[lammps-users] Ewald sum with Debye screening effect

Dear Steve,

Could you please tell me if LAMMPS can take are of Ewald sum with Debye screening effect? There is one reference here:
http://scitation.aip.org/getpdf/servlet/GetPDFServlet?filetype=pdf&id=JCPSA6000113000023010459000001&idtype=cvips&prog=normal

All the best!

Dongsheng

I don't know. Please read the LAMMPS doc pages about Ewald and PPPM
and see if they do what you want.

Steve

Hi Steve,

I checked the manual online. It seems that “pair_style cg/cmm/coul/long” can do it, but I am not certain about it.

From the manual, I can see that cg/cmm/coul/cut can take care of Debye screen effect in cut-off method if the parameter for the Deby screening is given. Then the manual says " Style cg/cmm/coul/long computes the same Coulombic interactions as style cg/cmm/coul/cut except that an additional damping factor is applied to the Coulombic term so it can be used in conjunction with the kspace_style command and its ewald or pppm option." I assume the damping factor means the Debye screening effect. However, when I checked the reference paper in Shinoda, DeVane, Klein, Mol Sim, 33, 27 (2007). I can’t find any relevent information redarding to the Deby screening effect.

Could you please confirm that “pair_style cg/cmm/coul/long” can use Ewald sum with the Debye screening potential? Thanks!

Dongsheng

Style cg/cmm/coul/cut adds a Coulombic pairwise interaction given by

where C is an energy-conversion constant, Qi and Qj are the charges on the 2 atoms, and epsilon is the dielectric constant which can be set by the dielectric command. If one cutoff is specified in the pair_style command, it is used for both the LJ and Coulombic terms. If two cutoffs are specified, they are used as cutoffs for the LJ and Coulombic terms respectively.

This style also contains an additional exp() damping factor to the Coulombic term, given by

where kappa is the Debye length (kappa=0.0 is the unscreened coulomb). This potential is another way to mimic the screening effect of a polar solvent.

Style cg/cmm/coul/long computes the same Coulombic interactions as style cg/cmm/coul/cut except that an additional damping factor is applied to the Coulombic term so it can be used in conjunction with the kspace_style command and its ewald or pppm option. The Coulombic cutoff specified for this style means that pairwise interactions within this distance are computed directly; interactions outside that distance are computed in reciprocal space.

hi dongsheng,

no. you are wrong. the wording in the manual is not clear
and that is my fault. i never ever expected that somebody
would even _want_ to do ewald with debye screening.

what you are looking for is currently not in LAMMPS, because
would you would need a matching kspace module for the debye
screened charges. this does not exist.

the explicit debye screening is going to vanish in the next
iteration of the cg/cmm package anyways (you can achieve the
same using pair hybrid, and the cg/cmm package needs a serious
cleaning up). it was included in the hope that we could avoid
having to use "proper" treatment of long-range charges, but
for inhomogeneous systems (e.g. lipid bilayers) there
"ain't no escape from the blues" (i.e. ewald/pppm). :frowning:

you will have to implement the algorithm in the paper, you
were referring to, yourself (or find/hire/bribe/force/persuade/trick
somebody else to do it for you) as a new kspace style, perhaps
as a modification of kspace/ewald?.

cheers,
   axel.

Hi there,

Does anybody know the interatomic potential dealing with the interface of CNT and Cu? I’m not aware of any available potentials can do this in the potential dir provided by LAMMPS. Basically, I’m interested in the adhesion of CNT and Cu interface and their thermal properties. Any comments or help are highly appreciated.

Thanks!

Best,

AC

Hi there,

Does anybody know the interatomic potential dealing with the
interface of CNT and Cu? I'm not aware of any available potentials
can do this in the potential dir provided by LAMMPS. Basically, I'm
interested in the adhesion of CNT and Cu interface and their thermal
properties. Any comments or help are highly appreciated.

hi ac,

not sure whether this is helpful, but a friend of mine has
been involved in simulations of tantalum atoms on top of
fullerenes. but they had to use car-parrinello MD for it
(i.e. DFT instead of explicit potentials). as far as i
remember, the electron density distribution played an important
role in the binding of the tantalum atoms/clusters and
their thermal stability.

i can dig out the reference, if needed. a web search on
tantalum and fullerenes will probably do, too.

cheers,
   axel.

Axel,

Sounds like that to do what I want require ab initio MD simulations and LAMMPS cannot be helpful anymore. That is beyond the scope of my research. Any other solutions?

Best,

AC

There is a pair_style lj/cut/coul/debye, that adds a exp() term
to the short-range forces, but it is not meant to be used with long-range
Ewald or PPPM solvers.

Steve

Dear DongSheng, Axel, Steve,

How about using the pair style Yukawa style described in
http://lammps.sandia.gov/doc/pair_yukawa.html
The form of the potential is:
\Phi = A * exp^{-\Kappa r} * 1/r
The exp^{-\Kappa r} where one takes \Kappa = 1/(Debye Screening
Length), would resemble a Debye screening and the 1/r term would be a
regular Culomb fall-off
(such a potential is used to simulate dusty plasma where the charged
dust is screened by a plasma).

I do not see any comments in the manual about Ewald summation being
restricted with such a pair-style.

I do wonder if I am missing any point!

Best Regards,
Manoj

2009/2/19 dongsheng zhang <[email protected]>:

Hi Axel,

I am glad that you can join this discussion. At the first thought, I don’t expect that somebody
would even want to do ewald with debye screening because of the damping factor from the Debye screening effect. I thought the cut-off method should be enough. However, a recent paper [1] reported that the cut-off method can introduce 10% difference of the electrosatic interaction potential comparing with the Ewald method [2]. From the references below, we can see that people had used the Ewald sum with the Debye screening effect a while ago.

  1. http://www.rsc.org/delivery/_ArticleLinking/ArticleLinking.cfm?JournalCode=SM&Year=2009&ManuscriptID=b818169a&Iss=Advance_Article

  2. http://scitation.aip.org/getpdf/servlet/GetPDFServlet?filetype=pdf&id=JCPSA6000113000023010459000001&idtype=cvips&prog=normal

  3. http://prola.aps.org/pdf/PRE/v71/i3/e031108

Hi Axel,

I am glad that you can join this discussion. At the first thought, I don't
expect that somebody
would even _want_ to do ewald with debye screening because of the damping
factor from the Debye screening effect. I thought the cut-off method should
be enough. However, a recent paper [1] reported that the cut-off method can

that depends, of course, on the amount of screening.

introduce 10% difference of the electrosatic interaction potential comparing
with the Ewald method [2]. From the references below, we can see that people
had used the Ewald sum with the Debye screening effect a while ago.

i don't have the time to look this all up, but i would say that:

a) for small debye screening - be it applied to coulomb directly, or
   as part of a yukawa potential - there is still significant coulomb
   contribution left at the cutoff, thus you need some way to adjust
   for it. either by using a larger cutoff, or by using an ewald sum
   type approach (or reaction field??).

b) you cannot use the regular ewald sum for this, as it will
   overestimate the long range contribution from charges, since
   it does not consider the screening.

technically, you are free to ignore either a) or b), and i would not
be surprised if there are people who have done it. but if you do so,
you will only get out of your simulation what you put in.

cheers,
    axel.

You can't use long-range Ewald or PPPM with pair yukawa.

Steve