Correcting Models with Long-Range Electron Interaction Using Generalized Cusp Conditions.

Sources of energy errors resulting from the replacement of the physical Coulomb interaction by its long-range erfc(μ r )/ r approximation are explored. It is demonstrated that the results can be dramatically improved and the range of μ giving energies within chemical accuracy limits significantly extended if the generalized cusp conditions are used to represent the wave function at small r . The numerical results for two-electron harmonium are presented and discussed.