Converts span and skew representation of a 3x3 interaction tensor into the corresponding matrix.
Isotropic part, span, and skew are often seen in the literature. It is not a good way of reporting an interaction tensor (the original matrix is best), but it is still being used.
iso - isotropic part of the interaction, defined as (xx+yy+zz)/3 in terms of eigenvaues sp - interaction span, defined as the difference between the largest and the smallest eigenvalue sk - interaction skew, defined as 3*(yy-iso)/sp where yy is the middle eigenvalue alp - alpha Euler angle in radians bet - beta Euler angle in radians gam - gamma Euler angle in radians
M - 3x3 matrix
The reverse transformation (from matrix into span, skew, and angles) is ill-defined.
Version 2.1, authors: Ilya Kuprov