Running FOCEX ============== .. FOrce Constant EXtraction (FOCEX) .. --------------------------------- .. role:: raw-math(raw) :format: latex html .. only:: html :math:`\\require{mediawiki-texvc}` Preliminaries ----------------- The input to FOCEX is a set of foce-displacement files, outputs of DFT calculation of supercells in which atoms have moved according to some scheme. There is a separate file for each supercell shape. Typically users use only one supercell, like 3x3x3, but you can generate more. The needed files by FOCEX are called ``POSCAR1`` and ``FORCEDISP1`` if there is only one supercell shape, otherwise you will also have ``POSCAR2`` and ``FORCEDISP2``, etc... These files are generated using the utility ``sc_snaps.x`` which has 3 input files. ``cell.inp`` contains the information on atoms in the primitive or conventional cell. ``supercell.inp`` contains the three translation vectors of the supercell in terms of those of the primitive cell as defined in ``cell.inp``. Finally in ``snaps.inp`` the user specifies the temperature for canonical sampling, a typical frequency scale and the number of required snapshots (typically, depending on the size of the supercell, and the number of required force constants 10 to 50 should be enough). 1-Run `sc_snaps.x` to generate the needed ``POSCAR`` files corresponding to each snapshot. They have the standard VASP format. After running VASP on each of these files, you collect a bunch of ``OUTCAR`` files, which you can rename ``outcar1``, ``outcar2``, ... ``outcarn``. 2-Run ``xtract.sh`` script to generate the ``FORCEDISP1`` file which will be used by FOCEX. The ``POSCAR1`` file contains the equilibrium atomic coordinates (force=0) in the supercell. Before running FOCEX you also need to prepare its other input files: ``default.params`` (you need it but don't have to touch it), ``structure.params``, ``dielectric.params`` containing the dielectric tensor and Born effective charges, ``kpbs.params`` containing the k-point paths for phonon band structure calculation, ``latdyn.params`` for density of states and thermodynamic properties calculations. Example are provided, and explanation of each line is in the file itself. The workflow of the FOCEX code is shown below to give a general understanding of how the code is structured and executed Workflow of FOCEX ----------------- The workflow of the FOCEX code is shown below to give a general understanding of how the code is structured and executed as a black box. Many of the intermediate steps in this process flow may even not be necessary to actually run the code. The one which are important are highlighted on the workflow diagram and are later explained in detail. .. image:: ./WorkFlow-FOCEX-Website.svg :width: 600 :align: center General steps for running FOCEX to get the force constants are given below: Description of input and output files ------------------------------------- * **Preparing FOCEX data files** FOCEX code accepts DFT force displacement files prepared by any software like VASP, or Quantum Espresso. We provide utility tools to extract forces and displacements in a supercell from VASP OUTCAR or Quantum Espresso output files. All the generated displacement-force data for each atomic snapshot are concatenated into a single file called FORCEDISP1. The equilibrium atomic positions in the supercell should be in a file called POSCAR1 which has the exact format of the VASP POSCAR file except atom type name. For example if the ``POSCAR`` file for Ge is this .. code-block:: python Ge8 1.0 5.6748542148543777 0.0000000000000000 0.0000000000000003 0.0000000000000009 5.6748542148543777 0.0000000000000003 0.0000000000000000 0.0000000000000000 5.6748542148543777 Ge 8 direct 0.0000000000000000 0.0000000000000000 0.5000000000000000 0.2500000000000000 0.2500000000000000 0.7500000000000000 0.0000000000000000 0.5000000000000000 0.0000000000000000 0.2500000000000000 0.7500000000000000 0.2500000000000000 0.5000000000000000 0.0000000000000000 0.0000000000000000 0.7500000000000000 0.2500000000000000 0.2500000000000000 0.5000000000000000 0.5000000000000000 0.5000000000000000 0.7500000000000000 0.7500000000000000 0.7500000000000000 Then ``POSCAR1`` file is this: .. code-block:: python Ge8 1.0 5.6748542148543777 0.0000000000000000 0.0000000000000003 0.0000000000000009 5.6748542148543777 0.0000000000000003 0.0000000000000000 0.0000000000000000 5.6748542148543777 8 direct 0.0000000000000000 0.0000000000000000 0.5000000000000000 