It was suggested that I posted a new thread to let you know about the problem with accuracy when EM modelling structures above a few THz. Seems that the message is not always getting through to users. We published an article that clearly explains the problem:
E. Episkopou, S. Papantonis, W. J. Otter and S. Lucyszyn, "Defining material parameters in commercial EM solvers for arbitrary metal-based THz structures”, IEEE Transactions on Terahertz Science and Technology, vol. 2, no. 5, pp. 513-524, Sep. 2012