Complexity (SolverMaxMemory) defines the maximum available memory for the stiffness matrix.
It corresponds to the resolution of the model and thus determines the calculation time.
Increasing this these values increases the resolution and thus calculation time!
It does not define the exact amount of memory the optimization optimisation uses, but the memory that is needed in the worst case if no material can be reduced due to the optimization optimisation setup.
You can either choose a percentage of your available RAM or an exact value.
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