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9:17:17 AM PST  Wed, Mar 5th 2014 

Dear All,
I would like to output both exchange(correlation) energy density and potential at each grid point.
I guess those quantities are definitely evaluated during the calculation for any DFT.
I looked at /PATH_TO_NWCHEM/src/nwxc/nwxc_x_pbe.F for PBE exchange.
I understand the lines for evaluating the exchange energy.
However, I did not see the codes for evaluating exchange potential.
To calculate GGA exchange potential, two quantities, the laplacian of density and $\nabla\rho \cdot \nabla \nabla \rho$, are required based on PRB, 86, 8800 (1986).
I am wondering could anyone with the experience of implementing density functionals in NWChem give me some advice?
Thanks a lot!




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3:37:04 PM PST  Thu, Mar 6th 2014 

Dear Shipenn,
Indeed originally people would calculate the laplacian of the density and evaluate the potential generated by a density functional. The paper you cite by Perdew and Wang (http://dx.doi.org/10.1103/PhysRevB.33.8800) uses this approach. However, the need to evaluate the second derivative of the density led to increased costs in evaluating functionals and was considered cumbersome. In 1992 it was shown that using an integrationbyparts the terms including the second derivatives of the density can be eliminated (provided you want to evaluate matrix elements in a finite basis, if I remember correctly). This is documented in: John A. Pople, Peter M.W. Gill, Benny G. Johnson, "Kohnâ€”Sham densityfunctional theory within a finite basis set", Chemical Physics Letters, Volume 199, (1992), Pages 557560, http://dx.doi.org/10.1016/00092614(92)85009Y. So in practice we never evaluate the actual potential we just calculate the required matrix elements. Now, for some functionals we can actually calculate the first and second order partial derivatives of the functional withrespectto its arguments (i.e. the density and the norm of the gradient of the density). I guess it should be possible to express the potential in those terms but I have never tried.



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