potential comparison for pair_style coul/long and pair_write output

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1

Dear users,

Trying to verify the output from pair_write command for coul/long pair_style with the analytic form of coulomb potential.

The comparison is inside cutoff distance. Both the potential values should match but they are differing with each other.

If I understand correct, pair_write must write the exact form of potential given by pair_style.

Also, what should be value of C and epsilon if wish to find correct coulomb potential. We are considering (1/4piepsilon) included with
potential form.
Anyway, tested with all possible permutations/combinations. No success. Attached a simple input file.

Can one please point out the missing links!

Regards

in.test_pair_write (1.49 KB)

2

I suggest you try using pair write with a cutoff LJ
potential first, e.g. pair lj/cut and verify that you

are able to compare it to an analytic formula.

They try a Coulomb potential. If you are doing
this in non-LJ units, e.g. real or metal, then
you will have to understand the energy units
that LAMMPS uses in each case for LJ and

Coulombic in order to compare to your formula.

Steve

3

Dear Steve,

Sending you the comparison u asked for. 3 cases taken into account.

  1. LJ/cut with LJ units
  2. Yukawa with SI units
  3. Coul/long with SI units

all plots are from 0.2 cutoff to cutoff distance in respective normalization.

The attached plots shows perfect matching with its analytic form for Yukawa and LJ/cut case while for Coul/long,
the results not matching.

I wonder if the problem related with value of C and epsilon in coul/long potential. As no suggestions for
providing these values given in pair_coeff.

Thanks

test_pot.pdf (14.2 KB)

4

Dear Steve,

Sending you the comparison u asked for. 3 cases taken into account.
1. LJ/cut with LJ units
2. Yukawa with SI units
3. Coul/long with SI units

all plots are from 0.2 cutoff to cutoff distance in respective
normalization.

The attached plots shows perfect matching with its analytic form for
Yukawa and LJ/cut case while for Coul/long,
the results not matching.

I wonder if the problem related with value of C and epsilon in coul/long
potential. As no suggestions for
providing these values given in pair_coeff.

Am I missing something here? How does coul/long have C and epsilon?

Ray

5

Hi Ray Shan,

I clarify my statement.

I was just saying from pot. energy form in manual: its E = CQ1Q2/epsilonr
As there is no needed value of pair_coeff for coul/long case, is the value (C/eps) is equal to (1/4pi*epsilon_0)!

I just posed my doubt because i am unable to match analytic form of this potential to pair write value. While for other potentials, as i have shown, both are matching.

6

Hi Ray Shan,

I clarify my statement.

I was just saying from pot. energy form in manual: its E = CQ1Q2/epsilonr
As there is no needed value of pair_coeff for coul/long case, is the value (C/eps) is equal to (1/4pi*epsilon_0)!

I just posed my doubt because i am unable to match analytic form of this potential to pair write value. While for other potentials, as i have shown, both are matching.

It cannot match because your formula is for coul/cut not coul/long.

7

Hi Axel,

Got the missing link. Thanks for pointing out. Its perfectly matching with coul/cut and must not
match as written in manual … due to kspace_style.

All other confusions buried with this solutions.

Thanks again.