Molecular Simulation/Interfacial properties

Surface Tension
The surface tension (γ) of a liquid is defined as, $$\gamma=\left( \frac{\partial G}{\partial A}\right)_{T,p,N}$$, where G is the Gibbs energy and A is the surface area of the liquid. At equilibrium, the surface tension of liquid is positive, indicating that the Gibbs energy of the system would increase if the surface area were increased. The SI unit for surface tension is N/m, but dyn/cm is sometimes reported instead.

Physically, this reflects that molecules at the liquid-vapor interface have fewer neighbors than molecules in the bulk liquid. This generally means that the intermolecular interactions the molecule engages in are weaker. Increasing the surface area would result in more molecules being at the interface, so this is thermodynamically disfavored. The surface tension is an indicator of the degree to which it is disfavored. This effect is particularly strong in associated liquids like water, which becomes more ordered and has fewer hydrogen bonds at the water-vapor interface.

The surface tension of a liquid can be calculated in a molecular simulation by measuring differences in the averages of the pressure tensor for an NVT simulation where the simulation contains a slab of the liquid surrounded by vapor layers along the z axis,

$$\gamma = \frac{1}{2} L_z \left[ \langle P_{zz} \rangle - \frac{1}{2} \left( \langle P_{xx} \rangle + \langle P_{yy} \rangle  \right) \right]$$