[Pw_forum] calculations in finite electric field

[Bo Xu]

Hello QE users:

I want to do calculations for the systems in the finite electric field, just similar to example30. However, when I tested the example30, I found some problems.
1.	When I use the input file “mgo.cp.cg.efield.in” to calculate the system’s properties keeping the position of the atoms fixed in the experimental equilibrium positions in the presence of an electric field E of 0.001 a. u. along the 3rd direction, the forces on the atoms are difference from those in “/emample30/reference/mgo.cp.cg.efield.out”.
For the calculated results (should be found in the “/tmp/mgo.for” file), the force on Mg atom in the z direction is 0.1955E-02 a.u., which is almost the same at that in “/emample30/reference/mgo.cp.cg.efield.out”, whereas the force on O atom in the z direction is -0.1096E-02 a.u., which is about half of that in “/emample30/reference/mgo.cp.cg.efield.out”. The forces on Mg and O atoms in the z direction are 0.1955E-02 and -0.2040E-02 a.u. in “/emample30/reference/mgo.cp.cg.efield.out”. So, I just want to know why there is so large difference between the calculated results and the reference results.
2.	Another problem is that the masses of O atom in “mgo.cp.cg.in”, “mgo.cp.cg.efield.in”, “mgo.cp.damp.efield.in” are 12.01, 12.01, and 1, respectively. Is that wrong?
I would appreciate if someone can response for this.

Regards,
Xu Bo