Erroneous results have been reported when using SpinDynamica v3.6.x with Mathematica version 13.
This is very surprising behaviour since Mathematica should be backwards compatible.
This has been pinned down to a surprising inconsistency in the handling of SparseArray objects between Mathematica versions 12 and 13. A bug report has been submitted to wolfram.com.
SpinDynamica v3.7 includes a workaround for known symptoms,
All SpinDynamica users using Mathematica version 13 should upgrade to SpinDynamica v3.7 or later.
MHL