In Abaqus, we have the ability to indicate to the solver that our constitutive models produce material stiffness matrices which are unsymmetric - and as such, Abaqus will use the unsymmetric solver rather than the symmetric one which often results in faster convergence. It seems that Calculix does not currently have such a facility.
Does Calculix automatically detect the unsymmetric terms, or does it go on to try and solve the system using a symmetric solver (which will probably get there, but could be pretty inefficient)?