How to use collinear functionals for non-collinear spins

We point out a flaw in the current use of collinear functionals for non-collinear spins, imposing a wrong constraint, local spin rotation invariance, instead of the correct constraint, global spin rotation invariance. This problematic treatment directly leads to serious problems, the incorrect collinear limit, vanishing local torque and poor numerical stability. In this work, the correct constraint is imposed, leading to a simple solution with all above-mentioned problems gone automatically.