The single-crystal diffractometer REciprocal Space Investigator (RESI) will be installed on the thermal beamline SR8 at the new neutron source FRM-II in Garching. The main features of the instrument will be high resolution at high Q-values and low background. The latter will be provided by the use of a neutron guide. For the optimisation of all optical components with regard to intensity, divergencies, peak profiles and wavelength resolution, comprehensive Monte Carlo simulations were carried out. The results of the simulation concerning the neutron guide will be presented.