Perturbation analysis with MPB

Dear Steve,

 In your 2002 paper on the perturbation theory with shifting material
boundaries, you formulate the problem in terms of parallel and normal
fields. I am hoping that I can follow that procedure to do some perturbation
analysis based on MPB result. Then the question is how I can determine the
normal vector as well as epsilon_parallel and epsilon_normal for each grid
point. The most direct way is of course, to use the same
spherical-quadrature method as implemented in MPB. But I am wondering if
there is some simpler way for me to extract such information. I have tried
to do the svd decompostion on the output epsilon tensor at each grid point,
but the problem is that I don't know how to automatically decide which
diagonal element corresponds to the parallel epsilon and which corresponds
to the normal epsilon. Do you have any suggestions on this?

Thanks a lot.

Regards,
Minghui Han