def ds2n(self):
        """Calculates the derivative of the neutron separation energies:

        ds2n(Z,A) = s2n(Z,A) - s2n(Z,A+2)
        """
        idx = [(x[0] + 0, x[1] + 2) for x in self.df.index]
        values = self.s2n.values - self.s2n.loc[idx].values
        return Table(df=pd.Series(values, index=self.df.index, name='ds2n' + '(' + self.name + ')'))
Calculates the derivative of the neutron separation energies:

        ds2n(Z,A) = s2n(Z,A) - s2n(Z,A+2)