Cutoff issue buckingham coulombic potential in LAMMPS

[sogol]

Hello everyone,
I have a question about cutoff in potentials when we apply
pair_style hybrid/overlay buck/coul/long/cs 10 12 zbl 0.5 2
the first one 10 is for buckingham and the next one 12 is for coulombic ?
so what should be defined- in kspace-modify and kspace-ewald ?
Thanks

[akohlmey]

All you are asking for is described in the LAMMPS documentation. Feel free to have a closer look.

[sogol]

Thanks for your reply. You are right, but I found something that made me unsure of what is exactly right. I want to attach something close to my question, in which you replied both cutoffs are for Buckingham potential and when I checked with LAMMPS it is mentioned the first one is for Buckingham and the next one is for Coulombic.

[akohlmey]

@sogol

I want to attach something close to my question, in which you replied both cutoffs are for Buckingham potential

I don’t understand what you are trying to say here and since you are not providing a reference or quote, I cannot comment since I write a lot of messages and don’t have eidetic memory.

[sogol]

Kspace ewald (coul/long potential) generates strange forces

You are only turning off the Lennard-Jones interactions. You still have Coulomb interactions and atoms with charges and thus there should be forces due to the charges from the Kspace style. That one always computes interactions between all atoms in the system. It is not a pair-wise computation, so how you assign pair styles in the hybrid pair style is irrelevant. This can be easily confirmed by changing your thermo_style command to: thermo_style custom step etotal epair evdwl ecoul elong W…

Here is the link I am talking about.

[akohlmey]

@sogol

Here is the link I am talking about.

You are misreading it. What I stated in that message is that coulomb is not completely turned off by setting its cutoff to zero, since this is using long-range coulomb treatment, where the Coulomb interactions are split between real space and reciprocal space. Thus you will still get Coulomb forces from the kspace style in that setup. By no means that says that the second cutoff is not related to Coulomb.

At any rate, if in doubt it is a bad idea to ask as generic questions as you have, but rather explain in detail why you are confused.

Beyond that, any explanation in the manual has a larger chance to be correct, especially for features that have been around for several years, since many people have looked at it, and if there was an error, someone would likely have noticed it. Forum posts and mailing list emails are not reviewed and typos or misunderstandings are much more likely.

<head_scratch>
Strangely enough, I am observing this for many years now, that some obscure message in an online forum or some statement in a YouTube video carries more weight and credibility than the manual authored by the people that wrote the software… I am curious if somebody could explain what are the possible reasons behind that phenomenon.
</head_scratch>

[bvirtual]

I reply to: …carries more weight … possible reasons …

I found it was people read the FIRST web page about the topic, and believe it. 100%. They do not check the author date, if recent or 10 years old. They do not seek second source of the same info.

When I search I open 3-10 match results tabs, and read them all. But then, I speed read.

So, the 2nd reason I suspect is they have not taken Speed Reading classes. I took 3, and doubled my speed each time, in just 8 weeks, each time. I must take another speed reading class.

And the 3rd reason, well, I do not like this one, but … they are lazy. And rather would post and not pay you for your writing time. I find this happens in person as well. I’ve learned to rapidly identify both situations, by their wording.

I have really enjoyed LAMMPS on MatSci due to two reasons:

1. Original Posters cover what they have already tried in detail, including the input file.

2. The replies are so very expert, concise, lucid, and easily understood … but only after I became seeped in the jargon that is special to LAMMPS, and the wide range of overloaded jargon, where other software uses it differently.

I like LAMMPS, having learned it 8 weeks ago, and have made many interesting simulations for my Fusion Motor plasma neutrals, ion and free electron motions (to explain to others with complex movies). I even implemented my own uniform magnetic field Lorentz Force, before finding the extension for bfield. I am about to integrate together many LAMMPS config files, to see if it will make a single movie. And overlay the movie onto a 3D model of the chamber and electrodes.