A new topological index F~1 was defined in this paper. The F~1 of AB~n molecules were calculated. It is found that F~1 and the standard molar enthalpies of formation, lattice enthalpies of AB~n molecules have good linear relationship, respectively. The relations between F~1 and cross sections of AB molecules (A=Na, K, Rb, Cs, B=F, Cl, Br, I) were discussed.

Key words: HALIDE, IODIDE, FLUORIDE, ENTHALPY OF FORMATION, TOPOLOGY, SECTION ANALYSIS, POLYATOMIC MOLECULE, LATTICE CONTRANTS