The raytracing settings for rendering can be customized under <key>Preferences –> Renderer –> Twilight</key>. The ray processor can be selected from the “Ray processor” drop-down menu. When changing parameter values in the preferences, you always have to leave the field you changed with your cursor before closing the preferences window (just click in a different field) for your changes to take effect. You can find an overview of the different raytracing algorithms to choose from besides their relevant settings in the algorithm section.