[CP2K-user] Proper use of ri_hfx.F module

[Nicholas Winner]

Hi devs,

I've been looking at the resolution of identy hfx module. I know that the 
forces are not implemented, so its not fully developed, but it promises to 
speed up HFX calculations so I'm interested in checking it out. 

My issue is that the performance is not very good, and I'm unable to find 
documentation on it because its experimental. What I have done is taken a 
normal ADMM input file and added the &RI section to the &HF section, and 
tried the two different RI flavors, MO and RHO, with no other 
modifications. The calculation runs fine, but the first step is much longer 
than a standard HF calculation, and what's more the subsequent SCF steps, 
which are normally very quick due to integral storage, are even longer than 
a normal HFX calc.

I'm wondering if this is due to the basis? I don't see any documentation 
for how to add an  RI basis for HFX, so I simply have my primary basis 
(DZVP) and then my BASIS_SET AUX_FIT cpFIT3 for the ADMM part. Is there a 
way to add a special RI AUX basis that would improve the speed? 

Any pointers are appreciated.

-Nick
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