Abstract: In the present investigation the theory and method to describe the MO's bonding ability with the Hellmann-Feynman electrostatic force are presented, the MO's bonding effects on a certain chemical bond are described according to the two viewpoints ——energy gradient and electrostatic force. The Mo's bonding properties can be reflected by two aspects of the H-F force vector, size and direction. The same MO has different chemical bonding abilities to different chemical bonds, MO's H-F force of ethylene and cyclopropane are calculated with the ab initio calculation program ABHF, the bonding properties are analysed. The relation between the H-F force and the MO's irreducible representation is presented, a method to decide the molecular geometric configuration variation of excited states by the frontier MO's H-F force is suggested.

Key words: H-F force method, Chemical bond, Ethylene, Cyclopropane