Should the presence of a d block element in the unit cell mandate the need for performing spin-polarized calculations? I wanted to find out the DOS, PDOS, and the band structure. I'm new to DFT and I'm using the Quantum ESPRESSO package (where spin polarized calculations correspond to nspin = 2. It would be nice if someone could guide me through doing it.
Thanks in advance.
I agree with Tristan that the fool proof method is to do a spin-polarized self consistent calculation and look at the final magnetic moment. If the final magnetic moment is non-zero you should include spin polarization in your calculations.
However, from a practical point of view you can have a look at the existing databases. In particular if you are working on bulk materials, you can look at the materials project database. For example according to the database $\ce{BaTiO_3}$ is non magentic and $\ce{BaFeO_3}$ is magnetic. This will give you an idea prior to doing calculations and might be useful if you are screening for materials.
PS: As suggested by Andrew, you should not rely on DFT databases too much with regards to magnetic behavior. They almost all use a high-spin magnetic initialization, which is entirely arbitrary. It is possible that the magnetic ground state is something entirely different (potentially even a state with no magnetic moments at all).
Unfortunately, without performing spin polarized calculations you cannot know if it will matter. Further, you must perform different magnetic configurations (AFM, FM, PM) to know which is stable.
If you can experimentally show if its magnetic or not this will help. Certain elements tend to be magnetic as well. If you have Ni, Co, or Fe then you almost definitely need spin polarization.
In short, you cannot know without checking. You can save time by starting from non-spin polarized calculations though.
