To find the critical point, lower the temperature. As the temperature goes down, the fluctuations in "H" go up because the fluctuations are more correlated. This means that the average of a large number of spins does not become small as quickly as if they were uncorrelated, because they tend to be the same. This corresponds to decreasing "A" in the system of units where "H" does not absorb "A". The phase transition can only happen when the subleading terms in "F" can contribute, but since the first term dominates at long distances, the coefficient "A" must be tuned to zero. This is the location of the critical point: