DENSITY-OF-STATES OF ALTERNANT CYCLIC POLYENES (CH)(N) BY A DIRECT LANCZOS METHOD

A direct full configuration interaction approach, previously used for studying individual low-lying eigenvalues, is combined with iterative Lanczos calculations, in order to obtain global properties of large He rmitean Hamiltonian matrices. To this effect systematic generation of random start vectors is used to compute statistical approximations to the density of states (DOS). Applications for cyclic polyenes in order to illustrate the viability of the scheme modeled by correlated, tigh t-binding Pariser-Parr-Pople (PPP) Hamiltonians of increasing complexi ty are presented. The degree of correlation in the solutions is contro lled by the parameter beta. Convergence properties of the DOS for N=10 and N=14 are studied varying the extent of correlation. (C) 1997 John Wiley & Sons, Inc.