  Sources of energy errors resulting from the replacement of the physical
Coulomb interaction by its long-range $\mathrm{erfc}(\mu r)/r$ approximation
are explored. It is demonstrated that the results can be dramatically improved
and the range of $\mu$ 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.
