Hi i am using Abaqus to implement Anisotropic yield (Hill's)for a 2D plane strain model .The potential parameters for hill's plasticity (R11,R22 ,R33,R12, R23 ,R13) are the inputs and when i assign values(R11,R22,R33,R12) the abaqus throws an error saying that other two values R13,R23 must also be assigned a positive value. so how to overcome this issue??