On the accuracy of intermolecular interactions and charge transfer: the case of TTF-CA

undergone by this charge transfer complex. Based on accurate X-ray structures at 105 and 15 K, topological analysis of both DFT and the experimental multipolar electron densities allowed detailed characterization of intra- and interstack intermolecular interaction s. Direct quantification of the intermolecular charge transfer and the dipole m oment are discussed. 15 20 grounds; it needs thorough investigations of interm olecular interactions (hydrogen bonds, van der Waals contacts, hydrophobicity, char ge transfer…) which is a very big challenge for both experimental and theoretical methods. This paper will concentrate on the characterization of intermolecul ar contacs and on the estimation of the molecular and atomic charges. Intermolecular interactions are usually 25 experimentally characterized by crystallographic or Raman-IR methods: conventional crystallography gives interatomic dist ances which should be compared to the sum of the so called "ionic" or "van der Waa ls" radii which are defined from statistical analysis of experimental interatomic di stances. IR or Raman methods correlate intermolecular interactions to the variat ion of typical phonon or molecular 30 group modes but the assumption are also empirical a nd only qualitative. Therefore one has to go a step further by modelling accuratel y the charge density in the intermolecular regions: this will give a direct est imation of the molecular charges. This can be achieved either by theoretical methods (Ab initio, DFT) or/and accurate high resolution crystallography.