[Wien] Questions about forces on nuclei in the presence of spin orbit coupling

[Zhiyong Zhu]

Dear Prof. Blaha,

I have two other questions:

>>* 4) If the forces calculated with spin-orbit coupling are indeed
incorrect and meaningless,
*>>* can we use PORT option in case.inM when doing atomic relaxation
(min_lapw)? This
*>>* question arises because I read in the wien2k user guide that "It
(PORT) minimizes the
*>>* total energy and NOT the forces (using the forces as derivative of E
vs. atomic positions).".
*>>* As I understand, the PORT method will use "total energy", instead of
"force on nuclei",
*>>* as a criterion to find the equilibrium positions of atoms, i.e., the
PORT method will find
*>>* an energy minimum by a real displacement of atoms, rather than by find
a structure in
*>>* which forces on atoms are zero. This means that the forces on nuclei
is NOT the essential
*>>* ingredient in the PORT method, and PORT method is still valid (as long
as the total
*>>* energy are calcuated correctly) even in the presence of spin-orbit
coupling. Is my
*>>* ratiocination correct?
*
> No, you can't use PORT. PORT uses the "gradiant of E-tot", i.e. the
forces in order to move
> atoms "in the proper direction" and if they do not fit with the resulting
E-tot, it will give up.
===============================================
Take a material, in which only one internal freedom x exists, as an example.
If I do a series of total energy calculations with difference x and find a
energy surface with respect to x, then I can obtain a force by taking a
energy derivative with respect to x. Is the force obtained by this method
meaningful (even in the presence of spin orbit coupling)?


> PS: As mentioned in UG, you can switch off SO for light atoms (eg.
oxygens), and then the forces for
> those atoms are still ok and meaningful.
====================================================
Does this mean that the spin-orbit term of Pulay correction for the
investigated atom, eg. oxygen, comes from the oxygen itself only,
and is irrelevant to those atoms around the oxygen?

Thank you very much.

Best Regards,
Zhiyong Zhu
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