[QE-developers] Coulomb Cutoff 2D

[Paolo Giannozzi]

I am always happy when it is possible to remove some code! Paolo

On Thu, May 10, 2018 at 6:09 PM, Andreussi, Oliviero <
Oliviero.Andreussi at unt.edu> wrote:

> Dear Developers,
>
> I am working on the code to make the calculation of the Hartree potential
> a bit more clean and code independent (PW vs CP). It may not be very useful
> per se, but it may help to reduce the cyclic dependencies of Environ.
>
> Having said this, while looking at the current version of the code I
> realized that there is a new feature related to the electrostatic
> interactions and the pbc corrections, the Coulomb Cutoff 2D. I double
> checked the formulas, and it seems to me that they are the same of the
> Martyna-Tuckermann approach in 2D (Mináry, P.; Tuckerman, M. E.;
> Pihakari, K. a.; Martyna, G. J. A New Reciprocal Space Based Treatment of
> Long Range Interactions on Surfaces. *J. Chem. Phys.* *2002*, *116* (13),
> 5351.), when an appropriate smoothing is added to avoid the cusp at the
> boundary, similar to what is done in the 0D Martyna-Tuckermann
> implementation in PW (the additional factor exp(-tpiba2*gg(:)*beta/4._dp)**2
> in wg_corr )
>
> Apart from the stress subroutines, which are totally new and very useful,
> the other subroutines in the Coul_cut_2D.f90 module can be merged inside
> the martyna-tuckermann.f90 module with very little changes. As a matter of
> fact, I did it already a few years ago, but did not submit it because I
> struggled -and failed- to make the 1D case work. My impression is that one
> just needs to change the way the wg_corr(ig) coefficients are computed,
> which for the 2D case would be something like
>
>      axis = at(3,3) * alat
>      DO ig =1, ngm
>        q2 = gg(ig)*tpiba2
>        IF (q2>1.e-6_dp) THEN
>          gxy = SQRT(g(1,ig)**2 + g(2,ig)**2)
>          wg_corr(ig) = - COS( 0.5D0 * g(3,ig) * tpiba * axis ) * &
>                      & EXP( -0.5D0 * gxy * tpiba * axis ) * &
>                      & fpi / q2
>        ELSE
>          wg_corr(ig) = 0.D0
>        END IF
>     END DO
>     wg_corr(:) =  wg_corr(:) * exp(-tpiba2*gg(:)*beta/4._dp)**2
>     if (gamma_only) wg_corr(gstart:ngm) = 2.d0 * wg_corr(gstart:ngm)
>
> I think having the two correction schemes with a more uniform
> implementation would be desirable. In particular, I find the implementation
> of the MT approach to be very clean and compact, I would actually suggest
> to move it to Modules, the only dependencies that forbid this are in a
> subroutines to print out debug stuff, which could be easily commented out.
>
> If the developers of this new feature want to fix this by themselves, I
> would not mind at all. Otherwise, if there are strong opinions on why the
> Coulomb Cutoff 2D should be implemented in a totally different way from the
> MT approach, despite being both G-space-based corrections, please let me
> know. At least we should change the capitalization of the module’s name, it
> is really hArd tO looK At.
>
> Best,
>
> Oliviero Andreussi
> --
> Assistant Professor
> Department of Physics
> University of North Texas
> Email: oliviero.andreussi at unt.edu <oliviero.andreussi at unt.edu>
> Skype: olivieroandreussi
> Web: https://sites.google.com/site/olivieroandreussi
>
>
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>


-- 
Paolo Giannozzi, Dip. Scienze Matematiche Informatiche e Fisiche,
Univ. Udine, via delle Scienze 208, 33100 Udine, Italy
Phone +39-0432-558216, fax +39-0432-558222
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