The CONOPT statement has no options. It is equivalent to specifying RESET SCRATCH;. The CONOPT statement should be used before stage 2 optimization commences. It indicates that the optimization performed next should consider the side constraints.

Usually, the optimal unconstrained network solution is used as a starting solution for constrained optimization. Finding the unconstrained optimum usually reduces the amount of stage 2 optimization. Furthermore, the unconstrained optimum is almost always "closer" to the constrained optimum than the initial basic solution determined before any optimization is performed. However, as the optimum is approached during stage 1 optimization, the flow change candidates become scarcer and a solution good enough to start stage 2 optimization may already be at hand. You should then specify the CONOPT statement.