Numerical estimates of the correlation function clumping factor ﹩\langle C\rangle =\xi ( 0) ﹩ for gas in adiabatic scale‐free simulations. The solid lines show the correlation function in the simulation at redshifts ﹩z=180﹩, 16, 4, 1, and 0 scaled to the appropriate nonlinear mass scale. The dashed line is the scale‐free correlation for ﹩n=-1.8﹩ and ﹩r_{0}=4﹩ h⁻¹ Mpc. We see that the simulations are still limited by resolution and give a lower bound of ﹩\langle C\rangle \gtrsim 900﹩. This rules out passively evolving IGM models and requires significant active heating to evade soft XRB constraints.