  The complex-source-point model are already used in the exact solution for the
urtrashort pulse and nonparaxial beam. In this letter we have used the
complex-source-point model to deduce the interaction potential equation for the
separation R between two atoms which is comparable with the size of the atoms.
We show the result and the characteristics of the numerical calculation. Since
the singular point around R=0 is removed by using the complex-source-point
model, so that we can obtain the result force around R=0. With the decreasing
of the distance between two atoms, the force switches from the electromagnetic
force to the strong force by use our equation.
