[Ipopt] Question about the "log" and "power" error

[张晨]

Hi all:
When I set "var a >=0.00000001 default 0.001" and solve the model by IPOPT with AMPL, IPOPT would return the error information "can't evaluate log(-3.05e-10)" (base number is very slightly less than zero) in equations including the "log(a)" term sometimes. Similarly, it would return "can't evaluate power(1.00035, 10e08)" (base number is very slightly more than 1) in equations including the "[1/(1+a)]^(10e08)" term.


So why would this phenomenon happen? It seems that the lower bound of "a" didn't take effect. How can I avoid this error? 


Besides, if there are some terms like exp(a) or a^b (a,b are variables) in AMPL equations, do we always need to change them to the logarithm form (e.g. a^b becomes b*log(a) )? Does this reformulation do make the solving process more efficient? 


Thank you very much! Looking forward to your reply.


Yours
Cavern
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