[QE-developers] Coulomb Cutoff 2D

Fri May 11 18:40:13 CEST 2018

Dear Paolo, 

Sorry for the obscure comment. It's because the 2D cutoff scheme doesn't work if the 2D cutoff distance is smaller than this rcut parameter. The rcut being the limit between short-range and long range parts of the pseudo, and the 2D cutoff acting only on the long range part, if the cutoff is shorter then we are not cutting off something that should be. 

The 2D cutoff distance is set to half the out-of-plane plane lattice parameter (c). So effectively this default rcut value sets a lower limit of 20 bohrs for c. For some strictly atom-thick materials like graphene, it can make sense to change it, but that should be done with care of course.

I will coordinate with Oliviero concerning a cleaning up Coulomb potential / PBC routines. 

Thibault Sohier
Post-doc
THEOS, EPFL
Lausanne, Switzerland 

> Le 11 mai 2018 à 11:00, developers-request at lists.quantum-espresso.org a écrit :
> 
> Send developers mailing list submissions to
>  developers at lists.quantum-espresso.org
> 
> To subscribe or unsubscribe via the World Wide Web, visit
>  https://lists.quantum-espresso.org/mailman/listinfo/developers
> or, via email, send a message with subject or body 'help' to
>  developers-request at lists.quantum-espresso.org
> 
> You can reach the person managing the list at
>  developers-owner at lists.quantum-espresso.org
> 
> When replying, please edit your Subject line so it is more specific
> than "Re: Contents of developers digest..."
> 
> 
> Today's Topics:
> 
> 1. Re: Coulomb Cutoff 2D (Paolo Giannozzi)
> 
> 
> ----------------------------------------------------------------------
> 
> Message: 1
> Date: Fri, 11 May 2018 09:47:20 +0200
> From: Paolo Giannozzi <p.giannozzi at gmail.com>
> To: General discussion list for Quantum ESPRESSO developers
>  <developers at lists.quantum-espresso.org>
> Subject: Re: [QE-developers] Coulomb Cutoff 2D
> Message-ID:
>  <CAPMgbCs5qfm=nczezWOJDCRXO+3Cvqr5GnRT62sfL-4pmFe=1A at mail.gmail.com>
> Content-Type: text/plain; charset="utf-8"
> 
> About 2D cutoff: just noticed this obscure comment in
> Modules/read_pseudo.f90
> 
> REAL(DP), parameter :: rcut = 10.d0
> -  !2D Coulomb cutoff: modify this (at your own risks) if problems with
> cutoff being smaller than pseudo rcut. original value=10.0
> 
> What does it mean?
> 
> 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 (Mina?ry, 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
>> 
>> 
>> _______________________________________________
>> developers mailing list
>> developers at lists.quantum-espresso.org
>> https://lists.quantum-espresso.org/mailman/listinfo/developers
> 
> 
> -- 
> 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
> -------------- next part --------------
> An HTML attachment was scrubbed...
> URL: <http://lists.quantum-espresso.org/pipermail/developers/attachments/20180511/ac2b20e7/attachment-0001.html>
> 
> ------------------------------
> 
> Subject: Digest Footer
> 
> _______________________________________________
> developers mailing list
> developers at lists.quantum-espresso.org
> https://lists.quantum-espresso.org/mailman/listinfo/developers
> 
> 
> ------------------------------
> 
> End of developers Digest, Vol 97, Issue 9
> *****************************************