An Approximate Spectral Density for the Estimation of some Topological Indices of A1ternant Systems

A symmetric two-delta-function model spectral density is used to estimate several topological indices of alternant hydrocarbons, namely: the total n-electron energy (E.), the modified topological index (Z), the HOlVIO-LUMO separation (XHL) and the spectral radius of adjacency matrix (R). It is found, that the invariants defined by integration (like E. and Z) are reproduced much better than the invariants defined as the Iimiting values of the spectral distribution (like XHL and R). The reason for the well known linear dependence between Er. and lnZ, is discussed.