If anyone is aware of any DFT codes with exact (i.e. not penalty energy) spin constraints, ideally ones that work with collinear spins (since it’s cheaper) that are available as ASE Calculators, I’d be very interested. From what I can tell VASP and pwscf implement spin constraints, but only with a quadratic penalty and only for noncollinear.
There appears to be at least one code, SPHInX https://sxrepo.mpie.de/, which does implement such constraints, so if anyone has thought about writing an ASE Calculator wrapping it, I’d be interested in hearing about that as well.