question about defining charges between similar atoms

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1

Hello,
I am trying to conduct a simulation with several charged molecules. I have been told that I should consider fixed charged for different atoms initially using the following code:

Potentials & Settings

group carbon type 1
group hydrogen type 2
group nitrogen type 3
group oxygen type 4
group sulfur type 5

#charge definition
#charge setting
set group carbon charge 0.0000
set group oxygen charge -0.7700
set group hydrogen charge 0.2200
set group sulfur charge 1.0300
set group nitrogen charge -0.4300

pair_style lj/cut/coul/long/cs 10.0
pair_modify tail yes
kspace_style pppm 1.0e-5

pair_coeff 1 1 0.305 4.18 # C
pair_coeff 2 2 0.010 3.20 # H
pair_coeff 3 3 0.415 3.99 # N
pair_coeff 4 4 0.415 3.71 # O
pair_coeff 5 5 0.305 4.24 # S
pair_coeff 1 4 0.356 3.94
pair_coeff 1 2 0.055 3.69
pair_coeff 2 4 0.064 3.46
pair_coeff 2 5 0.055 3.72
pair_coeff 3 4 0.415 3.85
pair_coeff 1 3 0.356 4.09
pair_coeff 2 3 0.064 3.58
pair_coeff 3 5 0.356 4.09
pair_coeff 1 5 0.305 4.21
pair_coeff 4 5 0.356 3.98
pair_coeff 2 5 0.356 4.12

this code works and the coul energy is calculated. but I need to define partial charges between every two atoms separately, not for one atom.
How can I do that?

2

Hello,
I am trying to conduct a simulation with several charged molecules. I have been told that I should consider fixed charged for different atoms initially using the following code:

don’t go after what somebody tells you. Understand the model and force field and know what you have to do. usually classical force field parameters are not defined per element, but per atom type according to tables of atom types (there are different parameters for carbon or hydrogen or nitrogen or oxygen atoms in different chemical environments). please pick up a text book on molecular dynamics simulations and look up how force fields are structured and derived.

[…]

this code works and the coul energy is calculated. but I need to define partial charges between every two atoms separately, not for one atom.

that is not how coulomb interactions are usually computed. the charges are not different for each pair, but they are set for each atom according to the force field and the topology.
only the lennard-jones parameters are set between atom types (again not individual atoms).

How can I do that?

you cannot. at least not with the coulomb interaction models implemented in LAMMPS. and I seriously doubt that that is what you should be doing.

axel.