Using shifted and non-shifted LJ potentials

Hello.

I have the next problem.
I am going to simulate 3 types of Lennard-Jones particles:
the 1st type interacts with 3rd type by shifted potential
and
the 2nd type interacts with 3rd type by non-shifted potential.

Can I use simultaneously shifted and non-shifted LJ potentials?
Is it possible to realize in LAMMPS ?
Thank You,
Sincerelly, Azat.

Is this the kind of shifted-potential you want to use?
http://lammps.sandia.gov/doc/pair_lj_sf.html

If you can find a pair_style that matches the one you want, you can
use "pair_style hybrid" to mix it with a different (Lennard Jones)
pair style. See
http://lammps.sandia.gov/doc/pair_style.html
http://lammps.sandia.gov/doc/pair_hybrid.html

Andrew

P.S.
If not, then you can always use "pair_style table":
http://lammps.sandia.gov/doc/pair_table.html

In fact, for only 3 types of particules you could use "pair_style
table" for all of your 6 of pairwise interactions (1-1, 1-2, 1-3, 2-2,
2-3, 3-3). I've noticed that if you try to use "pair_style hybrid" to
mix a table pair style with a different pair style, the results are
not always faster than if you simply use "table" for all pair
interactions. You can try it and see.

Alternately, I just noticed there seems to be a "pair_style lj/shift"
described briefly here which shifts the potential in the radial
direction (instead of in the energy direction):
http://lammps.sandia.gov/doc/99/force_fields.html

Perhaps this is what you are looking for?

Andrew
P.S.
Unfortunately, I could not locate the documentation for it or the
source code for it (It does not seem to be present in the version of
LAMMPS I downloaded). If you need it, please post a request.

Hello.
I have the next problem.
I am going to simulate 3 types of Lennard-Jones particles:
the 1st type interacts with 3rd type by shifted potential
and
the 2nd type interacts with 3rd type by non-shifted potential.

Can I use simultaneously shifted and non-shifted LJ potentials?
Is it possible to realize in LAMMPS ?

the shift of the potential (via pair_modify yes) only
affects the energy and not the forces, so it really
matters only for minimization calculations. however,
in that case, it would be recommended to shift the
potential for all three cases.

if you're referring to using a shifted force LJ potential,
then matters are a little bit different. you can use the
options andrew mentioned, but it would be interesting
to know, if there are any physical reason for having
a shifted potential for some and a truncated potential
for other interactions. i cannot imagine any, but i am
always curious to learn something new...

axel.

I'm guessing he is using it to simulate the LJ interaction between a
point particle and a thin spherical shell (or two thin spherical
shells?). In either case, it would be more accurate to integrate the
LJ interactions between the particle over the surface of the sphere.

(When you carry out the integral, a particle experiences an A/(r-R)^10
- B/(r-R)^4 type of interaction, where (r-R) is the distance between
the particle and the spherical surface. So, you're right that it's
not quite the same as a shifted Lennard Jones interaction. The
interaction between two spherical shells would be something like
A/((r-(Ri+Rj))^8 - B/(r-(Ri+Rj))^2 near the singularity). If anyone
cares, I can dig out the exact formula we used for this. Also,
there's an online example where we recently did this in LAMMPS using
"pair_style table", which I find as well.)

Alternately, I just noticed there seems to be a "pair_style lj/shift"
described briefly here which shifts the potential in the radial
direction (instead of in the energy direction):
Unfortunately, I could not locate the documentation for it or the
source code for it (It does not seem to be present in the version of
LAMMPS I downloaded). If you need it, please post a request.

It sounds like you found it already, but for the record, and for other
people who find this post, the shifted-force LJ pair_style is now
called "lj/expand", and it's documented here:
http://lammps.sandia.gov/doc/pair_lj_expand.html

Cheers