In the high frequency community there has been an increasing interest in so called frequency selective surfaces (FSS). These materials are built as large array of single unit cells. On the microscopic level, such a unit cell consists for example of a metal structure and air around. On the macroscopic level, they overall act as frequency selective material due to their microscopic resonances. In Figure 1 the dichroic filter  as an example for frequency selective surfaces is shown.
When simulating such structures, there is no need to simulate the whole array of unit cells, which would be time and memory consuming. Instead it is advantageous to simulate a single unit cell and use adequate boundary conditions. The dichroic filter shown above can be simulated by the unit cell depicted in figure 2 (left), where the gold areas are sufficiently accurate represented by PEC material (grey). The whole array is modeled by the magnetic and electric boundary conditions, which mirror the structure in x and y direction to infinity....
The results obtained with CST MWS are in good agreement with the measurements. The first passband (center frequency 1300 GHz) and second passband (center frequency 1750 GHz), which indicate the filter behaviour, are shown in the measurements and simulation results as well.
Within the MWS simulation also the electric and magnetic field pattern is obtained. Figure 5 shows the field plots, of the two modes which are able to propagate.
 C. Winnewisser, F. Lewen, J. Weinzierl, H. Helm, Transmission features of frequency-selective components in the far infrared determined by terahertz time-domain spectroscopy.", Applied Optics, Vol. 38, No. 18, pp. 3961-3967, 1999.