[SCIP] Constraint handlers and CONSEPALP callbacks

[Pippia Eleonora]

Good morning,
I'm using SCIP to solve an ILP model MP in order to test a conjecture related to graphs of low dimensions. Actually, the conjecture is equivalent to state that an integer linear programming model MP is unfeasible. I have few questions.

1) I've implemented two constraint handlers with separation callbacks. I don't understand in which order the callbacks CONSSEPALP for the different constraints are called. Assume that at a node of the branch-and-bound tree, four steps of the separation loop take place.  It happened that in the first three steps SCIP recalled the SEPALP callback from the first constraint handler to add a new violated cut so that the SEPALP from the second constraint handler was delayed in these steps. At the fourth step, sometimes SCIP called immediately the SEPALP from the second constraint handler without calling the callback from the first one. Why can this happen?

2) Moreover, I ran the same instance with the same model MP but on two different computers. It happened that at the same node of the branch-and-bound, on the two computers SCIP found two different fractional solutions of the LP. This had a strong impact on the number of nodes generated to solve the instance. Is it a normal behavior?

3) Finally, is there any way to use SCIP as an MIP-solver disabling all the constraint programming features?

Thanks for your time (and thanks for SCIP!!!)

Best regards,
Eleonora Pippia

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