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000849717 1001_ $$0P:(DE-HGF)0$$aEckold, G.$$b0$$eCorresponding author
000849717 245__ $$aUNISOFT: a program package for lattice dynamical calculations: user manual
000849717 260__ $$aJülich$$bForschungszentrum Jülich GmbH Zentralbibliothek, Verlag$$c1992
000849717 300__ $$aX, 117 p.
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000849717 4900_ $$aBerichte des Forschungszentrums Jülich$$v2639
000849717 520__ $$aLattice-dynamical investigations provide a powerful tool for the determination of interatomic forces and chemical bonding in crystals. In general the extraction of, say,potential parameters from phonon dispersion curves, as determined experimentally, is possible with the help of model calculations only. Phonons which are  cooperative excitations in many-body systems provide informations about interactions within the whole crystal. A quantitative interpretation of single phonons is therefore restricted to very simple crystal structures. In general, however, unambigious statements about interatomic forces require the knowledge of a good number of the phonon branches, at least in the symmetry directions. Often, the determination of phonon dispersion curves is a rather hard task since a crystal with N particles per unit cell exhibits as many as 3N phonon branches. The  only experimental method which allows the detection of phonons at arbitrary points within the Brillouin zone is the inelastic neutron scattering. Again, model calculations are very useful and sometimes even necessary in order to find phonons experimentally since phonon intensities vary strongly from one Brillouin zone to another and are determined by the corresponding eigenvectors. For complex structures, model calculations and neutron scattering experiments are thus complementary. The determination of interatomic forces in crystals requires both, the theoretical and the experimental approach which need to go hand in hand. The three-axes spectrometer UNIDAS at the FRJ-2 reactor at Jülich is available to external users and is especially designed to meet the requirements for phonon investigations. Details of this machine are described in the UNIDAS-Handbuch [1]. In the present paper we report an the complement to this spectrometer, namely the computer program package UNISOFT which allows to perform model calculations an phonon dispersion in general structures with up to 20 particles per primitive cell. UNISOFT has been developed in order to provide a tool for the optimization of experiments as well as for a first interpretation of the results. Certainly, a general lattice-dynamical program cannot deal with the most complex models for interatomic interactions. Until now the standard models such as $\bullet$ Born-von Kärmän model (longitudinal and transverse springs) $\bullet$ Born-Mayer potential $\bullet$ Lennard-Jones potentialvan der Waals potential $\bullet$ Coulomb potential (Ewald's method of summation) $\bullet$ shell model are implemented. The interaction between each pair of atoms can be chosen individually as an arbitrary combination of these model potentials. More sophisticated models must be developed by the user and are beyond the scope of UNISOFT as a universally applicable lattice-dynamical program. Special attention has been paid to the handling of this program package by external users. Offering the possibility of a tailored interatomic interaction set-up for each individual substance under consideration, the system is rather complex, of course.Universality of the programs, however, does not exclude an easy mode of operation.  Thus, UNISOFT can indeed be used in combination with UNIDAS by  experimenters which are not familiar with details of lattice-dynamical calculations and the interpretation of phonon dispersion curves. This combination of software and hardware is able to simplify the determination of interatomic forces in crystals even for nonspecialists. Chapter 2 of this report briefly reviews the theoretical background of lattice dynamics in the harmonic approximation along with group-theoretical considerations and symmetry constraints to the dynamical matrix. The response of a phonon system in neutron scattering experiments is also discussed. In section 3 the global structure of the program package is displayed before a detailed description of the individual programs is given in chapter 4. Section 5 deals with computer requirements. Some possible extensions of this program system are discussed in chapter 6. In order to illustrate all information available with UNISOFT, an example for a complete treatment of a crystal structure within this program system is presented in appendix A. Finally, a comprehensive list of all input cards, the alphabetical list of subroutines, the subroutine reference list and the list of symbols used in this manual are given in the appendices B, C, D and E. Concerning the notation, matrices are denoted by bold printed capital letters and vectors are represented by bold printed lower case letters.
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