[Pw_forum] unit of the polarization

[alan chen]

Dear PWSCF users,
     I have a question about the polarization unit which has confused me for
a long time.
     The atomic unit of electric field is: e/a0^2 where 'e' is the electron
charge and a0 is the Bohr radius. The unit of polarization (i.e. dipole per
unit volume) should have the same unit as electric field. However from the
source code c_phase_field.f90(version4.0.2), the output of dipole per cell
has a factor sqrt(2) which is the electron in the Ry units. So 'e' becomes
sqrt(2).
    Then if I want to calculate P/E which is dimensionless, shall I convert
the output of P into P/sqrt(2). Note when we input the electric field E, E
is in the atomic unit, i.e. 'e' is not converted into sqrt(2).
    I am just confused that since you use atomic unit, why you explicitly
convert 'e' into sqrt(2) rather than implicitly retain it in the unit?

Hanghui Chen
Department of Physics,
Yale University
-------------- next part --------------
An HTML attachment was scrubbed...
URL: <http://lists.quantum-espresso.org/pipermail/users/attachments/20081009/c3e30521/attachment.html>