o simplify any subsequent rotations. 

You can elect to use the default alignment vector or face, instead of 
specifying it.  The default vector will be the intersection of the face (hkl) 
with the face (100), whose indices are given by the cross product (hkl)x(100).  
If the face (hkl) is (100), the default alignment vector will be the vector 
[001].  For aligning a vector the algebra is just the same, with interchange 
of face and vector. 

If dealing with a molecule or polymer, using a cartesian axial system, the 
face/vector alignment option can be used to obtain a view down a given vector 
direction.  Use the vector alignment option, although when the structure axes 
are carte