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Re: [lammps-users] TIP4P interaction with other atoms (buckingham)
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Re: [lammps-users] TIP4P interaction with other atoms (buckingham)


From: Axel Kohlmeyer <akohlmey@...24...>
Date: Wed, 22 Nov 2017 14:28:13 -0500



On Wed, Nov 22, 2017 at 1:57 PM, jamal kazazi <jamalkazazi@...36.....1420...> wrote:
Dear all,
 
Thank you for the comments.
 
Is there anything wrong with defining the coulombic interaction separately like this:
 
kspace_style pppm/tip4p 1e-5
 
pair_style hybrid buck 8.5 tip4p/long 1 2 1 1 01546 8.5 lj/cut 8.5
 
pair_coeff  * *  tip4p/long
pair_coeff  1 3  buck       2.78e4   0.297         #TIP4P oxygen and type 3
pair_coeff  1 1  lj/cut    0.18521  3.1589       #TIP4P oxygen - TIP4P oxygen
 
It seems to me that this way, we define ALL the coulombic interactions with a tip4p aware style. Please let me know if this is a good way to handle the issue.

​no, it isn't. if anythin, you want hybrid/overlay, not plain hybrid. with hybrid​ the following pair_coeff commands will *override* previous settings, while hybrid/overlay will accumulate them.
also, pair_coeff 3 3 seems to be missing and what about 2 3?

axel.

 
 
Best,
Jamal
Sent: Wednesday, November 22, 2017 at 5:39 AM
From: "Hai Hoang" <hhoang052@...24...>
To: "Axel Kohlmeyer" <akohlmey@...24...>
Cc: "jamal kazazi" <jamalkazazi@...1420...>, "LAMMPS Users Mailing List" <lammps-users@...396...sourceforge.net>
Subject: Re: [lammps-users] TIP4P interaction with other atoms (buckingham)
Dear Dr. Axel and Ahmed,
 
Thank you so much for your explanations. It is clear to me, now!
 
Thanks,
Hai
 
On Wed, Nov 22, 2017 at 1:29 PM, Axel Kohlmeyer <akohlmey@...36.....24...> wrote:
 
 
On Wed, Nov 22, 2017 at 5:59 AM, Hai Hoang <hhoang052@...24...> wrote:
Dear Dr. Axel,
 
You have said that:
 
"​that depends on what your model is. with your setup, 1 3 interactions using the buck/coul/long ​potential, will "see" only the oxygen and hydrogen atoms and the charge on the M point will not be seen, but rather assumed to be on the location of the oxygen atoms, since this is a coul/long potential and not a tip4p/long potential."
 
So you mean that if one is using 
 
"pair_coeff  1 3  buck/coul/long       2.78e4   0.297         #TIP4P oxygen and type 3"
 
Lammps will assume that the charge on the M point moves to the O point, or it will not neglect the force due to this charge?
 
​this is not an "or" condition. it *will* consider the force due to the assigned charge, but it will consider it at the O location, because that is the only position that is known.​
​for coul/long styles there is no M point. the whole reason for tip4p/* styles is that in this case the 4-th position, i.e. ​the M point, can be inferred from the positions of the two hydrogens and the oxygen, yet this is only done *inside* the style. this significantly reduces the number of entries in the neighbor list and allows to use fix shake (instead of having to use fix rigid), which can work with a larger time step.
 
axel.
 
 
Thanks,
Hai
 
 
On Tue, Nov 21, 2017 at 11:41 PM, Axel Kohlmeyer <akohlmey@...24...> wrote:
 
 
On Tue, Nov 21, 2017 at 4:39 PM, jamal kazazi <jamalkazazi@...1420...> wrote:
Dear all,
 
I need to define buckingham interaction between the oxygen in TIP4P and an atom type 3. 
 
I define:
 
kspace_style pppm/tip4p 1e-5
 
pair_style hybrid buck/coul/long 8.5 lj/cut/tip4p/long 1 2 1 1 01546 8.5
 
pair_coeff  1 3  buck/coul/long       2.78e4   0.297         #TIP4P oxygen and type 3
pair_coeff  1 1  lj/cut/tip4p/long    0.18521  3.1589       #TIP4P oxygen - TIP4P oxygen
 
My question is: does specifying buck/coul/long take care of the shifted charge center for oxygen completely (both short and long range)?
But if that's the case, then using lj/cut/coul/long instead of lj/cut/tip4p/long for 1-1 interaction should make no difference.
 
it seems like it's okey to do this.
 
​that depends on what your model is. with your setup, 1 3 interactions using the buck/coul/long ​potential, will "see" only the oxygen and hydrogen atoms and the charge on the M point will not be seen, but rather assumed to be on the location of the oxygen atoms, since this is a coul/long potential and not a tip4p/long potential.
 
 
 
 
if you don't want this, you have to describe your TIP4P model as explicit 4-site potential, time integrate it with fix rigid (since you cannot apply SHAKE to that geometry) and then replace all tip4p variants (pair and kspace) with the the regular ones.
 
axel.
 
 
Thank you in advance
 
Jamal

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--
Dr. Axel Kohlmeyer  akohlmey@...24...  http://goo.gl/1wk0
College of Science & Technology, Temple University, Philadelphia PA, USA
International Centre for Theoretical Physics, Trieste. Italy.

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--
Dr. Axel Kohlmeyer  akohlmey@...24...  http://goo.gl/1wk0
College of Science & Technology, Temple University, Philadelphia PA, USA
International Centre for Theoretical Physics, Trieste. Italy.



--
Dr. Axel Kohlmeyer  akohlmey@...92......  http://goo.gl/1wk0
College of Science & Technology, Temple University, Philadelphia PA, USA
International Centre for Theoretical Physics, Trieste. Italy.