The low-lying sin&t ;md triplet stntcs of Il,CBc And HCBelI xc cxnnincd usingab initio molccul~r orbital theory_ In qrccmcnt with esrlier results. the lowest-lyin~structure of HzCBe hrrc Cpv swnmetry nnd is 3 triplet with one x dectron _ (3131)_ The resulrs presented hcrc su gx that the lowxtcnerw sin@ wttc~ure is the (t Bt) open-shell singJet, also with Czv symmetry.at leasr 2.5 kcril/tnol higher in energy. The sin&C Zvstructurcwitlt two s electrons ('At)is 15.9 kcxl/mol higher than 3Bt _ All of these structures nrc bound with rcsPcct to the ground State of rnethylcne nnd the beryllium stem. In HCBeH. linear equilibrium ~eomctrics arc found for the triplet ( 3~3 and singlet ('A) SI;LI~S_ The triplet is more smble rhn the singlet ('A) by 35.4 kcal/ mol. and is only 7.9 lx;ll/mol hi&x in encrg thtin triplet HzCBe. Since the rr.msition StNcIUrC connecting these two triplet molecules is found to be 50.2 lx~l/mol hi$cr in energ thJn HzCBc. both tril~lct equilibrium spccics might exist independently. The harmonic vibrationJ frequencies of all wttctures are also reported. \ c-as. 2 H To aid an experimental search for these CH, Be species. it is clearly important to characterize botkisomeric 0 009-2614/83/0000-0000/S 03.00 0 1983 North-Holland 265 Vdumc97, number 3 CHEMICAL PHYSICSLETTERS 2OMay1983 forms as welt as any possible transition structures. such ds 4. connecting them.