pair_style vashishta

Dear lammps experts,

I have modified the vashishta potential coefficient based on the available document for pair_style vashishta for AlN. it is actually a little confusing:

1- when the second and third atoms in the entry ARE the same, LAMMPS collect ONLY the 2 body parameters of the potential. So what about the 3 body parameters for this entry? I mean, should I also input the 3 body parameters?

2- when the second and third atoms in the entry are NOT the same, LAMMPS collect ONLY? the 3 body parameters of the potential?

any comment is highly appreciated.
Yours Sincerely,

Hi
Vashista potential is a three body potential for including angles. So it should include Al N N, N Al N, Al N Al, etc... contributions
Best
Pascal

Dear lammps experts,

I have modified the vashishta potential coefficient based on the available document for pair_style vashishta for AlN. it is actually a little confusing:

1- when the second and third atoms in the entry ARE the same, LAMMPS collect ONLY the 2 body parameters of the potential. So what about the 3 body parameters for this entry? I mean, should I also input the 3 body parameters?

2- when the second and third atoms in the entry are NOT the same, LAMMPS collect ONLY? the 3 body parameters of the potential?

any comment is highly appreciated.
Yours Sincerely,
Bahman Daneshian

thank you for your response. I know that actually. but I cannot understand lammps document explanation. this is what I have prepared according to the paper information:

X X X H eta Zi Zj lambda1 D lambda4 W rc B gamma r0 C cos(theta) #j=k–> 2 body

Al Al Al 507.6686 7 1.0708 1.0708 5.0 0 3.75 0 7.60 0.0 0.0 0.0 0.0 0.0 #j=k–> 2 body
Al N N 367.05546 9 1.0708 -1.0708 5.0 24.7978 3.75 34.58365 7.60 2.1536 1.0 2.60 20 -0.8843 #j=k–> 2 and 3 body ?!
N N N 1038.16334 7 -1.0708 -1.0708 5.0 49.5956 3.75 0 7.60 0.0 0.0 0.0 0.0 0.0 #j=k–> 2 body
N Al Al 367.05546 9 -1.0708 1.0708 5.0 24.7978 3.75 34.58365 7.60 2.1536 1.0 2.60 20 -0.8843 #j=k–> 2 body and 3 body ?!
N Al N 0 0 0 0 0 0 0 0 0 0 0 0 0 0 # j=/k–> 3 body
N N Al 0 0 0 0 0 0 0 0 0 0 0 0 0 0 # j=/k–> 3 body
Al Al N 0 0 0 0 0 0 0 0 0 2.1536 1.0 2.60 20 -0.8843 # j=/k–> 3 body
Al N Al 0 0 0 0 0 0 0 0 0 2.1536 1.0 2.60 20 -0.8843 # j=/k–> 3 body

Best regards,

Bahman Daneshian

If I well remember : X Y Z means X bonded to Y with Y bonded to Z

sorry, but it is not actually response to my question!

Reading the source code, with pair vashishta X is the center atom, while Y and Z are bonded (neighbors) of X.

All 8 lines are unique and each line defines 2 and 3 body interactions. It is just with AlN you won’t find Al-N-N bonds (the N Al N line) nor N-N-Al bonds (the N N Al line) – because they are outside the short range cutoff – so there is no need to define (or parameterize) them.

Al Al Al line has zeros for the 3-body terms simply because the developers didn’t need them when parameterizing this potential. Same for the N N N line.

This same concept is applied to Tersoff and Stillinger-Weber potentials.

Hope this helps.

Ray

Dear Ray,

Thank you for your reply. Actually, now I understood the lammps format for vashishta coefficients. thank you. However, I have pstill question about your explanation:

1- for Al Al Al and NNN lines the 3 body part should be zero.
2- for Al N N and N Al Al lines should we remove the 2 body part ?

3- I could not find bonds for the other lines like NALN-NNAl-AlAlN-AlNAl. So they all considered as zero.

#X X X H eta Zi Zj lambda1 D lambda4 W rc B gamma r0 C cos(theta)

Dear Ray,

Thank you for your reply. Actually, now I understood the lammps format for vashishta coefficients. thank you. However, I have pstill question about your explanation:

1- for Al Al Al and NNN lines the 3 body part should be zero.
Ans: There is no “should” – it is just the potential developers didn’t need them. 3-body attraction can be parameterized and added if necessary

2- for Al N N and N Al Al lines should we remove the 2 body part ?
Ans: No. Remember the “X Y Z” line refers to Y-X-Z bonds whereas the Y-X is the central bond. The “Al N N” refers to Al-N bond affected by an N neighbor of Al’s. the 2-body part defines the Al-N repulsion, whereas the 3-body defines the Al-N attraction (affected by the N neighbor of Al’s).

3- I could not find bonds for the other lines like NALN-NNAl-AlAlN-AlNAl. So they all considered as zero.
Ans: Yes.

#X X X H eta Zi Zj lambda1 D lambda4 W rc B gamma r0 C cos(theta)

Dear Ray,
Thank you very much for your response.so now all input lines are correct. So why when I run this program under NVE ensemble it explodes and the temperature rises up too much?! I applied the metal units. So, the units are similar to the vashishta paper.
should I also check my initial AlN structure again?