As a 2 > 0 , thermodynamic stability requires that the term in brackets be positive. The 2 κ κ q 2 is always positive but tends to zero at small wavevectors, large wavelengths. Since we are interested in macroscopic fluctuations, q → → 0 , stability requires that the second derivative of the free energy be positive. When it is, there is no spinodal decomposition, but when it is negative, spinodal decomposition will occur. Then fluctuations with wavevectors q < q c become spontaneously unstable, where the critical wave number q c is given by: