Monoclinic Cell Rotation for Surface Calculations

[SK2]

Dear All,
I have a monoclinic cell with
a=18.7,b=3.55,c=9.069

and beta=134 degrees, angle between a and c. Now I need to make the c axis parallel to z axis for surface calculations, ie to introduce a vacuum. What is the rotation matrix that would do this??
Thank you.

 

[M Quack]

Depends on what your initial coordinate system is...