Using hybrid/overlay to model (buck+coul) & (morse+coul) of PCRIM potential for SiO2

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1

Dear LAMMPS users,

I am trying to model Silica crystal at 1000k. The short range interactions ofSi-Si,Si-O and O-O atoms are modeled with Morse potential. The long-range electrostatic interactions are modeled with comb potential.

I wrote the following commands to model these interactions, but the temperature of the system from the NVT simulations is unreasonably high at the first step (over 3000K).

units metal
atom_style charge
boundary p p p

read_data SiO2.data

mass 1 28.0855
mass 2 15.9994

pair_style hybrid/overlay morse 9.0 comb
pair_coeff 1 1 morse 0.007695 2.0446 3.7598
pair_coeff 1 2 morse 1.99597 2.6518 1.628
pair_coeff 2 2 morse 0.23272 1.3731 3.791
pair_coeff * * comb ffield.comb Si O

neighbor 0.3 bin
neigh_modify delay 5

compute pe1 all pe/atom
compute ke1 all ke/atom
compute F_temp all temp

thermo 100
thermo_modify temp F_temp lost ignore norm yes

timestep 0.001

velocity all create 1000 213789
fix 1 all nvt temp 1000 1000 0.05 #iso 0.0 0.0 0.5 drag 0.2
fix 2 all qeq/comb 10 0.001

dump 1 all custom 1000 SiO2-melt.lammpstrj id type x y z c_ke1 c_pe1
restart 10000 SiO2.*.inp
run 50000

I wonder how to correct it?

Looking forward to your reply.

Liu Shuhui
Nanchang University

2

units metal
atom_style charge
boundary p p p

read_data SiO2.data

mass 1 28.0855
mass 2 15.9994

pair_style hybrid/overlay morse 9.0 comb
pair_coeff 1 1 morse 0.007695 2.0446 3.7598
pair_coeff 1 2 morse 1.99597 2.6518 1.628
pair_coeff 2 2 morse 0.23272 1.3731 3.791
pair_coeff * * comb ffield.comb Si O # It seem you overwrite the potential of Si-Si, Si-O, O-O with this line.

Asterisk mean it can be any number. In this case, * can be 1 or 2. So you have apply comb potential to Si-Si, Si-O, O-O. I am not sure you are applying comb to which atom, if you don’t want to apply it to all atom, this line is wrong.

Jocabi

3

Sorry but this is not correct. Pair_style comb must use “* *”.

COMB has more than just long range electrostatics so when you add Morse on top of COMB the resulting potential is not a reasonable one. If you want to add long range electrostatics on top of Morse, consider using coul/long.

Ray

4

Please note that ‘comb’ != ‘coul’. Axel