Wigner rotations of 2nd rank spherical tensor

This function will rotate a 2nd rank spherical tensor between different coordinate systems.

This function will rotate a a 2nd rank spherical tensor using reduced Wigner rotations.

example:
Chemical shift anisotropy is often described by a 2nd rank spherical tensor in the Principal Axis System(PAS):

R(2,m)(PAS)=sqrt(3/2)*aniso*[-sqrt(6)*eta/6 0 1 0 -sqrt(6)*eta/6]

This tensor will be described in the laboratory frame (LAB) by applying some wigner rotations, e.g.

R(2,0)(LAB)=R(2,m)(PAS)*dEuler(a1,b1,g1)*dEuler(a2,b2,g2)

The above could represent the following rotations

PAS->(a1,b1,g1)->MOL->(a2,b2,3b)->LAB

going from PAS coordinate system to LAB coordinate system via a MOL coordinate system.

source: Multidimensional Solid-State NMR and Polymers. Schmidt-Rohr & spiess. Academic press 1994