  Controlling the water contact angle on a surface is important for regulating
its wettability in industrial applications, which involves developing ab initio
prediction scheme of accurately predicting the angle. The scheme requires
structural models for the adsorption of liquid molecules on a surface, but
their reliability depend on whether the surfaces comprise insulating or
metallic materials. Previous ab initio studies have focused on the estimation
of the water contact angle on insulators, where the periodic-honeycomb array of
water molecules was adopted as the adsorption model for the water on the
insulating surface and succeeded in the insulating cases. This study, however,
focus on the water contact angle on a metallic surface, and propose a simple ab
initio based estimation scheme. We not only adopt the previously proposed
structural modeling based on the periodic-honyecomb array, but also consider an
ensemble of isolated water oligomers that have different molecular coverage
(ML) values. We established a statistic model to predict a contact angle of the
water wetting on a Cu(111) surface: The coverage-dependent contact angles
obtained from each of the isolated clusters was fit to a quadratic regression,
and the contact angle was interpolated by referring to a ML value of water
layer in literature. This interpolated value lay within the deviation of
experimental angles. In addition, the Boltzmann-average over the isolated
clusters was found to agree well with the interpolated one. This indicates that
the Boltzmann-average is useful for estimating the contact angle of other
metallic surfaces without knowing a ML value a priori.
