what's the value of energy-conversion

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1

Dear Mr Tiwari,

Several days ago, I asked a question that I was confused by the formula for potential in some MD paper based on LAMMPS. The interaction between atomic sites are expressed as

C92E6A70@...5907....jpg

For the Coulombic potential, It seems that both of the energy-conversion constant C and the dielectric constant equal 1.

Thanks for your reply and I have done a test run as you recommend: two atoms with the charge of 1 C are put in a box, and the distance is set to 1m, in addition, I take the “coul/cut” potential, and the “units” is set to SI. Coulomb potential is export by the thermos_style command, and the result is 8.9876109. This value is equal to (1/4piepsilon). Therefore, I think 1/4piepsilon cannot be omitted in the Potential equation. However, I have found some published paper that omit 1/4pi*epsilon. Can you give me some clear explanation.

Thanks!

Zhang Chao

2

Hi Zhang,

1/(4piepsilon0) is surely part of Coulomb potential. I tested it many time, even with your chosen form of potential. I am again repeating the steps which may help you somewhat, else let me know, i will share the example input after digging my example repository.

  1. Take # Force field with SI units

pair_style lj/cut/coul/cut {cutf} pair_coeff * * {elj} ${sig}

  1. Take as many as particles anywhere in box (just remember box size and take cutoff smaller than box size for the check purpose)

  2. use command pair_write to write potential energy between any two particles at range of distances. Plot it.
    The text should be something like
    pair_write 1 2 2000 r {icf} {ocf} pot12.dat lj/coul {Q1} {Q2}

  3. Analytically plot the same potential in that spacial range on the top of it. Both must match.

This clearly will reflect that 1/4piepsilon0 is part of potential code.
I remember Axel gave one more hint to look into some file for the same thread.

Hope it works.

Sanat

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