# lammps 'real units' and conversion

Hello,

I have come across a paper dealing with the simulation of ionic liquids where
they use the Born Mayer Huggins potential (or born/coul/long in “lammps language”). Some of the parameters for the potential are given in Joules*A^6 units.

The system consists of 256 anions and 256 cations so a total of 512 ions in the simulation.

Now, suppose I want to work with the “real” units in lammps so the energy should be in kcal/mol.

Then, in this case what the mol refers to ? the total number of moles of the ions ?
or the cation-anion pairs ? Or, should I just multiply the given energy*distance^6 parameter by Avogadro’s number to convert from J to J/mol.

On the other hand, I think the potential parameters should be intensive quantities and then I don’t really understand how to work with the “real” unit system of lammps where the energy is normalized to the moles of (something) in the system.

Any help will be appreciated !

Hello,

"mol" refers to 6*10^23.

Regards,
Oleg.

06.07.2013, 14:49, "david furman" <[email protected]...>:

Dear Oleg,

Of course a mol is 6.02E23. However, is it a mol of ions or a mol of anion-cation pairs ? so for example if the simulation
used 0.5 mol of ions, then should I divide the parameter by 2 ? My question refers to whether the parameter A, C, D and alfa in the Born/coul/long pair potentials are “intensive” or “extensive”

Thanks

Energy is always an extensive property that depends upon the size or extent of the system, so that energy are reported with normalizations.

That is: 1 kcal/mol = 1 kilocalorie per a mole of molecules/atoms/particles = 4.184 kilojoules per a mole of molecules/atoms/particles = 0.043 eV per molecules/atoms/particles.

The normalizations, since well understood, are sometimes neglected thus reported energies can be in units of kcal, kJ, or eV.

Ray

In other words, in David's case it's ions.

Oleg.

06.07.13, 20:13, "Ray Shan" <[email protected]...>":

Said another way, LAMMPS uses the energy
param when a single pair of atoms interact.
The energy of that single interaction can be
in eV or Kcal/mole. The former is a small
amount of energy. The latter is a big
amount of energy normalized by another big number, to yield
the interaction energy of the single I,J pair.

Steve