calculate Gruneisen parameter for the Tange equation

    :param v: unit-cell volume in A^3
    :param v0: unit-cell volume in A^3 at 1 bar
    :param gamma0: Gruneisen parameter at 1 bar
    :param a: volume-independent adjustable parameters
    :param b: volume-independent adjustable parameters
    :return: Gruneisen parameter

def tange_grun(v, v0, gamma0, a, b):
    """
    calculate Gruneisen parameter for the Tange equation

    :param v: unit-cell volume in A^3
    :param v0: unit-cell volume in A^3 at 1 bar
    :param gamma0: Gruneisen parameter at 1 bar
    :param a: volume-independent adjustable parameters
    :param b: volume-independent adjustable parameters
    :return: Gruneisen parameter
    """
    x = v / v0
    return gamma0 * (1. + a * (np.power(x, b) - 1.))