Pure translation instantaneous normal modes: Imaginary frequency contributions vanish at the glass transition in CS2
1997; American Institute of Physics; Volume: 107; Issue: 18 Linguagem: Inglês
10.1063/1.474968
ISSN1520-9032
Autores Tópico(s)Thermodynamic properties of mixtures
ResumoWhile the connection between imaginary frequency instantaneous normal modes (INM) and barrier crossing in liquids is well established, nondiffusive modes exist which must be removed in theories of the self-diffusion constant, D. It is shown that in molecular systems rotation is the primary source of such modes. A new type of pure translation (TR) INM is introduced, describing center-of-mass motion. TR densities of states, 〈ρTR(ω)〉, are obtained at several T for CS2. The TR Im-ω modes vanish at the glass transition and their fraction, fuTR, has a T dependence identical to that of D. Correspondingly, the Re-ω density looses the usual INM linear low-ω dependence and attains conventional ω2 behavior. It is proposed that the unmodified TR INM have a negligible nondiffusive character and naturally describe translational dynamics in molecular systems. Some general ideas are given for how INM properties, including the importance of nondiffusive modes, vary among different materials.
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