Simplified method for calculating the energy of weakly interacting fragments

A method for calculating approximately the coupling energy of weakly interacting fragments is presented. The method is a simplified version of the density-functional scheme of Kohn and Sham and is applicable whenever the electron density of the coupled fragments does not deviate too markedly from a sum of isolated fragment densities. The coupling energy is expressed directly in terms of properties of the isolated fragments and the only nontrivial computational step is the determination of an eigenvalue sum for the coupled system with a fixed potential. Neither self-consistency cycling nor a solution of Poisson's equation for the coupled fragments is required. The method is therefore particularly appropriate when full density-functional calculations are tractable for the isolated fragments but difficult for the coupled system, e.g., a molecule interacting with a surface. Explicit calculations for dimers illustrate that the approach is very accurate for weakly interacting systems, and that reasonable results can be obtained even for strong covalent bonds.