Results of the vertical gradient freeze (VGF) growth of Si-doped (3 ") and semi-insulating (4 ") GaAs crystals are shown. The VGF process conditions were optimised with the aid of numerical simulations using the so-called inverse modelling. Experimental results and predictions from the computer simulation (software CrysVUN + +) are quantitatively compared with respect to the power versus time profiles of the heaters, and qualitatively with respect to the shapes of the solid-liquid interface and the growth rate.