def _get_radius(site):
        """
        An internal method to get the expected radius for a site with
        oxidation state.
        Args:
            site: (Site)

        Returns:
            Oxidation-state dependent radius: ionic, covalent, or atomic.
            Returns 0 if no oxidation state or appropriate radius is found.
        """
        if hasattr(site.specie, 'oxi_state'):
            el = site.specie.element
            oxi = site.specie.oxi_state

            if oxi == 0:
                return CrystalNN._get_default_radius(site)

            elif oxi in el.ionic_radii:
                return el.ionic_radii[oxi]

            # e.g., oxi = 2.667, average together 2+ and 3+ radii
            elif int(math.floor(oxi)) in el.ionic_radii and \
                    int(math.ceil(oxi)) in el.ionic_radii:
                oxi_low = el.ionic_radii[int(math.floor(oxi))]
                oxi_high = el.ionic_radii[int(math.ceil(oxi))]
                x = oxi - int(math.floor(oxi))
                return (1 - x) * oxi_low + x * oxi_high

            elif oxi > 0 and el.average_cationic_radius > 0:
                return el.average_cationic_radius

            elif oxi < 0 and el.average_anionic_radius > 0:
                return el.average_anionic_radius

        else:
            warnings.warn("CrystalNN: distance cutoffs set but no oxidation "
                          "states specified on sites! For better results, set "
                          "the site oxidation states in the structure.")
        return 0