One cannot accurately simulate dynamic gain control while performing an .AC analysis. This is because SPICE is performing a small signal analysis on the devices based on how they are biased after the DC bias portion of the simulation. Large signal behavior, such as the exponential relationship between VBE and IC are not modeled, and this exponential relationship is fundamental to the operation of THAT Corporation's log/anti-log VCAs. Performance with static gain can be simulated with the .AC analysis, and dynamic gain control can be simulated with TRAN (transient) analysis.
Also, the results of .AC analysis with THAT Corporation's RMS detectors can be misleading, since the result only shows the ripple of the RMS detector's log filter, and not the actual DC output. Transient analysis yields the correct DC output, but the user may want to experiment with the .IC statement to speed settling times for the log filter.