0.2500000000000000 0.2500000000000000 0.7500000000000000 0.0000000000000000 0.5000000000000000 0.0000000000000000 0.2500000000000000 0.7500000000000000 0.2500000000000000 0.5000000000000000 0.0000000000000000 0.0000000000000000 0.7500000000000000 0.2500000000000000 0.2500000000000000 0.5000000000000000 0.5000000000000000 0.5000000000000000 0.7500000000000000 0.7500000000000000 0.7500000000000000 If there are other supercells, the equilibrium positions and displacement-force data should be stored in POSCAR2 and FORCEDISP2 ... etc. The detailed description of the ``FORCEDISP1`` file is given in the example section :ref:`example-focex` .. collapse:: utility tool To prepare the ``FORCEDISPi`` file there is a shell script inside ``utility`` folder. The shell script ``process_dft.sh`` executes the ``read_outcar.x`` and ``read_qe.x`` binary to generate force-displacement file for VASP and QE respectively. * **Preparing input files** The input file(s) required to run FOCEX are ``structure.params``, ``dielectric.params``, ``kpbs.params``, ``latdyn.params``, ``defaults.params``, ``POSCAR1`` and ``FORCEDISP1``. .. collapse:: structure.params something about structure.params file .. collapse:: dielectric.params something about dielectric.params file .. collapse:: kpbs.params something about kpbs.params file .. collapse:: defaults.params something about defaults.params file .. collapse:: latdyn.params something about latdyn.params file .. collapse:: POSCAR1 something about POSCAR1 .. collapse:: FORCEDISP1 The format of this file is : First line is a comment and should contain " # POSITION ...", second line should contain an integer followed by the total energy of the supercell snapshot in eV. Units for positions or displacements should be in Angstrom, and forces in eV/Ang. .. collapse:: kpbs.params The first line contains a flag. If 0, the kpoints are given in reduced units of the primitive vectors of the reciprocal space, else they should be in reduced coordinates of the conventional lattice vectors of the reciprocal space. The second line contains the number of kpoints along each direction, The third line contains the number of direction paths. The following lines contain the name of the special point followed by the 3 reduced components of the special kpoint in units of primitive (if flag=0) or conventional (if flag is non-zero) vectors of the reciprocal lattice. ``kpbs.params`` .. code-block:: python 30 # number of kpoints along each direction 4 # number of directions G 0 0 0 K 0.75 0.75 0 X 1 1 0 G 0 0 0 L 0.5 0.5 0.5 * **output files** * fc1.dat and fc2_irr.dat * fc2.dat and fc2_irr.dat * fc3.dat and fc3_irr.dat * lat_fc.xyz * log***.dat Example of Running FOCEX ----------------------- .. _example-focex: Ge ^^^^ FOrce Constant Extraction (FOCEX) is a code to extract force constants from force-displacements data, the output of which can be used as input to the following codes. The installation of FOCEX has to be done before using it and is given in section :ref:`focex-install`. This code, FOCEX (FOrce Constant EXtraction) included in ALATDYN (Anharmonic LATtice DYNamics) employs the force constant calculation, 2nd, 3rd and 4th order to be latter used for other thermodynamical properties. The installation of FOCEX is simple and just require the fortran compiler. An example of **Ge** is provided inside **FOCEX/example** which contains the needed input files described above, and the FOCEX output. In this calculation a single Ge atom in the 2x2x2 Ge supercell (64 atoms) is displaced by 4% to evaluate the forces. The force-displacement data is stored in ``FORCEDISP1`` file. The equilibrium coordinates are in ``POSCAR1`` in the older VASP POSCAR format as below. .. code-block:: python Ge8 # this is a comment 1.0 # scale factor 11.5257244110 0.0000000000 0.0000000000 # supercell 0.0000000000 11.5257244110 0.0000000000 0.0000000000 0.0000000000 11.5257244110 64 # number of atoms in the supercell Cartesian 2.881431103 0.000000000 2.881431103 2.881431103 0.000000000 8.644293308 2.881431103 5.762862206 2.881431103 2.881431103 5.762862206 8.644293308 8.644293308 0.000000000 2.881431103 8.644293308 0.000000000 8.644293308 8.644293308 5.762862206 2.881431103 8.644293308 5.762862206 8.644293308 1.440715551 1.440715551 1.440715551 1.440715551 1.440715551 7.203577757 1.440715551 7.203577757 1.440715551 1.440715551 7.203577757 7.203577757 7.203577757 1.440715551 1.440715551 7.203577757 1.440715551 7.203577757 7.203577757 