GRAPHIC RULES FOR REDUCING THE ORDER OF THE HUCKEL DETERMINANT

McClelland's rules are generalized to construct characteristic polynom ials of molecular graphs with a symmetry plane. A new graphic represen tation of the adjacency matrix is suggested Diagonal matrix elements a re assigned to graph nodes with directed loops; the number of edges co nnecting the adjacent nodes is equal to the product of the symmetric o ff-diagonal elements. Graphic rules for reducing the order of the Huck el determinant are suggested. The rules can be used in Huckel, vibrati on, and perturbation theories.