Modelling the Air-Water Interface

Water is an incredibly important molecule for chemical and biochemical modelling of complex systems. Despite this, many of its emergent properties are not fully understood. Recent ellipsometry experiments into probing the surface of water-air interface have shown the presence of a water-like surface film with a much higher refractive index of light than the bulk solvent.$^{[1]}$ This runs counter to molecular dynamics calculations, which propose a tapering off of molecular density in this region over sub-nm thickness, and of refractive index as well. It has been suggested that there is some emergent behaviour of water molecules at the air-water interface that give rise to the high electric field at the surface, causing the observed higher refractive index.
Ellipsometry experiments can be simulated by studying the arrangement of molecules at an interface during a molecular dynamics (MD) simulation in terms of order parameters to calculate surface polarisation. Usually hyperpolarisabilities are then taken from quantum simulations to describe the molecular response to an applied electric field, however these are normally parametrised to a single molecule and so may neglect complex surface interactions. In addition, performing quantum MD simulations on large scale systems to simulate physical experiments is highly time consuming, so a full ensemble of states may not be generated within the computational resources available. Therefore, it is advantageous to perform as much of the simulation as possible using classical polarisable force field methods.
Using the commercially available AMBER 12 molecular mechanics software we have so far implemented ways to deal with long range Lennard-Jones force corrections based on the methodology of Jane\v{c}ek.$^{[2]}$ These corrections to particle forces and energies can be accurately dealt with during post-processing in systems containing homogeneous densities, but it becomes necessary to include them ``on-the-fly'' during each simulation time-step in the presence of interfaces. We present an overview of how these force corrections are important for replicating surface behaviour in Lennard-Jones fluids, and also the development of a novel way to thermodynamically estimate surface energetic and entropic terms to assess their accuracy.
\begin{thebibliography}{99}
\bibitem{greef08}
R. Greef, J.G. Frey; The Water-like Film on Water, \textit{ Phys. Stat. Sol.}; \textbf{5}, 1184 (2008)
\bibitem{janecek06}
J. Jane\v{c}ek; Long Range Corrections in Inhomogeneous Simulations, \textit{J. Phys. Chem. B}; \textbf{110}, 6264 (2006)
\end{thebibliography}