  Evidence for capillary waves at a liquid/vapor interface are presented from
extensive molecular dynamics simulations of a system containing up to 1.24
million Lennard-Jones particles. Careful measurements show that the total
interfacial width depends logarithmically on $L_\parallel$, the length of the
simulation cell parallel to the interface, as predicted theoretically. The
strength of the divergence of the interfacial width on $L_\parallel$ depends
inversely on the surface tension $\gamma$. This allows us to measure $\gamma$
two ways since $\gamma$ can also be obtained from the difference in the
pressure parallel and perpendicular to the interface. These two independent
measures of $\gamma$ agree provided that the interfacial order parameter
profile is fit to an error function and not a hyperbolic tangent, as often
assumed. We explore why these two common fitting functions give different
results for $\gamma$.
