Lj/sf/dipole/sf formula

[Saqlain_Raza]

Hello,

I am trying to use a potential provided in the following paper:
Orsi, M., 2014. Comparative assessment of the ELBA coarse-grained model for water. Molecular Physics , 112 (11), pp.1566-1576.

The pair style provided in this paper matches with lj/sf/dipole/sf pair style in LAMMPS except for one factor which I am confused about. The dipole-dipole (Epp) interaction for this pair style is defined in LAMMPS as:

However, in the paper, it is defined as:

The first factor is missing in the LAMMPS (or atleast I think so and maybe wrong). Can you please guide me if the factor is actually missing or my understanding is incorrect?

Thank you.

[stamoor]

It depends on the unit system, for some unit systems that prefactor is 1.0. So the two formulas are equivalent.

For example, see https://www.reddit.com/r/askscience/comments/19pnw8/in_planck_units_why_is_the_coulomb_constant_set/.

[Saqlain_Raza]

Thank you for your response, Dr. Moore. I am using real units for my simulation and not sure if the prefactor will be 1.0 in this case. In the case of coulombic pair style, a prefactor C is defined which is the same as 1/(4piepsilon_0). See:

what's the value of energy-conversion constant in lammps

Dear Lammps users/developers, In manual, I have found that the coul/cut style computes the standard Coulombic interaction potential given by E = Cqiqj/(epsilon*r). where C is an energy-conversion constant, and epsilon is the dielectric constant which can be set by the dielectric command (the constant is unitless). I want to know the value and unit of C. In the mail list, I have found a mail talking about how lammps deals with C. however, the exact value isn’t shown. I need the value and unit o…

The value of this prefactor C for real units is defined in update.cpp as:
force->qqr2e = 332.06371; // see also force->qqr2d_lammps_real

Thank you for your guidance.

[stamoor]

LAMMPS automatically does the right thing depending on the unit system that you choose. Just make sure the units are correct for the potential parameters that you input into LAMMPS.

[Saqlain_Raza]

Thank you again, Dr. Moore. I understand that both the formulas are same (in my original post) and LAMMPS will take care of the prefactor. I will make sure that the units of the potential parameters are correct (all in real units in my case).