I have been using PARDISO for a while and it is really good.
I have the solver linked up to an old Fortran program that generates a standard stiffness matrix for a beam.
I was looking at the output for a timber beam that is modelled as 4 uniform lengths -- I get the first four modes as the same value, but if I perturb the lengths slightly I get 4 different values that are now closer to the measured values provided I play with the constants a bit.
Interesting problem, I suppose I should print out the structures matrix and look for some form of symmetry that is providing the first answer.
Any thoughts would be appreciated?
John