  A modified static exchange model is introduced, it could be useful to control
the elastic s-wave scattering length in studying cold-atomic interaction. The
theory includes the long-range van der Waals interaction in addition to
short-range exchange force. The model is applied on Ps-H system to study the
Feshbach resonances e.g. the s-wave elastic phase shifts and the corresponding
cross sections at ultracold energies. These are compared with the data obtained
using the static exchange model. A very interesting features are observed when
we vary the interatomic separation i.e. the strength of dipole-dipole
interaction.
