Focused ion beam columns with liquid metal ion sources should be optimized to give the highest possible current in a probe of given dimension. A procedure for this optimization is presented, which includes the influence of Coulomb interactions between the particles in the beam. The column is first optimized without including the Coulomb effects and it is found that the optimized magnification and aperture size are independent of the gun brightness. The magnification approaches a constant value in the high current domain, where the gun lens aberrations seriously influence the probe size. The Coulomb effects can be characterized by pointing out a domain in the probe current-probe size plane which is inaccessible, no matter how high the brightness of the gun may be. The approximate position of this inaccessible area can be found with a relatively small number of calculations. It is then demonstrated how the beam current as a new function of probe size can be reoptimized by the choice of magnification and aperture size. Coulomb effects turn out to be dominating the probe size except for very small probes and for very large probes.