SIC in qs_ks_utils.F

[Xiaoming Wang]

Hello,

In the lines 613 and 614 of the subroutine qs_ks_utils.F, key operations of 
the SIC correction are shown:
CALL pw_poisson_solve(poisson_env, orb_rho_g%pw, ener, work_v_gspace%pw)  
energy%hartree = energy%hartree-dft_control%sic_scaling_a*ener 
The first line calculates the electrostatic energy of the unpaired electron 
(self-interaction) and the second line subtracts this energy from the total 
Hartree energy. For open boundary calculations, this is correct, since the 
electrostatic energy calculated this way is the self-interaction energy. 
However, for periodic boundary conditions, the electrostatic energy 
calculated by the first line is not the self-interaction energy. Instead, 
the interaction energy of the unpaired electron and its periodic images is 
also included!! (Please correct me if I am wrong.)
 So it seems that to do a correct SIC calculation for PBC systems, one 
needs to either add back the interaction energy of the unpaired electron 
and its periodic images using perhaps Ewald summation method (not sure) or 
correct the self-interaction term by changing a poisson solver. Is it 
possible to calculate the Hartree energy using non-periodic poisson solver, 
say multipole, for a periodic system?

Best,
Xiaoming


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