# LJ 10 - 4

Hi I am working with a simulation the requires the use of a LJ 10-4 potential “epsilon*((sigma/r)^10 - (sigma/r)^4)” instead of the 12-6 or 9-6. I saw the files pair_lj_cut.cpp and pair_lj96_cut.cpp. I looked at the functions “PairLJCut::init_one” and “PairLJ96Cut::init_one” and I know how to change the coefficients to obtain L-J 10-4. However, in MPI_Allreduce there are more coefficients and I do not know what they mean. For LJ 9-6 the function looks like this MPI_Allreduce(count,all,2,MPI_DOUBLE,MPI_SUM,world); double PI = 4.0atan(1.0); double sig3 = sigma[j]sigma[j]sigma[j]; double sig6 = sig3sig3; double rc3 = cut[j]cut[j]cut[j]; double rc6 = rc3rc3; etail_ij = 8.0PIallallepsilon[j] * sig6 * (sig3 - 2.0rc3) / (6.0rc6); ptail_ij = 8.0PIallallepsilon[j] * sig6 * (3.0sig3 - 4.0rc3) / (6.0rc6); and for LJ 12- 6 it looks like MPI_Allreduce(count,all,2,MPI_DOUBLE,MPI_SUM,world); double PI = 4.0atan(1.0); double sig2 = sigma[j]sigma[j]; double sig6 = sig2sig2sig2; double rc3 = cut[j]cut[j]cut[j]; double rc6 = rc3rc3; double rc9 = rc3rc6; etail_ij = 8.0PIallallepsilon[j] * sig6 * (sig6 - 3.0rc6) / (9.0rc9); ptail_ij = 16.0PIallallepsilon_[j] * sig6 * (2.0sig6 - 3.0rc6) / (9.0*rc9); Can you explain me how do you obtain these coefficients Regards_

Hi I am working with a simulation the requires the use of a LJ 10-4
potential "epsilon*((sigma/r)^10 - (sigma/r)^4)" instead of the 12-6 or 9-6.
I saw the files pair_lj_cut.cpp and pair_lj96_cut.cpp. I looked at the
functions "PairLJCut::init_one" and "PairLJ96Cut::init_one" and I know how
to change the coefficients to obtain L-J 10-4. However, in MPI_Allreduce
there are more coefficients and I do not know what they mean. For LJ 9-6 the
function looks like this
MPI_Allreduce(count,all,2,MPI_DOUBLE,MPI_SUM,world); double PI =
4.0*atan(1.0); double sig3 = sigma[j]*sigma[j]*sigma[j]; double sig6 =
sig3*sig3; double rc3 = cut[j]*cut[j]*cut[j]; double rc6 = rc3*rc3; etail_ij
= 8.0*PI*all*all*epsilon[j] * sig6 * (sig3 - 2.0*rc3) / (6.0*rc6);
ptail_ij = 8.0*PI*all*all*epsilon[j] * sig6 * (3.0*sig3 - 4.0*rc3) /
(6.0*rc6); and for LJ 12- 6 it looks like
MPI_Allreduce(count,all,2,MPI_DOUBLE,MPI_SUM,world); double PI =
4.0*atan(1.0); double sig2 = sigma[j]*sigma[j]; double sig6 =
sig2*sig2*sig2; double rc3 = cut[j]*cut[j]*cut[j]; double rc6 = rc3*rc3;
double rc9 = rc3*rc6; etail_ij = 8.0*PI*all*all*epsilon[j] * sig6 *
(sig6 - 3.0*rc6) / (9.0*rc9); ptail_ij = 16.0*PI*all*all*epsilon[j] *
sig6 * (2.0*sig6 - 3.0*rc6) / (9.0*rc9); Can you explain me how do you
obtain these coefficients Regards

are you using?

pair_modify tail yes

if not, don't worry about those.

axel.

Dr. Kohlmeyer

I am not using pair_modify tail, is “PairLJCut::init_one” the only function that I need to change?

Regards

Dr. Kohlmeyer

I am not using pair_modify tail, is "PairLJCut::init_one" the only function
that I need to change?

to go from 12-6 to 10-4 lennard jones?

no. there is more. this is easiest seen in the
src/USER-CG-CMM/pair_lj_sdk.{cpp,h} files
and src/USER-CG-CMM/lj_sdk_common.h
those support 12-6, 9-6 and 12-4 lennard jones
all in one pair style.

if i were you, i'd probably not program a new pair style
unless i was certain, that the simulation will work, but
would rather first start with generating an input for
pair style table.

if you want to go beyond that, i'd probably extend
lj/sdk to 10-4 rather than writing yet another lj/XXX
pair style. there are too many of those already... cheers,
axel.

Dr. Kohlmeyer

Ok. I think I will work with pair_style table.

Thank you