When a large number of basis functions is used for dense bulk systems with fcc, hcp, and bcc like structures, the basis set tends to be overcomplete. In such a case, you may observe erratic eigenvalues. To avoid the overcompleteness, a small number of optimized basis functions should be used. Another way to avoid the problem is to switch off the keyword 'scf.ProExpn.VNA' as
scf.ProExpn.VNA off # on|off, default = onIn this case, you may need to increase the cutoff energy for the numerical grid in real space by the keyword 'scf.energycutoff'.
For large-scale systems with a complex (non-collinear) magnetic structure, a metallic electric structure, or the mixture, it is quite difficult to get the SCF convergence. In such a case, one has to mix the charge density very slowly, indicating that the number of SCF steps to get the convergence becomes large unfortunately.
For weak interacting systems such as molecular systems, it is not easy to obtain a completely optimized structure, leading that the large number of iteration steps is required. Although the default value of criterion for geometrical optimization is Hartree/Bohr for the largest force, it would be a compromise to increase the criterion from to in such a case.