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.