Json-serializable dict representation of Molecule

def as_dict(self):
        """
        Json-serializable dict representation of Molecule
        """
        d = {"@module": self.__class__.__module__,
             "@class": self.__class__.__name__,
             "charge": self._charge,
             "spin_multiplicity": self._spin_multiplicity,
             "sites": []}
        for site in self:
            site_dict = site.as_dict()
            del site_dict["@module"]
            del site_dict["@class"]
            d["sites"].append(site_dict)
        return d