Dear Eric, I do not want define the weight of model as load case because but I need the deformation only cause by external loads.
One solution is define the weight in first load case and external loads in second and by subtracting I will have the deformation only cause by external loads. Another method usual method in geomechanic is defining the stress as the initial condition. As you know in material like soil (because of the internal friction) the strength of material (for example Drucker-Prager material model) is related to normal stress and we have to consider the effect of weight on stresses during analysing