# pair class2 potential

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.