DFT+U dE[U]/dP(i,j)

[Marco]

Hello,

Sorry about this simple question but I was hoping someone could explain to 
me how the potential is calculated in the DFT+U Mulliken method.
In lines 1379-1383 of the dft_plus_u.f90 file, the potential dE[U]/dP(i,j) 
is defined as:

IF (isgf == jsgf) THEN
  v_block(isgf,isgf) = u_minus_j*(0.5_dp - fspin*q_matrix(i,i))
ELSE
  v_block(isgf,jsgf) = -u_minus_j*fspin*q_matrix(j,i)
END IF

Why is the v_block(isgf,jsgf) not defined as:
v_block(isgf,jsgf) = u_minus_j*(0.5_dp - fspin*q_matrix(j,i))

Best regards,
Marco
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