calculate pressure from vinet equation

    :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
    :return: pressure in GPa

def vinet_p(v, v0, k0, k0p):
    """
    calculate pressure from vinet equation

    :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
    :return: pressure in GPa
    """
    # unumpy.exp works for both numpy and unumpy
    # so I set uncertainty default.
    # if unumpy.exp is used for lmfit, it generates an error
    return cal_p_vinet(v, [v0, k0, k0p],
                       uncertainties=isuncertainties([v, v0, k0, k0p]))