A donor-acceptor model with randomly distributed N donor and N acceptor sit
es and with finite charge transfer energy Delta is investigated at zero tem
perature by means of computer simulations on two-dimensional samples of dif
ferent sizes, We demonstrate that the density of one-electron excitations g
(epsilon) for accepters in the vicinity of the Fermi energy mu depends esse
ntially on the size of the sample investigated and that g(epsilon) further
away from mu exhibits a size-independent power-law with the non-universal e
xponent decreasing with increasing Delta. Rigorous relations for mu and for
the mapping of g(epsilon) for donors into g(epsilon) for accepters are als
o given. (C) 2000 Elsevier Science B.V. All rights reserved.