To
facilitate more reliable descriptions of transport properties
in liquids, molecular dynamics (MD) simulations are performed based
on the effective fragment potential (EFP) method derived from first-principles
quantum mechanics (in contrast to MD based upon empirically fitted
potentials). The EFP method describes molecular interactions in terms
of Coulomb, polarization/induction, dispersion, exchange-repulsion,
and charge-transfer interactions. The EFP MD simulations described
in this paper, performed on hexane and acetone, are able to track
the mean-square displacement of molecules for sufficient time to reliably
extract translational diffusion coefficients. The results reported
here are in reasonable agreement with experiment.
Molecular Dynamics (MD) simulations based upon the Effective Fragment Potential (EFP) method are utilized to provide a comprehensive assessment of diffusion in liquid n-hexane. We decompose translational diffusion into components along and orthogonal to the long axis of the molecule. Rotational diffusion is decomposed into tumbling and spinning motions about this axis. Our analysis yields four corresponding diffusion coefficients which are related to diagonal entries in the complete 6 ´ 6 diffusion tensor accounting for the three rotational and three translational degrees of freedom, and for the potential coupling between them. However, coupling between different degrees of freedom is expected to be minimal for a natural choice of molecular body-fixed axis, so then off-diagonal entries in the tensor are negligible. This expectation is supported by a hydrodynamic analysis of the diffusion tensor which treats the liquid surrounding the molecule being tracked as a viscous continuum. Thus, the EFP MD analysis provides a comprehensive characterization of diffusion, and also reveals expected shortcomings of the hydrodynamic treatment particularly for rotational diffusion.
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