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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: "Ahmed E. Ismail" <aeismail@...24...>
Date: Wed, 22 Nov 2017 12:29:13 +0000

Hello, Hai:

TIP4P water is normally specified like TIP3P water, with the M charge “assigned” to the O atom. The charge is moved only if a tip4p “aware” style is used. Otherwise, the force will be calculated, but that calculation will assume the charge is on the oxygen, not the displaced location assumed by tip4p.

—AEI
On Wed 22. Nov 2017 at 06:00, 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?

Thanks,
Hai


On Tue, Nov 21, 2017 at 11:41 PM, Axel Kohlmeyer <akohlmey@...36.....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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