BKS potential of a-silica

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1

Dear LAMMPS users,

When using the BKS potential in the attached 1996 PRB to simulate glassy silica, is it right that I should choose pair style ‘buck/coul/long’ and k_space style ‘ewald’ with parameters in the PRB?

But if using pair style ‘buck/coul/long’ , how to add a harmonic potential at short distance as suggested in the PRB? There seems no any parameters about the harmonic potential in the PRB. Could anyone please give me some advice?

Thanks in advance!
Zhao

1996PRB_moifiedBKS_Cooling-rate effects in amorphous silica- A computer-simulation study.pdf (631 KB)

2

please search the mailing list archives. there were tabulated
potential files posted here for this potential multiple times.

axel.

3

Thanks, Axel, very much for your advice.

I have the BKS table and used it. But I got some weird result about shear modulus. It seems the table is for original BKS potential, which was published in the 1990 PRL, instead of the modified BKS in the 1996 PRB. http://lammps.sandia.gov/threads/msg32211.html

So is it possible to use pair style ‘buck/coul/long’ and then add a harmonic potential as suggested in the 1996 PRB to use the modified BKS in LAMMPS?

Thanks,

Zhao

4

Thanks, Axel, very much for your advice.

I have the BKS table and used it. But I got some weird result about shear
modulus. It seems the table is for original BKS potential, which was
published in the 1990 PRL, instead of the modified BKS in the 1996 PRB.
http://lammps.sandia.gov/threads/msg32211.html

So is it possible to use pair style 'buck/coul/long' and then add a harmonic
potential as suggested in the 1996 PRB to use the modified BKS in LAMMPS?

then you have to build new tables.

axel.

5

there is no way to use pair style ‘buck/coul/long’ with an additional harmonic potential?

6

there is no way to use pair style 'buck/coul/long' with an additional
harmonic potential?

what LAMMPS can do is described in the manual.
what is not described in the manual it cannot do unless you program
(and document) it.

axel.

7

I see. Thanks very much.
Zhao

8

What is a “harmonic potential” in the context of pairwise

interactions?

Steve

9

I’m not sure for it either. They just said in the PRB that

In order to prevent, in the rare cases in which the particles cross the barrier, the particles from fusing together, we have substituted the potential given by Eq. ~1! by a har- monic potential when rij is smaller than the location of the barrier, i.e., for rij<1.1936 Å and rij<1.439 Å in the case of the Si-O and O-O interactions. Note that for intermediate and low temperatures this modification does not affect the potential given by Eq. ~1! and that in this limit we are thus working with the usual BKS potential.

And no other information about the harmonic potential.

Thanks,

Zhao

10

I'm not sure for it either. They just said in the PRB that

In order to prevent, in the rare cases in which the particles cross the
barrier, the particles from fusing together, we have substituted the
potential given by Eq. ~1! by a har- monic potential when rij is smaller
than the location of the barrier, i.e., for rij<1.1936 Å and rij<1.439 Å
in the case of the Si-O and O-O interactions. Note that for intermediate and
low temperatures this modification does not affect the potential given by
Eq. ~1! and that in this limit we are thus working with the usual BKS
potential.

And no other information about the harmonic potential.

since the buckingham potential becomes attractive, if you make it
across the barrier, it means, that they just add a soft repulsion.
i think you can choose it any way you like, as it is not supposed to
be hit frequently, but it is required to avoid particles collapsing on
top of each other and causing a "coulomb catastrophe".

at any rate, using tabulation (or pair style python for testing) is
the way to move forward on this.

axel.

11

Previously, I directly used the buckingham potential, but it always lost atoms when temperature is above 3000 K.

Then I used the attached table and there was no lost of atoms even I heated sample to 7000 K. So I think in the table potential file, some repulsion was added at short distance . But there are something weird in my result:

I quenched several sample with cooling rate between 1e12 and 1e15 K/s under NPT ensemble from 7000 to 300 K.

The shear module of a-SiO2 with cooling rate of 1e12 K/s is smaller than some samples with faster cooling rate. The shear modulus of other samples increases with cooling rate decreasing. But from uniaxil tension stress-strain curves, the strength of 1e12 K/s is highest. I got the shear modulus from shear stress-strain curves.

And another problem is during uniaxil tension, sample of 1e14 K/s fractured at smaller strain compared to sample of 5e13 K/s.

So now I’m not sure the problems above is due to the potential or others?

Thanks,
Zhao

silica.tabulated (121 KB)

12

Well, can certainly spline any term you wish to the short-range
part of a Buckingham potential and formulate it as
a pair_style table. But you have to work out the formula
they are using and do the splining correctly. Pair_write
and help you see that you’ve done it correctly.

Steve

13

OK. I will try as your suggestion. Thanks very much.
Zhao