calculate Kunc EOS
    see Dorogokupets 2015 for detail

    :param v: unit-cell volume in A^3
    :param v0: unit-cell volume in A^3 at 1 bar
    :param k0: bulk modulus at reference conditions
    :param k0p: pressure derivative of bulk modulus at reference conditions
    :param order: order for the Kunc equation
    :return: pressure in GPa

def kunc_p(v, v0, k0, k0p, order=5):
    """
    calculate Kunc EOS
    see Dorogokupets 2015 for detail

    :param v: unit-cell volume in A^3
    :param v0: unit-cell volume in A^3 at 1 bar
    :param k0: bulk modulus at reference conditions
    :param k0p: pressure derivative of bulk modulus at reference conditions
    :param order: order for the Kunc equation
    :return: pressure in GPa
    """
    return cal_p_kunc(v, [v0, k0, k0p], order=order,
                      uncertainties=isuncertainties([v, v0, k0, k0p]))