DFT+U does not work with non-collinear spin (not spin-orbit)
After the recent work by @jjunquer , DFT+U is compatible with spin-orbit-coupling, but apparently cannot be used in a simpler non-collinear spin calculation. It is set explicitly in the code:
if ( spin%NCol ) then
call die('DFT+U cannot be used with non-collinear spin.')
else if ( spin%SO ) then
....
Why is that?
(Issue reported by Andrés Téllez)