Computer simulation study of the structure of the liquid-vapor interface of mercury at 20, 100, and 200°C

We report the results of self-consistent quantum Monte Carlo simulations of the liquid-vapor interface of Hg. Our calculations are intended to answer two questions: (i) What is the quality of agreement between experimental data and calculations of the structure of the liquid-vapor interface of Hg based on the best available treatment of an inhomogeneous liquid metal? (ii) How well does a theory of the liquid-vapor interface of a metal that uses a simple model local pseudopotential reproduce the predictions obtained from a theory that uses an accurate nonlocal pseudopotential? As for the liquid-vapor interfaces of other metals, we find that the density distribution along the normal to the liquid-vapor interface of Hg (the longitudinal density distribution) is stratified, with a penetration depth into the bulk liquid of about three layers. The use of a nonlocal pseudopotential in the self-consistent Monte Carlo simulations leads to very good agreement between the calculated and observed longitudinal density distributions. We also show that the use of a model local pseudopotential leads to a qualitatively correct description of the longitudinal density in the liquid-vapor interface of Hg. When the model local pseudopotential is used, the locations of the strata in the longitudinal density distribution are correct but the peak heights are too large and the peak widths are too small. The largest part of the error arising from the use of the model local pseudopotential can be traced to the error in the predicted surface tension. The quality of the agreement between simulations based on the best nonlocal pseudopotential and those based on a simple model local pseudopotential is sufficient to make the latter a very useful tool for inferring the qualitative properties of a broad class of inhomogeneous liquid metals and alloys. We also report the results of calculations of the transverse (in-plane) structure of the liquid-vapor interface of Hg, for several slices of the interface. Except in the outermost layer, the transverse pair correlation function is found to be indistinguishable for the bulk liquid pair correlation function, in agreement with the available experimental data. Unlike the case of the liquid-vapor interface of alkali metals, our results imply there is a nontrivial difference between the transverse pair correlation function in the outermost layer of the liquid-vapor interface of Hg and that in the bulk liquid. The difference takes the form of an exaggerated shoulder on the first peak of the pair correlation function and is of the type which some investigators believe is associated with dimerization. However, we can fmd no direct evidence for dimer formation in images from our simulations of the positions of the ion cores in the outermost layer of the liquid-vapor interface of Hg. [S1063-651X(99)04401-3].