Calculations of the phonon dispersion curves of C2H2, OC(ND2)(2), and Na2CO3 from generic force field

The phonon dispersion curves of cubic acetylene, tetragonal urea, and monoc linic sodium carbonate are calculated from the generic force field using th e direct method. The force constants of the dynamical matrix are determined from the forces induced by the displacement of an atom in the simulated su percell. The obtained dispersion curves are displayed along many high-symme try lines of the Brillouin zone. For sodium carbonate, a soft mode has been found. The dispersion curves of urea fit quite well to the measured phonon s. The results show that the force field approach leads to reasonable descr iption of the lattice dynamics of crystals. (C) 1999 American Institute of Physics. [S0021-9606(99)50211-0].