Three-flavor instantaneous normal mode formalism:: Diffusion, harmonicity, and the potential energy landscape of liquid CS2

Instantaneous normal modes are classified by their one-dimensional potential energy profiles, U(q), into single well (SW), double well (DW), and shoulder potential (SH) modes. It is proposed that the resulting three-flavor description replace the current two-flavor division into real or imaginary frequency modes, and that the DW modes replace Im omega in theories of diffusion. Calculations of the three-flavor densities of states are presented for normal and supercooled liquid CS2, and the self-diffusion constant, D(T). is related to the DW modes. Indicators of strength/fragility an given based on the relative numbers of different mode types, and indicators of harmonicity are constructed by comparison of simulated instantaneous normal modes properties with the predictions of the harmonic approximation. It is found that the SW modes an harmonic in an intermediate Re omega range 20 ps(-1) > omega > 2.5 ps(-1), and the anharmonicity at high and low omega is explained in terms of the potential energy "landscape." DW modes are remarkably harmonic over the full range of Im omega. The T dependence of the diffusion constant is also interpreted in terms of the landscape, as manifested primarily in the properties of the energy barriers to diffusion. Diffusion is clearly associated with the strongly T-dependent crossing of barriers with omega similar to 3 ps(-1). (C) 1998 American Institute of Physics.