(X+d1*x/a, Y+d1*y/b, Z+d1*z/c)
B12X 3 x y z 10.0000 10.0000
i.e. having fixed zero occupancy and temperature factors. Put in HFIX as for closo structures but not including the dummy atom of course!
Links to the other X-Ray Crystallography web pages are also listed here for your convenience:-