  We calculated the Madelung energies of both the ground state and excited
states in tetrathiafulvalene - p-chloranil (TTF-CA) by taking into account
intramolecular charge distribution. The distribution is found to be significant
in the neutral-ionic (NI) transition. In the ionic phase, the Madelung energy
depends more strongly on the intermolecular distance perpendicular to the
$\pi$-stacking chains than on that along the chains. This indicates that simple
single-chain models neglecting interchain electrostatic coupling are not
adequate. The gain of the Madelung energy due to dimerization is concluded to
be small compared with the other structural changes. We also calculated the
formation energy of excited state domains, which appear in the initial process
of the phase transition. A one-dimensional excited domain has the smallest
energy among the possible domains with the same number of molecules, and the
energy per molecule is considerably reduced in increasing the domain size. It
is consistent with the experimental suggestion that a large number of
excitations were generated by only one photon.

Strongly Correlated Electrons;Materials Science