On the basis of structural data of the disordered tetragonal phase of tanane, the Van der Waals intermolecular forces are computed for the 16 nearest neighbours of a given molecule. Considering first the molecular ordering in a « rigid » tetragonal lattice, the pair energies between neighbouring molecules are analysed within a pseudo-spin formalism which shows that the main ordering interactions define linear chains of molecules along the ferroelectric Z-axis. As there is no exact procedure for a statistical treatment of the 3-d Ising Hamiltonian, we just calculate the transition temperature To in the mean-field approximation. The predicted result : T0 ~ 200 K is in good agreement with the ordering temperature in the « rigid » lattice deduced from experimental investigations : To = 235 K. In a second step, we consider the « piezo-electric-like » coupling between the molecular ordering and the uxy shear of the lattice. Using a « Bragg-Williams-like » treatment of the strain-dependent Ising Hamiltonian, we build a free energy expansion whose bilinear terms agree well with those determined from experiments (shear modulus C66 and « piezo-electric-like » coefficient D), and which yields for the predicted transition temperature : T c ~ 240 K (actual value : Tc = 287 K). This analysis also reveals an important biquadratic coupling term which is supposed to enhance the instability of the structure in the vicinity of T c.