Contribution of intra- and intermolecular interaction to nonlinear opticalsusceptibilities

We present a new model of intermolecular interactions taking into account t he anharmonic coupling between the tunneling inter-conformation molecular m ode and the long-range crystalline displacement. The coupling of the anharm onic phonon modes and local mode disordering causes the non-centrosymmetry in the electrostatic potential distribution. Symmetry of the molecular diso rder has a point group C-3 The model is explained within a framework of tun neling soft mode conception and including molecular dynamics ab initio mole cular dynamics simulations. The optical second harmonic generation (SHG) se nsitive to local potential non-centrosymmetry is used as an experimental me thod for detection of manifestation the intermolecular disordering. The res ults are considered to be general importance for systems with electron-phon on anharmonic ordering, which is common to many organic materials.