  The present paper is related to a talk presented during the Symposium on
Coherent Control and Ultracold Chemistry held during the Sixth Congress of the
International Society for Theoretical Chemical Physics (ISTCP-VI, July 2008).
The talk was entitled "Electronic structure properties of alkali dimers and
trimers. Prospects for alignment of ultracold molecules". Here we report on the
electrostatic repulsion forces of the ionic cores at short separation, involved
when the potential energy surfaces of alkali trimers are calculated with a
quantum chemistry approach based on effective large-core potentials for ionic
core description. We demonstrate that such forces in the triatomic molecule can
be obtained as the sum of three pairwise terms. We illustrate our results on
the lowest electronic states of Cs$_3$, which are computed for the first time
within a full configuration interaction based on a large Gaussian basis set. As
a preliminary section, we also propose a brief introduction about the
importance of alkali trimer systems in the context of cold and ultracold
molecules.
