Many-body exchange effects in clusters of rare gases with a chromophore:: He2CO2

The many-body exchange effects up to the 5-body have been analyzed in terms of possible mechanisms by which the intermolecular electron exchange is carried out. The terms corresponding to single (SE), triple (TE), double (DE), and quadruple (QE) exchanges are identified, and their diagrammatic interpretation given. It is demonstrated that they require different physical interpretations, as well as display different angular and radial behaviors. The 3-body SE and TE terms were calculated ab initio, and analyzed for a variety of geometries of the He2CO2 cluster. The anisotropy of SE over the wide range of configurations is consistent with the electrostatic model of the exchange quadrupole of He-2, interacting with the permanent quadrupole of CO2. The behavior of TE may be rationalized by considering the Pauli-principle-induced distortion of the monomers involved. The influence of the CO2 asymmetric stretch on the SE, TE, and on the total exchange non-additivity has also been evaluated and discussed. The TE 4-body term in He3CO2 has been calculated and compared with the SE 3-body terms. (C) 1998 Elsevier Science B.V. All rights reserved.