American Physical Society copyright is acknowledged URL: http://link.aps.org/doi/10.1103/PhysRevB.67.035120 ; International audience ; The prototype compound for the neutral-ionic phase transition-namely, TTF-CA-is theoretically investigated by first-principles density functional theory calculations. The study is based on three neutron diffraction structures collected at 40, 90, and 300 K [M. Le Cointe , Phys. Rev. B 51, 3374 (1995)]. By means of a topological analysis of the total charge densities, we provide a very precise picture of intrachain and interchain interactions. Moreover, our calculations reveal that the thermal lattice contraction reduces the indirect band gap of this organic semiconductor in the neutral phase and nearly closes it in the vicinity of the transition temperature. A possible mechanism of the neutral-ionic phase transition is discussed. The charge transfer from TTF to CA is also derived by using three different techniques.