Dear developers,

I think that the way you are writing the lj/class2 potential is not correct. The sigma value (where V(R)=0) should be substituted with Ro where V'(Ro)=0. Do you define sigma in another way? If I use this potential but instead of sigma use Ro is the implementation influenced someway?

Many thanks.

Kind regards,

Christos

Dear developers,

I think that the way you are writing the lj/class2 potential is not correct.

hmm... how is it possible then, that people have used

it correctly for such a long time?

The sigma value (where V(R)=0) should be substituted with Ro where V'(Ro)=0.

Do you define sigma in another way?

sigma is the minimum of the potential. the potential is 0 only for infinite R.

this can be easily validated using a function plotting tool like

gnuplot or grace.

If I use this potential but instead of sigma use Ro is the implementation influenced someway?

just run some tests.

axel.

i should perhaps point out in addition, that the definition

of sigma in the class2 LJ potential differs from the regular LJ.

for which the minimum is at: 2^1/6 * sigma

this is reflected in the functional form that is provided.

cheers,

axel.

Hi christos

it is correct that the definition of sigma is the radius in which force=grad(v) is zero but

note that in regular Lj formula if you set r=sigma, then v=0

this is because we can shift the plot of v® up and down in y direction without affecting

the results because the important quantity is force=v’, not v itself

(derivative of a constant is zero)

so it is prevalent to shift the V® up until in r=sima we have both v®=v’®=0

best,

Dear Axel,

Thanks for the reply. So if I understand well the sigma in the class2 potential is actually the distance at the minimum (where the V'(x)=0). In the case of the 12-6 instead sigma is where V(x)=0. I wanted to be sure that in the case of the 9-6 the sigma was not converting to Ro in the code.

Kind regards,

Christos

Dear Axel,

Thanks for the reply. So if I understand well the sigma in the class2 potential is actually the distance at the minimum (where the V'(x)=0). In the case of the 12-6 instead sigma is where V(x)=0. I wanted to be sure that in the case of the 9-6 the sigma was not converting to Ro in the code.

if you want to make absolutely certain about your parameters,

use the pair_write command to plot out the potential and forces

for a given set of parameters.

http://lammps.sandia.gov/doc/pair_write.html

axel.