How to covert the renormalized force constant file xxx.xml at a finite temperature to the format xxx.fcs

[LiuHaoXuan0403]

Hi Dr. Tadano,
I try to use ALAMODE to compare the renormalized force constant with the harmonic constant. Could you please give me some advice on how to convert the xxx.xml file to xxx.fcs file?

[LiuHaoXuan0403]

Hello Dr. Tadano
Could you please guide me what is the unit of FC2 in of xxx.xml. I want to compare the renormalized force constant at different temperature. Could you please give me some advice?

[gangliu-github]

Hi, I also want to change xml file to FORCE_CONSTANTS file for phonopy. Do you clear the format of xml file?

[LiuHaoXuan0403]

The is same as XXX.fcs. But the pair1, pair2 and the unit in is not clear. That's all I know

[gangliu-github]

Where can I found the format of xxx.fcs?

[ttadano]

The unit of force constants in *.xml is Ry/bohr^n for the n-th order force constant. The same unit is used also for the values in *.fcs.

Only irreducible sets of force constants are shown in *.fcs, where complete set of force constants are save in *.xml. To convert force constants in *.xml into the *.fcs format, we need to recover the information of force constant symmetry, so it would be quite tedious to achieve.

If you want to compare the difference between force constants in two different *.xml files, I recommend that you parse the values under <HARMONIC> using python, store them in a dictionary or Pandas dataframe, and then compare by taking a diff.

The meaning of pair1 and pair2 is a bit complicated:

• pair1 = "atom1 xyz1"

  atom1 is the index of atom in a primitive cell, and xyz1 labels the xyz component (1: x, 2: y, 3: z).

• pair2 = "atom2 xyz2 icell"

  atom2 is the index of atom in the supercell, and xyz2 labels the xyz component.

  icell is additional index necessary to judge which periodic image of the supercell atom2 is located.
  Specifically, the icell is labelled as follows:
  1: ( 0, 0, 0)
  2: (-1, -1, -1)
  3: (-1, -1, 0)
  4: (-1, -1, +1)
  5: (-1, 0, -1)
  ....
  27: (+1, +1, +1)

[ttadano]

About converting the format into FORCE_CONSTANTS of phonopy, please see #192.

[LiuHaoXuan0403]

Hello, Tadano,

I get it. Thank you very much.

Best regards,