Computes the couplling constant C_q as defined in:
            Wasylishen R E, Ashbrook S E, Wimperis S. NMR of quadrupolar nuclei
            in solid materials[M]. John Wiley & Sons, 2012. (Chapter 3.2)

        C_q for a specific atom type for this electric field tensor:
                C_q=e*Q*V_zz/h
            h: planck's constant
            Q: nuclear electric quadrupole moment in mb (millibarn
            e: elementary proton charge

        Args:
            specie: flexible input to specify the species at this site.
                    Can take a isotope or element string, Specie object,
                    or Site object

        Return:

            the coupling constant as a FloatWithUnit in MHz

def coupling_constant(self, specie):
        """
        Computes the couplling constant C_q as defined in:
            Wasylishen R E, Ashbrook S E, Wimperis S. NMR of quadrupolar nuclei
            in solid materials[M]. John Wiley & Sons, 2012. (Chapter 3.2)

        C_q for a specific atom type for this electric field tensor:
                C_q=e*Q*V_zz/h
            h: planck's constant
            Q: nuclear electric quadrupole moment in mb (millibarn
            e: elementary proton charge

        Args:
            specie: flexible input to specify the species at this site.
                    Can take a isotope or element string, Specie object,
                    or Site object

        Return:

            the coupling constant as a FloatWithUnit in MHz
        """
        planks_constant=FloatWithUnit(6.62607004E-34, "m^2 kg s^-1")
        Vzz=FloatWithUnit(self.V_zz, "V ang^-2")
        e=FloatWithUnit(-1.60217662E-19, "C")

        # Convert from string to Specie object
        if isinstance(specie, str):
            # isotope was provided in string format
            if len(specie.split("-")) > 1:
                isotope=str(specie)
                specie=Specie(specie.split("-")[0])
                Q=specie.get_nmr_quadrupole_moment(isotope)
            else:
                specie=Specie(specie)
                Q=specie.get_nmr_quadrupole_moment()
        elif isinstance(specie, Site):
            specie=specie.specie
            Q=specie.get_nmr_quadrupole_moment()
        elif isinstance(specie, Specie):
            Q=specie.get_nmr_quadrupole_moment()
        else:
            raise ValueError("Invalid speciie provided for quadrupolar coupling constant calcuations")

        return (e * Q * Vzz / planks_constant).to("MHz")