[CP2K-user] Nuclear energy contributions in GAPW scheme

[pavan kumar behara]

Hello CP2K developers,

I am trying to separate the electronic and nuclear energy contributions in 
the GAPW scheme. I could do so successfully for a molecule with 
non-periodic calculation by passing nuclear-only density to the pw_poisson 
and hartree_1c energy calculation routines. By adding nuclear_pw, 
nuclear_1c with the self energy term I get exact same value as analytical 
value (q1*q2 / r ). I did this by passing rho0_s_gs, with only the core z 
part and ignoring the hard and soft coefficients, to pw_poisson_solve to 
get nuclear_pw. And, by considering only the rhoz terms in 
hartree_local_methods to get nuclear_1c. When it comes to periodic 
calculations doing the same results in negative nuclear energies for small 
cell sizes. If I do a very big box (of 100A) by placing the molecule at the 
center, I do see that this energy value is positive and it is converging 
towards the non-periodic value but still not so close. 

My question is whether I am not accounting for the ewald summation effects 
correctly? Or my assumption or way of separating the nuclear and electronic 
densities is not correct? Looking back to the GAPW paper by Lippert, et 
al., 1999, I feel I do not completely comprehend the compensation charge 
and rho0_multipoles in GAPW scheme. It seems a single hard exponent 
Alpha0_Hard is supposed to counteract the multipoles of [nA_hard - 
nA_smooth + nA_core] but again I see Qlm_tot contains the sum of 
contributions from Qlm_h, Qlm_s, Qlm_z. Any lead on understanding this part 
better is highly appreciated.

I also saw a very old discussion about parameter tuning (link 
<https://groups.google.com/d/msg/cp2k/RUFQScjSDn0/lWdtYskTm9IJ>) and 
there's a mention about periodic interactions not being correct for small 
cells and system type. I am testing the periodic case on a hematite 
supercell (3x3x2) structure and facing difficulties. Is it possible that 
the errors I am encountering are due to bad input parameters? 

I am sorry for such a long post. Thank you very much for your time.

Best regards,
Pavan.
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