Theoretical Calculations on Nonlinear Susceptibilities of Organic Crystals
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THEORETICAL CALCULATIONS ON NONLINEAR SUSCEPTIBILITIES OF ORGANIC CRYSTALS YUZO ITOH, TOMOYUKI HAMADA, ATSUSHI KAKUTA, AND AKIO MUKOH Hitachi Research Laboratory, Hitachi Ltd., 4026 Kuji-cho, 319-12 Japan
Hitachi,
Ibaraki,
ABSTRACT Theoretical calculations on nonlinear susceptibilities of organic crystals are made. Under the oriented gas model approximation, nonlinear susceptibility of a crystal can be calculated both from the molecular hyperpolarizability and crystal structure data. Molecular hyperpolarizabilities are calculated by an ab initio molecular orbital (MO) method and the energy minimum crystal structures are obtained by an empirical atom-atom pairwise potential method. Finally the effect of intermolecular interactions on molecular hyperpolarizabilities, which is neglected in the above approximation, is investigated quantitatively by using a "super molecule" method of ab initio MOcalculations. INTRODUCTION Most organic crystals are molecular with the molecules being bound by weak van der Waals forces. Accordingly, the oriented gas model approximation, in which intermolecular interactions caused by molecular packing in the crystal are neglected, is usually good. Under this approximation we can calculate nonlinear susceptibility of a crystal from molecular hyperpolarizabilities and the crystal structure information. There have been many theoretical investigations on hyperpolarizabilities of molecules[l-20]. However, most of them used empirical or semi-empirical methods. Theoretical calculations by ab initio MO methods on hyperpolarizabilities of relatively large molecules[7,17,19], which are interesting for nonlinear optical applications, are very few. Semiempirical methods are very convenient to evaluate relatively accurate hyperpolarizabilities with a relatively small cpu time, but the ab initio method is necessary to analyse them more deeply and comprehensively. First, some calculation methods of hyperpolarizabilities by the ab initio MO method were investigated and compared to each other. Secondly, since very few theoretical studies on crystallization properties of organic nonlinear materials have been done, these properties were investigated by crystal energy calculations using empirical atom-atom pairwise potentials. Finally, the effect of intermolecular interactions by molecular packing in the crystal on hyperpolarizabilities was quantitatively investigated by a "super molecule" method of ab initio MO calculations. So far only one paper from this viewpoint[7] has been presented, in which the effect of hydrogenbonding in the crystal on hyperpolarizabilities was investigated. However, only the electrostatic interaction was treated for intermolecular interactions. Then, in the present treatment all short range intermolecular interactions were included. THEORETICAL CALCULATIONS ON HYPERPOLARIZABILITIES BY THE AB INITIO METHOD There were several approaches to the calculation of molecular hyperpolarizabilities of organic nonlinear optical materials. (1) The hyperpolarizability is obtained from the diff
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