Solving a Model with FDTD To solve a model with the finite difference time domain (FDTD), you must activate the solver for the model. On the Solve/Run tab, in the Solution Settings group, click the Solver Settings icon. On the Solver Settings dialog, click the FDTD tab. Figure 1. The Solver Settings dialog (FDTD tab). Under Time domain solver, select the Activate the finite difference time domain (FDTD) solver check box. Click the OK to close the dialog.