  We have studied the neutral-ionic transition in organic mixed-stack
compounds. A unified model has been derived which, in limiting cases, is
equivalent to the models proposed earlier, the donor-acceptor model and the
ionic Hubbard model. Detailed numerical calculations have been performed on
this unified model with the help of the density-matrix renormalization-group
(DMRG) procedure calculating excitation gaps, ionicity, lattice site entropy,
two-site entropy, and the dimer order parameter on long chains and the unified
phase diagram has been determined.
