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FEBio often struggles with problems that have an initial separation. This is especially so when the problem is force-driven, meaning that the deformation of the model is driven by a force (opposed to displacement-driven problems). In general, when the contact interface is necessary to define a well-constrained problem it is best to avoid initial separation if possible. Another way around this issue is to first use a displacement to bring the surfaces in contact and then replace the displacement with a force.