  In this paper we discuss how the information contained in atomistic
simulations of homogeneous nucleation should be used when fitting the
parameters in macroscopic nucleation models. We show how the number of solid
and liquid atoms in such simulations can be determined unambiguously by using a
Gibbs dividing surface and how the free energy as a function of the number of
solid atoms in the nucleus can thus be extracted. We then show that the
parameters of a model based on classical nucleation theory can be fit using the
information contained in these free-energy profiles but that the parameters in
such models are highly correlated. This correlation is unfortunate as it
ensures that small errors in the computed free energy surface can give rise to
large errors in the extrapolated properties of the fitted model. To resolve
this problem we thus propose a method for fitting macroscopic nucleation models
that uses simulations of planar interfaces and simulations of three-dimensional
nuclei in tandem. We show that when the parameters of the macroscopic model are
fitted in this way the numerical errors for the final fitted model are smaller
and that the extrapolated predictions for large nuclei are thus more reliable.
