  We propose a computational scheme to evaluate Hamaker constants, $A$, of
molecules with practical sizes and anisotropies. Upon the increasing
feasibility of diffusion Monte Carlo (DMC) methods to evaluate binding curves
for such molecules to extract the constants, we discussed how to treat the
averaging over anisotropy and how to correct the bias due to the
non-additivity. We have developed a computational procedure for dealing with
the anisotropy and reducing statistical errors and biases in DMC valuations,
based on possible validations on predicted $A$. We applied the scheme to
cyclohexasilane molecule, Si$_6$H$_{12}$, used in 'printed electronics'
fabrications, getting $A \sim 105 \pm 2$ [zJ], being in plausible range
supported even by other possible extrapolations. The scheme provided here would
open a way to use handy {\it ab initio} evaluations to predict wettabilities as
in the form of materials informatics over broader molecules.
