[CP2K-user] [CP2K:16469] Energy explosion when atom count increases with HFX. Issue with auxiliary basis sets?

[Nicholas Winner]

I've been looking at a couple of systems that are atomically pretty dense. 
For example, alpha boron and diamond.

For these systems, when I increase the supercell size too large, the energy 
in the SCF loop explodes to ridiculous values. For example, I am 
calculating diamond with 54 atom and 128 atom supercells, truncated PBE0 
with 2A cutoff. 

The 54 atom case converges with no real issues. In that case I have used 
OT, and against common wisdom for really accurate results I screen on the 
initial density matrix with scf_guess atomic. The convergence seems robust 
to that and the aggressive (1e-6) integral screening.

Then, I move to 128 atom supercells, and I am unable to converge as the 
energies diverge rapidly after a few SCF steps. For this case, I have 
stopped screening on the initial density matrix. I was able to narrow the 
issue down to the auxiliary basis sets used for C. I tried cFIT3, pFIT3, 
cpFIT3, and FIT3, which all result in the rapid divergence of the energy. 
Example:


     1 OT CG       0.15E+00  141.5     0.01764397      -673.6622732538 
-6.74E+02

     2 OT LS       0.60E+00   35.1                     -806.4853195301

     3 OT CG       0.60E+00   38.5    12.03779293     -4186.8334161380 
-3.51E+03

     4 OT LS       0.30E+00   33.7                    -8657.0632214970

Since these all failed, but the primary basis is prohibitively large, I 
tried setting BASIS_SET AUX_FIT SZV-MOLOPT-SR-GTH, and this remedies the 
problem, and the calculation finishes.

My questions then are four-fold.
(1) Is this behavior for dense systems like diamond expected? I have not 
had this issue with any other system besides alpha boron.
(2) Is this un-fixable? i.e. we can't use the FIT basis sets for such 
systems?
(3) Would someone have another idea for why this is the case? i.e. maybe 
FIT basis sets can still be used if some other setting was changed. 
and finally (4) Does anyone see an issue with proceeding with using the SZV 
basis as the auxiliary basis?

-Nick

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