pair style lj/cut/coul/wolf ? pair style lj/cut/coul/dsf ?

Hello lammps users,

lammps version 11 august 2017.

I have two questions

  1. pair style lj/cut/coul/wolf doest not work. I get the following error unknown pair style. Can someone please verify this.

  2. I am trying to verify the energies form pair style lj/cut/coul/dsf with the calculation that I have done offline.

Consider the following input script.

Two LJ particles with charge +1 and -1 at a distance of 1A.

units real
atom_style full
boundary f f f
region 1 block 0 3 0 3 0 3
create_box 1 1
create_atoms 1 single 1 0 0
create_atoms 1 single 2 0 0

mass 1 1.0
set atom 11 charge 1
set atom 2
2 charge -1
pair_style lj/cut/coul/dsf 0.2 2
pair_coeff 1 1 0 0

thermo 1
thermo_style custom pe evdwl ecoul elong
run 0
write_data data.test

The output I get from lammps is -412.4329 kcal/mole.

PotEng E_vdwl E_coul E_long
-412.4329 0 -412.4329 0

However when I calculate it offline using the expression in the doc pages I get -83.8224 kcal/mole

To further verify my offline calculation I tried with pair style lj/cut/cou/cut 2 2 I get

PotEng E_vdwl E_coul E_long
-332.06371 0 -332.06371 0

and my calculations give Ecoul = -332.06 which matches with lammps.

I would appreciate if anybody could clarify how pair style lj/cut/coul/dsf calculations are done or point out if there is any obvious mistake that I am making.

The following octave script might be useful if anyone is trying to verify.

clc;clear all;
q= [-1 1] 18.2223;
alpha = 0.2;
r=1;
rc = 2;
Ecoul = (q(1)q(2))/r
t1=erfc(alpha
rc)/(rc
rc);
t2=(2alphaexp(-alphaalpharcrc))/(sqrt(pi)rc);
Esf = q(1)q(2)(erfc(alpha
r)/r - erfc(alpha
rc)/rc + (r-rc)*(t1 + t2))

save as dsf.m

and run as

$octave dsf.m

Thank you
Arun

Figured out the second part of the question after going through the source code and the paper of Fennell.

pair style_lj_cut_coul_wolf is missing in the source files.

Please add them

Thank you

Arun

Figured out the second part of the question after going through the source
code and the paper of Fennell.

pair style_lj_cut_coul_wolf is missing in the source files.

Please add them

I think Axel means that on the bug.html page it shows that pair
style was added in the 22 Feb 18 version. So if you
have an older version, you won’t have the source files.

Steve

Thank you !.

I am still trying to verify the calculations from lj/cut/coul/dsf with the following script that I posted earlier.

I am not sure what this prefactor is in the source code.

prefactor = factor_coul * force->qqrd2e * atom->q[i]*atom->q[j]/r;

phicoul = prefactor * (erfcc - re_shift - rsqf_shift);

I would appreciate any help.

Thanks
Arun

It’s the same prefactor used in every Coulombic potential in LAMMPS.

If you want the value of qq22e, then look in update.cpp where it is

set for all choices of LAMMPS units.

Steve