  A method is proposed for manipulating with diagrammatic expansion of Green's
function of Frenkel's exciton random walks on the perfect lattice. The method
allows one to select diagrams, to supply diagrams with factors containing
information about the number of sites the diagram has passed through, etc.
Simple problems related to the defect lattices are considered using the
proposed method. The new criterion of localization of Frenkel exciton - the
number of sites covered by the wave function - is established. The number of
sites covered by the zero state of 1D non-diagonally disordered chain is
studied. It is shown that this problem can be solved by calculating the random
walks Green's function with modified diagrammatic expansion. By means of the
developed method an exact analitical expression for the number of sites covered
by the zero-state is obtained and zero-state is shown to be localized.
