The dynamical structure factor S (Q,ω) of solid β-N2

Abstract

A classical computer simulation study of the dynamical structure factor S (Q,ω) for a simple model of solid β-N2 is reported. We utilized a hexagonal system composed of 288 molecules interacting via a (12–6) atom–atom potential. Periodic boundary conditions were used to simulate the infinite crystal. The molecular half-bond length d was assumed to be given by d=0.1646σ, where σ is the zero of the atom–atom potential. The molecular density ρ=Nσ3/V=0.836 and the temperature T=1.27 ε/kB (ε is the potential well depth) were such that using recently proposed potential parameters our thermodynamic state corresponded to V=26.1 cm3/mole, T=47.4 K. When compared with real β-N2 under these conditions our calculated pressure is a little too high, and the transverse optic, Raman active frequency too low. For small wave vectors Q≲2 Å−1, the full dynamical structure factor shows phonon peaks which are virtually indistinguishable from those due to the center of mass motion, while at larger Q (between 4.8 and 6.4 Å−1) the spectra are dominated by the reorientational motion of the molecules. The computer simulation phonon energies are compared with lattice dynamical calculations employing an intermolecular potential derived from the spherical average of the atom–atom potential. There is particularly poor agreement for long wavelength shear modes, probably due to the neglect (in the lattice dynamical calculations) of the coupling of these modes to molecular reorientational motion.