Solving Excited States From Downfolding And Exact Diagonalization Of Effective Molecular Hamiltonian

Abstract

We present a new numerical technique and implementation of downfolded configuration interaction method for molecular excited states in a sequentially compressed model space comprising only a small number of determinant clusters for which a fast state-specific exact diagonalisation can be carried out. The effective molecular Hamiltonian is enlarged, built and solved until convergence of a state which recovers accurate dynamic electron correlation in an iterative procedure by (1) mapping important determinants from the model space to an impurity state on selected clusters, (2) then creating an exact resolvent coupling operator of Schur complement form in a substantially reduced dimension, and finally (3) pulling selected outer determinants of strong coupling. The algorithmic implementation and performance will be demonstrated for computing accurate molecular excited state energetics that were previously difficult. A tentative application of this method to simulations of energy and state dissipation from ab-initio molecular Hamiltonian will be discussed for square H4 molecule.