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I am new VASP user (moving from Gaussian to VASP), and learning it pretty much by myself. I went through most of Hands on examples, and started my own calculations. I have number of questions, but this is the first one.

I'd like to optimize only spin for Fe crystal while fixing cell parameters. Does ISIF=2 do this job, and where I can see the results? Also if I want to fix spin and optimize geometry, is NUPDOWN the tag to fix it?

I appreciate if someone could help me out.

Thank you very much.

1) to optimize the spin for a fixed geometry, please simply set
ISPIN=2
ISIF=2; PREC=Accurate
the total magnetic moment of the cell is given in OSZICAR, please search for the 'mag= ' entry at the end of each ionic step.
as you moved from GAUSSIAN to VASP: please note that the local moments are not uniquely defined in all plane wave or mixed basis set codes, in contrast to codes based on local orbitals. This may be new to you, please have a look at the various forum postings concerning this issue.
2) fixing the spins is in principle done by setting NUPOWN, you are correct. however, for metallic systems (bands crossing the Fermi level) it may be difficult to achieve convergence, please choose your k-point set carefully in that case (NUPDOWN is best suited to fix spin multiplet states in atoms, molecules or clusters (using the Gamma point only).