= Solution
Creating a liquid surface changes the molecular environment and costs interfacial <free energy>. In the dispersion-only cleavage model, the attractive <Van der Waals force> per area has magnitude $A/(6\pi h^3)$. Separating two contacting halves from a molecular cutoff $h_0$ to infinity takes work per area
$$
\frac WA=\int_{h_0}^{\infty}\frac{A_H}{6\pi h^3}dh=\frac{A_H}{12\pi h_0^2}.
$$
Here $A_H$ denotes the <Hamaker constant>, to distinguish it from an area. Cleavage creates two free surfaces, giving the <Hamaker estimate of liquid surface tension>
$$
\boxed{\gamma\simeq\frac{A_H}{24\pi h_0^2}.}
$$
The continuum attraction diverges at zero separation and neglects molecular impenetrability and short-range repulsion. The cutoff is therefore an effective contact separation of molecular size, commonly of order $0.1$–$0.3\,\mathrm{nm}$, not a limit that can be taken to zero.
An experimental limitation is the strong temperature dependence of <surface tension>. For water in equilibrium with its vapour, the recommended experimental values decrease from $71.98\,\mathrm{mN\,m^{-1}}$ at $25\,{}^\circ\mathrm C$ to $58.92\,\mathrm{mN\,m^{-1}}$ at $100\,{}^\circ\mathrm C$, and the fitted tension vanishes at the critical point. A fixed pairwise cleavage energy misses interfacial entropy, changing density and many-body correlations.<Revised Release on Surface Tension of Ordinary Water Substance by International Association for the Properties of Water and Steam>[2014 release, printed pp. 3–4: temperature-dependent equation and Table 1]{ref} The dispersion-only model is unsuitable when cohesion is dominated by <hydrogen bonds>, ionic interactions or <metallic bonding>; liquid metals are a clear example of the latter failure.
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