I just wanted to offer a few additional comment about the second point, how to
a) know that there is a problem, and
b) how to solve
It is strongly recommended to always start out small, when you approach a new material. If you use house-hold elements like carbon this is less necessary, but even for gold, platinum etc, and especially spin-dependent elements like manganese and more rarely used transition elements, one should make a habit of first investigating the basic properties of the material. That means, look at simple dimers, the bulk band structure, how does the energy scale with the lattice constants, etc.
That is - precisely what you have done (I assume the Mn-Bi molecule dimer is not your real project?
)
This way you learn which mesh cut-off that is required, if LDA is sufficient or GGA is needed, how many k-points, which basis set to use, etc, etc.
So, what to do if problems are encountered? Well, I must admit that tuning the basis set parameters is not the first thing you would try. It is, however, the place you probably end up if all other things fail
In this particular case it also helped to plot the Mn basis functions (not a trivial exercise).
And then, once you do end up having to tweak the basis set, the only thing that typically helps is to tune the charge (slightly!); the basis set is not very sensitive to the other parameters, but it does respond nicely to the charge setting.
Now we have informed the community about that!
