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 (1/4*pi*epsilon) 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)

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

Dear Steve,

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

- LJ/cut with LJ units
- Yukawa with SI units
- 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)

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

Hi Ray Shan,

I clarify my statement.

I was just saying from pot. energy form in manual: its E = C*Q1*Q2/epsilon*r*

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.

Hi Ray Shan,

I clarify my statement.

I was just saying from pot. energy form in manual: its E = C*Q1*Q2/epsilon*r*

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.

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.