Abstract: For the first time,the geometries of the two lacking electron fragments HXLIF(X =B, Al) are investigated by ab initio method at RHF/6-31G and RHF/6-31G^*levels.The results indicate that the different basis have trivial effect on the equilibrium geometries and their energy orders.Both HBLiF and HAlLiF have four types of equilibrium geomtries,(3) >(4)>(2).For HAlLiF, they decrease in the order of (1) >(2)>(4)>(3).To analyse characteristics and stabilities of the geometries the Mulliken bond orders have also been reported.In the geometry la (a denotes the geometries of HBLiF, b those of HAlLiF) the bond order of BLi (0.138, at 6-31G) is much smaller than those of 2a (0. 399) and 4a(0.330), while the bond length in 1a (0. 2394 nm, 6-31G) bigger than those of 2a(0. 2248) and 4a (0.2093), and stabilizing energy of la relative to the free entities, HBand Fli,is also smaller than the latter two geometries, which implies that the BLi bond of the geometry la is weaker than that in 2a and 4a. The negative bond order of AlLi in 1b(-0. 097/6-31G, 0.000/6-31G^*) indicates that no real chemical bond is present between the Al and Li atoms, the stabilizing energy of 1b ought to be attributed to the van der Waals interaction. Therefore, among all the geometries, it is preliminarily certain that la and 1b are the most unstable. About 2a and 2b, the bond orders of FLi are negative at the two levels, the bond lengthes much bigger than the ordinary FLi bond.The bond orders of the HX, Xli, FLi and HLi are positive, to some extent, which states that 3a and 3b are definitely four-membered ring.4a and 4b are types of three-membered ring, in which the bond orders of HX, XLi and FLi are positive. H₂CLiF, H₂SiLiF and HNLiF have also the stable three -membered ring geometries.the relative stabilities of the geometries are to be further studied in terms of the isomerization barriers.

Key words: HBLiF fragment, HAlLiF fragment, Geometries, Ab initio study