positive value for pair potential

Hello all,

I tried to output the values of pair potentials I have used in my simulation. I used 3 pair potentials: lj/cut, lj/cut/coul/long, and rebo. I used “compute pair” command for this purpose. The scalar value I get for lj/cut/coul/long pair potential is positive. Is this right?

Here are the thermo outputs:

PotEng Press Temp Temp_water pair_LJ pair_LJ_Coul pair_rebo

-168261.95 24105.803 162.29057 296.49234 -231.34277 38506.996 -161036.51

Regards,

Farshad

PS. Here is my input file:

read_restart restart.equil_298K.100411

neighbor 3.0 bin
neigh_modify delay 0 every 10 check yes

set potential function and parameters

Create Groups

group hy type 8 # H_water
group ox type 9 # O_water
set group ox charge -0.8476
set group hy charge 0.4238

pair_style hybrid/overlay rebo lj/cut 7.5 lj/cut/coul/long 8.5

pair_coeff * * rebo CH.airebo C H C H C C C NULL NULL
pair_coeff * * lj/cut 0.0 0.0
pair_coeff * * lj/cut/coul/long 0.0 0.0
pair_coeff 1 9 lj/cut 0.013 2.71
pair_coeff 2 9 lj/cut 0.0036 1.7753
pair_coeff 3 9 lj/cut 0.013 2.71
pair_coeff 4 9 lj/cut 0.0036 1.7753
pair_coeff 5 9 lj/cut 0.013 2.71
pair_coeff 6 9 lj/cut 0.013 2.71
pair_coeff 7 9 lj/cut 0.013 2.71
pair_coeff 9 9 lj/cut/coul/long 0.0067 3.166

bond_style harmonic
bond_coeff 1 7.669 1.0
angle_style harmonic
angle_coeff 1 1.829 109.47
kspace_style pppm 1.0e-5 #final npt relaxation
kspace_modify slab 3.0

compute pair_LJ all pair lj/cut
compute pair_LJcoul all pair lj/cut/coul/long
compute pair_rebo all pair rebo

group slab type 1 2 3 4 # slab
group water type 8 9 # water
compute temp_water water temp
thermo 1
thermo_style custom step pe press temp c_temp_water c_pair_LJ c_pair_LJcoul c_pair_rebo
thermo_modify norm no

timestep 0.001

fix freeze slab setforce 0.0 0.0 0.0
fix nvt_1 all nvt temp 300.0 300.0 0.1
fix_modify nvt_1 temp temp_water
fix shake_1 water shake 1e-5 20 0 b 1 a 1
dump dcd_1 all dcd 500 equil.dcd
run 30000
write_restart restart.equil.*

Hello all,

I tried to output the values of pair potentials I have used in my
simulation. I used 3 pair potentials: lj/cut, lj/cut/coul/long, and rebo. I
used "compute pair" command for this purpose. The scalar value I get for
lj/cut/coul/long pair potential is positive. Is this right?

LAMMPS will compute what you tell it to. it is your model, so you
should be the person best set up to debug it.

axel.

Why should it be wrong? There is no rule that any energy component
must be negative. Take a look at what atom type pair is
lj/cut/coul/long applied to, and think about how Coulomb energy works.

Ray