  We present a simulation scheme to calculate defect formation free energies at
a molecular crystal/water interface based on force-field molecular dynamics
(MD) simulations. To this end we adopt and modify existing approaches to
calculate binding free energies of biological ligand/receptor complexes to be
applicable to common surface defects, such as step edges and kink sites. We
obtain statistically accurate and reliable free energy values for the
aspirin/water interface, which can be applied to estimate the distribution of
defects using well-established thermodynamic relations. As a show case we
calculate the free energy upon dissolving molecules from kink sites at the
interface. This free energy can be related to the solubility concentration and
we obtain solubility values in excellent agreement with experimental results.
