Regarding pair_coeff * * ffield.reax C H O N

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1

Dear Lammps users,
I have a question regarding reaxff implementation in lammps and I appreciate any help in advance,
I like to switch the intractins off between a part of atoms in the simulation system.
But unlike the other force fields I get error where I try to modify per_coff as follow:
{pair_coeff * * ffield.reax C H}
to some thing like
{ pair_coeff * 40* ffield.reax C H}
so a part of atoms do not interact with each other but interact with the rest of system.
I appreciate any comments on how I could modify lammps input to perform this.
Thanks in advance
Simin

2

Dear Lammps users,
I have a question regarding reaxff implementation in lammps and I appreciate any help in advance,
I like to switch the intractins off between a part of atoms in the simulation system.
But unlike the other force fields I get error where I try to modify per_coff as follow:
{pair_coeff * * ffield.reax C H}
to some thing like
{ pair_coeff * 40* ffield.reax C H}
so a part of atoms do not interact with each other but interact with the rest of system.
I appreciate any comments on how I could modify lammps input to perform this.

it is not recommended for ReaxFF based calculations to do such a thing. it is not a pairwise additive force field and the charge equilibration requires to “see” all atoms to work correctly.
you can wipe out the computed forces after the fact using fix setforce.

axel.

3

Thanks for your reply.
I have a system of (100 hydrogen radicals + a graphene surface which has also hydrogen termination).
In my simulation this Hydrogen radicals should not attack each other and form a H2, but they should attack the surface.
If I set force On them to Zero, I think they would not move at all.

I appreciate any thought and help.
Regards
Simin

4

Thanks for your reply.
I have a system of (100 hydrogen radicals + a graphene surface which has also hydrogen termination).
In my simulation this Hydrogen radicals should not attack each other and form a H2, but they should attack the surface.

well, your model isn’t very physical. it would be more physical to gradually deposit the hydrogen atoms.

if you absolutely want to stop those hydrogen atoms to get too close to each other, you could set up a simulation with pair style hybrid/overlay, where you add to the hydrogen-hydrogen interaction an additional, repulsive term with a suitable diameter, e.g. via pair style soft.

If I set force On them to Zero, I think they would not move at all.

correct. your description said you didn’t want forces; it didn’t say that you would want to remove only certain inter-atomic forces. it wouldn’t have worked with the pair_coeff example you have given, even if ReaxFF would be pairwise additive and could be selected like lj/cut or similar.

axel.

5

Thanks so much Alex for your comments and help, helped me a lot.
Regards
Simin