Electron units documentation issue

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1

Hi everybody,

I found the following description for the electron units in the LAMMPS doc.

For style electron, these are the units:

  • mass = atomic mass units
  • distance = Bohr
  • time = femtoseconds
  • energy = Hartrees
  • velocity = Bohr/atomic time units [1.03275e-15 seconds]
  • force = Hartrees*Bohr
  • temperature = degrees K
  • pressure = Pascals
  • charge = multiple of electron charge (+1.0 is a proton)
  • dipole moment = Debye
  • electric field = volts/cm

I assume that there is a typo in the force units (It should be Hartrees/Bohr, I think) but what I don’t understand is
where the “1.03275e-15 seconds” comes from since 1 atomic time unit is 2.418842605e-17 seconds.

Is the time unit really 1 femtosecond?

Thank you for all your work.

Cheers.

Pablo.

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2

yes, the * should be a “/” - I’ll change it.
I’ll leave it to Andres to answer the time Q.

Steve

3

Pablo,

when selecting the “electron” units in EFF the code uses atomic units internally, but with the modification that the unit mass is taken to be one atomic mass unit (i.e. 1/12 of the mass of an unbound neutral atom of carbon-12) and it is scaled by m_e^(-1/2), where m_e corresponds to the dynamic electron mass that relates the forces on the electron wave packets to the rate of change of their translational and radial velocities.

Thus the internal time unit (1 atu) is

dt = [hbar/Eh]*sqrt(amu/me) = 1.03275e-15 s (close to 1fs)

Where the bracketed term (1st) corresponds to the standard atomic unit of time, i.e. 2.418884326502e-17. This time conversion factor is applied internally to allow true femtosecond dynamics time steps.

This enables m_e to be modified by the user to reproduce some characteristic known time-scale of electron motion in the system being modeled/simulated. Keep in mind that m_e is separate from the electron mass present in the kinetic energy term in Schrodinger’s equation (which is fixed and corresponds to the true electron mass), and changing m_e does not affect electron sizes or bonding in the ground state (i.e. chemistry).

Andres

4

Hi Steve,

I just noticed that the last example in the pair_style eff/cut command description (pair_eff.html) is missing the ecp type label, i.e.

pair_coeff 3 22.721015 0.728733 1.103199 17.695345 6.693621

should be

pair_coeff 3 p 22.721015 0.728733 1.103199 17.695345 6.693621

Thanks and best,

Andres