Calculates Young's modulus (in SI units) using the
        Voigt-Reuss-Hill averages of bulk and shear moduli

def y_mod(self):
        """
        Calculates Young's modulus (in SI units) using the
        Voigt-Reuss-Hill averages of bulk and shear moduli
        """
        return 9.e9 * self.k_vrh * self.g_vrh / (3. * self.k_vrh + self.g_vrh)