Results of ab initia configuration interaction ulculations are presented
for s simple model of molecuIar ewimers. (H4)*-Preliminary SCF cafcufations ;1s treli as several berets of CI are discussed for the Ioant three singler and triplet states of H4. Square, rectzmguIzr_ and reg&r tmpezoidal geometries of He zr= e_xami.ned. The results of this study are sigrMiuntIy better in s quantitative sense thzm eartier work on the excited St.&es of H+ I_ Fnt~o~uction The interaction of two ground state hydrogen molecules has been extensively studied by ab initio caiculations either to study the intermolecular forces (e-g. ref. [I ]) or in an attempt to rationaIize the observed Iow activation energy of the hydrogen-deuterium exchange reaction (e-g_ reF_ [2]). The excited states of H4 provide an interesting mode1 for photochemical processes such as excimer formation and photocycioaddition reactions [3-g] _ In the present work, more ehtborate moiectdar orbital calctdations in an extended gaussian basis set inciuding polarization functions and several levels of configuration interaction are reported for the ground and Iow-lying excited states of Hd_ The recent pioneering work of MicbI et al. [3,4,6.8] on excited state energy hypersurfaces involved ab initio #minimal basis set valence bond calculations on Hz. One of the most important results of Michi's work [3] was the prediction of a bound low-lying excited singlet state of I-Z4 which was cIassiEed as an extimer state since upon molecular dissociation it correlates with one ground state H2 and one Hz. in its lowest singlet excited state.