Electric Fields and Polarisation

In equation (91) the force $\mathbf F$ may also come from an electric field $\mathbf E_{\rm el}$, i.e.

  $$\mathbf F = q \mathbf E_{\rm el} a_0$$ (96)

The Bohr radius $a_0$ is included in order to yield $\mathbf F$ in units of meV.

The resulting electric polarisation $\mathbf P_{\rm el}$ can be calculated by mcphas and is stored in output file results/mcphas.fum. This electric polarisation refers to the orientational polarisation generated by moving ions from their equilibrium positions. It does not include the molecular electric dipole moment induced by the electric field.

  $$\mathbf P_{\rm el} = q_i \langle \mathbf \Delta r_i \rangle$$ (97)

Note that in addtion there may be a static polarisation $\mathbf P_{\rm el0}$(i.e. electret matter), which is calculated using the Wigner Seitz unit cell of the primitive lattice and summing the dipolar moment contributions of all charges, this static polarisation is output in the header of results/mcphas.fum.

  $$\mathbf P_{\rm el0} = q_i \langle \mathbf r_i \rangle$$ (98)