Partial ionicity, cohesion, and charge correlations in narrow-band solids

A many-electron, correlated-state representation is introduced for narrow-band solids of donors and acceptors. The electrostatic energy for partial ionicity 0⩽q⩽1 is related to zero bandwidth Wigner lattices and to wide-band Hartree–Fock results for both simple orthorhombic lattices of point charges and for the organic conductor TTF–TCNQ at q = 1/2. Partial ionicity requires repulsive as well as attractive Coulomb contacts in the first coordination sphere, a condition that is met by organic conductors. The configuration interaction among correlated crystal states washes out the fixed neutral and ionic sites of a Wigner lattice, while retaining most of its electrostatic binding. Short-range charge correlations in partly ionic lattices substantially improve the Hartree–Fock cohesion and rationalize the shallow minimum (relative to neutral solids) in representative partly ionic organic solids.