The authors performed simulations of Penning discharge at the low magnetic field using a particle-in-cell computer code to derive the optimum cell geometry of a sputter ion pump (SIP). The energy and angle distributions of incident ions on the cathode of a Penning cell were directly obtained from the simulation. Based on these data, the authors calculated the amount of deposited Ti on the inner surface of the cell, which was found to be proportional to intrinsic pumping speed of a SIP, and then estimated the effective pumping speed for various cell geometries. In this calculation, the incident ions on the outer region of the cathode as well as those at the center were analyzed, and the effect of the ionizationless region on Penning discharge was also taken into account. The calculated values agreed well with the existing experimental data, especially in the region of a small cell radius. The results also show that the cell radius should be at least 5 mm for effective pumping, and the optimum geometrical parameters for a conventional sputter ion pump have the radius of 8.5 mm and the gap of 7 mm at the applied voltage of 5600 V.