Analyze forces

You must prepare a working directory for analyzing phonon property. The files needed in your working directory are a setting file (INPHON), a unit cell position file (POSCAR) and a force information file (FORCES). INPHON and FORCES are newly prepared though POSCAR must be same as used previous step. In VASP case, we can derive forces from vasprun.xml or OUTCAR file. FORCES is like as follows,

	6
	1   0.001267   0.000000   0.000000
	 -0.09791747     0.00000000    -0.06166513
	  0.03115951     0.00000000    -0.06039806
	  0.00748203     0.00000000     0.07084983
	1   0.000000   0.002535   0.000000
	  0.00000000    -0.06411762    -0.06150771
	  0.00000000    -0.00036659    -0.06033161
	  0.00011170     0.00637913     0.06042420
	1   0.000000   0.000000   0.002067
	  0.00000000     0.00000000    -0.19703426
	  0.00000000     0.00000000    -0.06497917
	  0.01041537     0.00000000     0.08319831
	  ....

The first line is the number of calculated supercells. In the second line, first column is the number of the displaced atom. The second to fourth columns are the displacement amount for each axis in reduced coordinates in supercell. The third to fifth lines are forces for each axis on atoms. The order follows the position file and the unit is free. In VASP case, the native output is eV/Å.

Then you must prepare a setting file (INPHON). An example is as follows,

	MASS =  207.200 16.000
	NDIM = 2 2 2
	ND = 1
	NPOINTS = 51
	QI = 0.0 0.0 0.0
	QF = 0.5 0.0 0.0
	EIGENVECTORS = .TRUE.

By this setting, fropho outputs the band structure from (0, 0, 0) to (0.5, 0, 0) in the reciprocal space. The mass of two atoms are 207.2 and 16.0, respectively. The unit of mass is free. In this case, the unit is atomic mass. You must prepare POSCAR file which is same as that used in the first step. These are minimum setting to analyze. Type,

	% fropho < INPHON
   

If the fropho finishes without error, you have several output files of FORCE_CONSTANTS, QPOINTS, EIGENVECTORS, BAND and frophorun.yaml by this setting. FORCE_CONSTANTS file contains the force constants calculated by fropho. QPOINTS file contains the q-points in reduced coordinates where the dynamical matrices are solved. EIGENVECTORS file contains the eigenvectors for each eigenvalue. BAND file appears only when the band structure analysis is expected. This file contains the frequencies at each q-point. You can draw the band structure by using this file. frophorun.yaml file contains the most information in YAML [14] syntax.