This paper is concerned with the use of time- and frequency-domain codes for computing the interaction of electromagnetic waves with simple and complex structures. An example chosen for this study is a cubic box with the top open. The Finite Difference Time Domain (FDTD) method is used for computing time-domain responses to an electromagnetic pulse (EMP), a Gaussian pulse, and a sine wave. Frequency-domain results are obtained by using a moment method solution of the electric field integral equation (EFIE). Comparison is then made, both in the frequency and time domains, on corresponding quantities using Fourier transforms. Effects of various factors - the shape of the incident waveform, discretization of the structure, and Fast Fourier Transformation - on the CPU time and the accuracy of the solution are demonstrated. Guidelines are established for obtaining an accurate response.