Abstract: The a_N-index based on the quantum theory performs well but it can only be used for saturated hydrocarbons. An extended a_N-index called the general a_N-index has been developed and a new scheme for its calculation is adapted to obtain a set of parameters of orbital interactions. The general a_N-index can be used for almost all kinds of organic compounds including those containing heteroatoms and multiple bonds. The procedure for computing the general a_N-index is described and some examples of applications of the general a_N-index are given in the paper.

Key words: general a_N-index, CNDO/2, QSPR