Hi,

I am wondering what is the unit for the stress tensor that is dumped by LAMMPS when using real units. Dump does not have units option as far as I know. units real is my command at the beginning of the input.

When I look at the website:

S = m.vi.vj + 0.5*SUM{ (ai-bi)Fij }

m in real units is given as g/cm3 in unit command, (ai-bi) is in angstroms and force is defined in kcal/mol.Angstroms in units command. v is in angstroms/fs.

When we divide by the volume later, volume is in angstroms cube.

These units are really not consistent, LAMMPS should be doing some conversions in the final output. I have found efactor, pfactor and boltz in initialize.h. But I am not sure in which units it writes in dump file for the stress tensor.

I have the similar unit problem with the velocities that are dumped. They really should be in angstroms/fs. But when I calculate temperature elsewhere using the initial velocities created by create gauss command to give 300.15K, I don’t get 300.15K but much high T.

When I calculate temperature, I use the following formula which is the same principle as for LAMMPS.

Ekin = ekfactor*(0.5*m*v^2)

T = 2*Ekin/(3*boltz)/n

I use eV for kinetic energy, A/fs for volume and amu for atom mass. K for temperature. And I have ekfactor for conversion which I calculated to be 103.64269. I calculate a much higher temperature for the same velocities but LAMMPS print 300K in the output.

I would appreciate if someone can clarify me the units in stress tensor and the velocity.

Thank you,

Burcu