# Gradients calculation and search for critical points:ΒΆ

The script make_TDS_Gradients calculates the inverse gradients of the phonon dispersion relation in a specified energy window and allows for the search of critical points. The following steps must have been performed beforehand:

- Symmetrysation
- Fourier Interpolation
- Debye-Waller factors for the temperature of choice

The usage is

make_TDS_Gradients castep_filename input_filenamethe file

castep_filenamecan be either the name of the original castep output our the associated hdf5 file. In any case the associated hdf5 file must exist already.The input_file must set the variables:

- Mandatories

- subN1
- subN2
- subN3
- N1
- N2
- N3
- Nfour_interp
- Temperature
- Lambda
- Optional

- branchWeight
- energyWindow
- outputname
The inverse gradients of each phonon branch are written to individual .h5 and .ccp4 files. The auxilary script find_maxima.py can be used to filter the results and search for maximas,

`make_TDS_Gradients.`

`subN1`

= 20Subdivision, in the elementary brillouin zone, along the Z dimension of the reconstructed volume, which is parallel to the first axis of the brilloin zone.

`make_TDS_Gradients.`

`subN2`

= 20Subdivision, in the elementary brillouin zone, along the Y dimension of the reconstructed volume, which is parallel to the second axis of the brilloin zone.

`make_TDS_Gradients.`

`subN3`

= 20Subdivision, in the elementary brillouin zone, along the X dimension of the reconstructed volume, which is parallel to the third axis of the brilloin zone.

`make_TDS_Gradients.`

`N1`

= 4Besides being subdivised, the Brillouin zone, is replicated alonx the first axis for -N1 up to N1-1 included. In reciprocal cell vector units. N1 can be either an integer or a pair of integer. In the latter case the first element is the starting shift, the second is the ending shift+1.

`make_TDS_Gradients.`

`N2`

= 4same thing as N1 but along the second reciprocal cell vector

`make_TDS_Gradients.`

`N3`

= 4same thing as N1 but along the third reciprocal cell vector

`make_TDS_Gradients.`

`Nfour_interp`

= 4The number of points in each direction of the 3D reciprocal grid. The hdf5 file must contain a previous pretreatement done with the same parameter.

`make_TDS_Gradients.`

`Lambda`

= 0.2For X-ray scattering : the wavelenght in Angstroems.

`make_TDS_Gradients.`

`outputname`

= Nonea name to design the output file with data from calculation considering branchWeight or energyWindow

`make_TDS_Gradients.`

`branchWeight`

= Nonea list of weights : one per branch.

`make_TDS_Gradients.`

`energyWindow`

= Nonea list of two numbers : minimum and maximum in meV