At very low voltage, 0.6V and below, circuit effects and variation break all of the estimations that PT and Tempus rely upon to calculate timing. Worst of all the corner values that everyone uses do not really tell you where the timing will end up. Just like analog designers use the MC SPICE to determine the average and the distribution for a circuit, path based timing needs to do the same thing. In fact, the difference between the average delay and the corner delay on a path at low voltage can be 10% or higher - bigger than the variance around the average itself.
Path FX is the only solution that determines timing at low voltage accurately and efficiently. Because it is based on FX, a transistor level model, it is able to both find the true average delay just like MC SPICE, and take into account all of the circuit level effects that PT and Tempus struggle with (Miller Capacitance, nonlinear waveforms, slew variation, local voltage effects and IR drop...).
At low voltage, Path FX is the answer.
To learn more, we have created a white paper on low voltage timing. Please provide us your name and where you work. We promise not to send you loads of unsolicited emails (though we may harass you briefly before DAC). The form will take you to the link.