# add a constant force to each atom in real units(correct post)

Dear all
Excuse me. My last e-mail was wrong. Now i correct it.
I want to model poisell flow in lammps. I am using “real units”. I want to add a constant force (i.e. 5 pN) in x direction to each atom .
I know 5 pN is equal to 0.0011e-22 Kcal/Angstrom. But we know the force unit in “real units” is Kcal/Angstrom-mole. So, what is the correct force when i use “fix addforce fx 0 0” command in “real units” if i want to add 5pN in x direction to each atom in my group? (i mean what is fx? 0.0011e-22 or 0.0011e-22 /6.022e23 or 0.0011e-22 *6.022e23?)

Excuse me. My last e-mail was wrong. Now i correct it.
I want to model poisell flow in lammps. I am using "real units". I want to
add a constant force (i.e. 5 pN) in x direction to each atom .
I know 5 pN is equal to 0.0011e-22 Kcal/Angstrom.

How did you get that value? 5 pN = 5e-12 J/m = 5e-12 * (1/4184 kcal) /
(1e10 A) = 1.20e-25 kcal/A.

This is the value per atom, so per mol of atoms it will be N_A times
larger: 1.20e-25 * 6.02e23 = 0.0719 kcal/mol-A.

Best,
Laurent