in the entire space. We demonstrate the efficiency and accuracy of this theory for several molecules of the multireference nature relevant to chemistry.