Dear LAMMPS community,
I am trying to simulate diffusion of water molecules (TIP4P model) in MIL125-MOF at the limit of zero loading (infinite dilution).
For that, I need to switch off both Lennard-Jones and electrostatic interactions between all water molecules in the system, while I will have to keep these interactions between water molecules and the MOF framework. Doing this is easy for the Lennard-Jones interactions where one can explicitly define which atom pairs interact with each other. However, I have no idea how this can be done for electrostatic interactions when using TIP4P model in LAMMPS. This is because LAMMPS makes use of a special treatment for TIP4P model which does not allow electrostatic interactions to be explicitly defined for specific atom pairs. Does anyone have a clue on how this can be done?
When both fluid-fluid and fluid-framework interactions are on, I use the following treatment:
pair_style lj/cut/tip4p/long 1 2 1 1 0.1546 12.0 8.5
pair_modify shift yes
kspace_style pppm/tip4p 1.0e-6
However, this applies electrostatic interactions between all atom pairs in the system and cannot be used for the case that water-water interactions are required to be switched off.
Thank you in advance for any helpful assistance.
The situation is not as simple as you put it.
part of the problem is, that you are using long-range electrostatics. that means, that part of the coulomb interaction is not computed as pairwise interactions (only the damped real-space part), but as a everything with everything, so you would have to use a cutoff coulomb or an approximated long-range coulomb like coul/dsf or coul/wolf.
next, when you turn off coulomb completely, you don’t need a tip4p style to handle the interactions, as you have no coulomb at all. so you could build this model with pair style hybrid (under the consideration, that you cannot use a kspace style).
also, if you want to turn off the coulomb interaction gradually (or just switch between on and off), you can use fix adapt and a compatible pair style that exports a “scale” factor array, that could be tuned. for tip4p this doesn’t exist yet, but it would be straightforward to implement it in the corresponding c++ pair styles by looking at the corresponding coul/cut or equivalent styles.
finally, when turning interactions off gradually or via pair_coeff statements (and not by replacing pair styles in a hybrid style), you have to consider the the discontinuity at r ~ 0. with no or very weak interactions atoms can overlap, which can cause problems due to the forces becoming infinity with with very small distances. in that case you need to use soft core potentials, like those in the USER-FEP package.
so there is a lot more to consider and a lot more possibilities of making mistakes and getting bogus results. for the most part this is unrelated to using tip4p water.