Stop Condition of Liberate trio in characterizing a DFF

Hi all,

Currently i am characterizing setup time of a DFF, and i have a question regarding the stop condition of Liberate trio. 

- Testbench : 

 The criteria for setup time is degradation and i sweep variable "constraint_delay_degrade" from 0.1 to 1 with the step of 0.1 to observe the setup time reported by the tool. (I purposely set the variable "constraint_delay_degrade_abstol " small  such that it cannot impact the criteria)

- Result :

 There are many cases that i think Liberate stops "too early".

 For instance :

 When "constraint_delay_degrade" is 0.9, the setup time is 17.64ps and ck2q is 4.26ps. The relative difference percentage of this ck2q compared to "relaxed" ck2q is only about 62.6%, which is much less than 90%.

 Furthermore, in the case when "constraint_delay_degrade" is 0.8, the relative difference percentage is higher than the case of 0.9 and equal to 77.8%. (I expect that the 0.9 case should give a higher value of ck2q or relative difference percentage than the 0.8 case)

- Question : 

 So my question is, what is the criteria for Liberate to stop searching ? I read in the Liberate manual that it will stop when the number of iteration or simulation time exceeds specified value, but none of these cases is likely to happen in my testbench. The manual also mentions about Brent's algorithm, but i cannot find more information about this in the manual. Therefore, i think it would be great if i can know more about this topic. Please give me any hints for this one.

Thanks for your support