Permanent or induced dipole moment

Dear all,

I have a question about the simulation of the dipole moment by LAMMPS.

As we all know, the thermalized Drude model can be used to simulate the induced dipole moment. from my understanding, when I simulate the Drude model without an external electric field at a finite temperature, the “compute dipole” command gives the permanent dipole moment. if the external field is set, the computed dipole moment is permanent+induced.

is it correct? if not, how can I simulate permanent and induced dipole moments separately?

all the best

Not really. When you are applying a polarizable model your dipole moment(s) always depend on the degree of polarization. Most of that is coming from the interaction between the atoms in neighboring molecules. So you may be able to determine some kind of “average dipole”. Please note that compute dipole will compute the total dipole. Dipoles of subsets like molecules requires compute dipole/chunk.

When adding fix efield you are adding forces to the charges, including those forming the Drude oscillators and they should (on average) pull them further apart, i.e. change the resulting dipole further.

You can tell the differences between with or without fix efield only by computing the average values for each case and then get the difference.