Electron correlation by exchange mapping in electronic structure calculations

A method for increasing the accuracy of configuration interaction (CI) calculations of molecules and other electronic systems is proposed. The energy defect of a given calculation is associated with the electron pair origin of configurations not yet generated, and this defect is mapped onto the exchange interaction for the corresponding pair of spatial molecular orbitals. The orbitals can have opposite spins, and thus, the contribution includes the self-energy and differs from fermion exchange due to antisymmetry. A single parameter, γ, multiplying the exchange integral, is determined from the exact thermodynamic energy of a few reference molecules. The value of γ depends on the basis and level of configuration interaction but is the same for all molecules. Calculated energies are compared with experimental thermodynamic data for a set of forty mainly organic molecules, representing a wide range of bonding environments. Results are reported for two types of multi-reference CI calculations: (1) a triple-zeta basis plus d-type functions for C, N, O, and F and an s, p basis for H, and (2) a severely truncated virtual space in which higher spherical harmonic basis functions are removed. The error of the initial CI calculations is large; however, including the exchange-based contribution brings calculated CI energies into much closer agreement with exact values.