Dense and ultra-cold atomic gases are now commonly produced by laser c
ooling and trapping techniques. In these gases, the long-range interac
tions which are usually negligible become very important, and light-sh
ifts also play an important role. In the present paper, we investigate
the dominant cross-terms of long range interactions and light-shifts,
and we discuss the consequences of these terms for collisions or opti
cal lattices.