  Ab initio quantum chemistry calculations are performed for the mixed alkali
triatomic system. Global minima of the ground and first excited doublet states
of the trimer are found and Born-Oppenheimer potential energy surfaces of the
Li atom interacting with the LiCs molecule were calculated for these states.
The lithium atom is placed at various distances and bond angles from the
lithium-caesium dimer. Three-body nonadditive forces of the Li$_2$Cs molecule
in the global minimum are investigated. Dimer-atom interactions are found to be
strongly attractive and may be important in the experiments, particularly
involving cold alkali polar dimers.
