Using electric field for solute+solvent system

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1

Dear lammps developer and users,

I am currently modelling the effects of electric field on solute+solvent systems using lammps. It will be great if anyone can kindly provide some suggestions for following questions:

  1. I am wondering if the solute molecule can be fixed in the centre of the simulation box and does not move, or solute molecule can move but its structure keeps the same during the simulation process. Because I intend to use a hydrogen transfer transition state structure as the solute in the simulation, the TS structure can easily change to reactant or product state without constrain. According to the manual, fix setforce command and fix string command might be two options, but I am not sure which one is more suitable?

  2. Is it possible to specify the direction of electric field using atoms in the solute molecule? For example, can I set the field along the direction from atom1 to atom2 of the solute molecule? Otherwise, if I use electric field along x Cartesian direction, can I set the direction from atom1 to atom2 of the solute molecule is in the x Cartesian direction during the whole simulation?

  3. Is it practical to set the condition that electric field only affects the solvent molecules while does not affect the solute?

Many thanks!

2

Dear lammps developer and users,

I am currently modelling the effects of electric field on solute+solvent systems using lammps. It will be great if anyone can kindly provide some suggestions for following questions:

  1. I am wondering if the solute molecule can be fixed in the centre of the simulation box and does not move, or solute molecule can move but its structure keeps the same during the simulation process. Because I intend to use a hydrogen transfer transition state structure as the solute in the simulation, the TS structure can easily change to reactant or product state without constrain. According to the manual, fix setforce command and fix string command might be two options, but I am not sure which one is more suitable?

the easiest way to immobilize a group of atoms, is to do time integration only on the group of the remaining atoms and not on group “all”.

  1. Is it possible to specify the direction of electric field using atoms in the solute molecule? For example, can I set the field along the direction from atom1 to atom2 of the solute molecule? Otherwise, if I use electric field along x Cartesian direction, can I set the direction from atom1 to atom2 of the solute molecule is in the x Cartesian direction during the whole simulation?

if you can find an expression in x, y, and z that can be specified in equal style variables, this should be duable.

  1. Is it practical to set the condition that electric field only affects the solvent molecules while does not affect the solute?

fix efield only operates on a group of atoms. most common is to use the group “all”. if you set up a different group then only atoms in that group will experience the force from fix efield. whether this is meaningful or not, is a different question, that i cannot answer.

axel.