THE PPP MODEL OF ALTERNANT CYCLIC POLYENES WITH MODIFIED BOUNDARY-CONDITIONS

The extension of the PPP Hamiltonian for alternant cyclic polyenes to noninteger values of the pseudomomentum by imposing modified boundary conditions is discussed in detail. It is shown that a computer program for periodic boundary conditions can be easily adapted to the new boundary conditions. Full CI computations are carried out for some low-lying states of the PPP model of alternant cyclic polyenes (CH)(N) (N even) at half-filling. The energy values obtained by using periodic (Bloch) and antiperiodic (Mobius) orbitals are used to perform energy extrapolations for N --> infinity. (C) 1995 John Wiley and Sons, Inc.