You can define stiffness along any vector by adding the vector in parentheses following the X, Y, or Z "placeholder" when defining the restraint "Type".
For example, X(1,0,1) or Y(1,0,1) or Z(1,0,1) would define stiffness along a 45 degree line in the horizontal plane. (The "Type" of X(1,0,-1) is perpendicular to this example.)
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Dave Diehl