Pseudo-potential calculation of atomic interactions

The use of atomic pseudo-potentials in calculations of atomic interactions is briefly discussed. It is shown that an appropriate choice of pseudo-potential leads to the appearance of the van der Waals interactions in first order. Care is needed in the choice of the core-core interaction potential lest spurious long-range forces appear. Explicit calculations are reported for the (2)Sigma(+)(g) and (2)Sigma(+)(u) ground states of Li-2(+) and comparision made with the results of conventional calculations. High accuracy is obtained over a wide range of internuclear separations.