The partitioning of interaction energy between a closed-shell and an open-shell system is proposed. This allows us to describe the unrestricted Moller-Plesset interaction energy as a sum of fundamental contributions : electrostatic, exchange, induction and dispersion. The supermolecular energies derived within unrestricted Moller-Plesset perturbation theory are analyzed in terms of perturbation theory of intermolecular forces. The latter has been generalized to allow for the description of monomer wave functions within the unrestricted Hartree-Fock approach. The method is applied to the potential energy surfaces for the first excited triplet states, (3)A’ and (3)A ", Of the He+Cl-2((3) Pi(u)) complex. The (3)A’ and (3)A " potential energy surfaces have different shapes. The lower one, 3A’, has a single minimum for the T-shaped structure. The higher one, (3)A ", has the global minimum for the T-shaped structure and the secondary minimum for a linear orientation. The calculated well depth for the (3)A’ state is 31.1 cm(-1) at the 3.75 Angstrom intersystem separation ar the UMP2 level with extended basis set involving bond functions. The (3)A " well depth is approximately 2.3 cm(-1) smaller at this level. This order is reversed by higher correlation effects. The angular and radial behaviors of the individual components of the (3)A’ and (3)A " interaction energies are compared to reveal the different nature of interaction energies in both states. A comparison with the ground state reveals that the A " state has a typical van der Waals character similar to thar : of the ground state. The A’ state, on the other hand, differs considerably from the ground state, The A’ and A " states differ primarily in different role of the intramonomer correlation effects.