THE HELLMANN–FEYNMAN THEOREM AND CHEMICAL BINDING

The advantages of the electrostatic interpretation of chemical binding are illustrated. It is shown that the forces exerted by the electrons in a molecular orbital may be determined by general expressions. These are functions of only one parameter, the value of which is easily specified for a particular case. From the general expressions it is possible to obtain quantitative measures of the bonding or antibonding power of a molecular orbital. It is shown that the usual interpretations of bonding and antibonding properties in molecular orbital theory are misleading and that few orbitals are, in actual fact, antibonding. If molecular orbitals are classified as netbonding or netantibonding, terms which are precisely defined and have a definite physical significance, a much closer correlation with previous concepts of bonding is obtained. The effects of mutual orthogonalization within a set of orbitals are studied. It is found that this procedure decreases the bonding ability of bonding orbitals, but increases that of antibonding orbitals. The dangers of disregarding such orthogonalization in a conventional calculation are clearly illustrated.