MCNP TR Transform Card Question

[MadGander]

TL;DR Summary

Question regarding the surface transform card in MCNP.

I'm trying to rotate an RPP 45 degrees around the y axis (BUT NOT THE ORIGIN Y AXIS, rather the y axis at x=a, z=b). Is there a way to do this in MCNP? I've tried every single possible combination of angles and inputs to no avail. Again, I have an RPP that is not centered at the origin and I want to rotate it around the y axis that travels through the middle of the RPP cell.

 

[Alex A]

A rotation around the origin followed by a translation is equivalent to a rotation around any arbitrary centre. Work out a rotation matrix for the 45 degree turn, then feed x=a, z=b through that. The difference between a -> a', b -> b' gets you the amount you need to subtract to put it back in it's place. The translation vector. The rotation matrix needs to be transposed and the translation numbers go in as they are. If you don't transpose the rotation will be in the wrong direction because of the way the command is defined (how to create the alt co-ordinate system from the global one).

The gritty details are here. Are you familiar with matrix transforms?