7.203577757 1.440715551 7.203577757 7.203577757 7.203577757 2.881431103 2.881431103 0.000000000 2.881431103 2.881431103 5.762862206 2.881431103 8.644293308 0.000000000 2.881431103 8.644293308 5.762862206 8.644293308 2.881431103 0.000000000 8.644293308 2.881431103 5.762862206 8.644293308 8.644293308 0.000000000 8.644293308 8.644293308 5.762862206 1.440715551 4.322146654 4.322146654 1.440715551 4.322146654 10.085008860 1.440715551 10.085008860 4.322146654 1.440715551 10.085008860 10.085008860 7.203577757 4.322146654 4.322146654 7.203577757 4.322146654 10.085008860 7.203577757 10.085008860 4.322146654 7.203577757 10.085008860 10.085008860 0.000000000 0.000000000 0.000000000 0.000000000 0.000000000 5.762862206 0.000000000 5.762862206 0.000000000 0.000000000 5.762862206 5.762862206 5.762862206 0.000000000 0.000000000 5.762862206 0.000000000 5.762862206 5.762862206 5.762862206 0.000000000 5.762862206 5.762862206 5.762862206 4.322146654 1.440715551 4.322146654 4.322146654 1.440715551 10.085008860 4.322146654 7.203577757 4.322146654 4.322146654 7.203577757 10.085008860 10.085008860 1.440715551 4.322146654 10.085008860 1.440715551 10.085008860 10.085008860 7.203577757 4.322146654 10.085008860 7.203577757 10.085008860 0.000000000 2.881431103 2.881431103 0.000000000 2.881431103 8.644293308 0.000000000 8.644293308 2.881431103 0.000000000 8.644293308 8.644293308 5.762862206 2.881431103 2.881431103 5.762862206 2.881431103 8.644293308 5.762862206 8.644293308 2.881431103 5.762862206 8.644293308 8.644293308 4.322146654 4.322146654 1.440715551 4.322146654 4.322146654 7.203577757 4.322146654 10.085008860 1.440715551 4.322146654 10.085008860 7.203577757 10.085008860 4.322146654 1.440715551 10.085008860 4.322146654 7.203577757 10.085008860 10.085008860 1.440715551 10.085008860 10.085008860 7.203577757 Here, only the type of atom is not present in ``POSCAR1`` as compared to the new format of VASP POSCAR file. Similarly, ``FORCEDISP1`` is a force-displacement data format accepted by FOCEX code and its format for example in the case of Ge is given below. .. code-block:: python # POSITION (ang) TOTAL FORCE (eV/Ang) 1 -289.18629538 =t, Etot(eV) # snapshot #1 2.8929600000000000 0.0000000000000000 2.8814299999999999 -0.11758299999999999 -0.0000000000000000 -0.0000000000000000 2.8814299999999999 0.0000000000000000 8.6442899999999998 4.9600000000000002E-004 -0.0000000000000000 -0.0000000000000000 2.8814299999999999 5.7628599999999999 2.8814299999999999 4.9600000000000002E-004 -0.0000000000000000 -0.0000000000000000 2.8814299999999999 5.7628599999999999 8.6442899999999998 -4.5640000000000003E-003 -0.0000000000000000 -0.0000000000000000 8.6442899999999998 0.0000000000000000 2.8814299999999999 3.2899999999999997E-004 -0.0000000000000000 -0.0000000000000000 8.6442899999999998 0.0000000000000000 8.6442899999999998 -1.5699999999999999E-004 -0.0000000000000000 -0.0000000000000000 8.6442899999999998 5.7628599999999999 2.8814299999999999 -1.5699999999999999E-004 -0.0000000000000000 -0.0000000000000000 8.6442899999999998 5.7628599999999999 8.6442899999999998 2.6699999999999998E-004 -0.0000000000000000 -0.0000000000000000 1.4407200000000000 1.4407200000000000 1.4407200000000000 2.8677000000000001E-002 -1.9474999999999999E-002 1.9474999999999999E-002 1.4407200000000000 1.4407200000000000 7.2035799999999997 -4.8099999999999998E-004 -7.1400000000000001E-004 3.2800000000000000E-004 1.4407200000000000 7.2035799999999997 1.4407200000000000 -4.8099999999999998E-004 -3.2800000000000000E-004 7.1400000000000001E-004 1.4407200000000000 7.2035799999999997 7.2035799999999997 2.4350000000000001E-003 -4.0000000000000003E-005 4.0000000000000003E-005 7.2035799999999997 1.4407200000000000 1.4407200000000000 -1.8400000000000000E-004 -3.3000000000000000E-004 3.3000000000000000E-004 7.2035799999999997 1.4407200000000000 7.2035799999999997 8.3999999999999995E-005 -4.3999999999999999E-005 3.4000000000000000E-005 7.2035799999999997 7.2035799999999997 1.4407200000000000 8.3999999999999995E-005 -3.4000000000000000E-005 4.3999999999999999E-005 7.2035799999999997 7.2035799999999997 7.2035799999999997 -2.1599999999999999E-004 -3.4400000000000001E-004 3.4400000000000001E-004 2.8814299999999999 2.8814299999999999 0.0000000000000000 -4.0619999999999996E-003 7.2599999999999997E-004 -7.2599999999999997E-004 2.8814299999999999 2.8814299999999999 5.7628599999999999 -4.0780000000000000E-003 -7.3099999999999999E-004 -7.3099999999999999E-004 2.8814299999999999 8.6442899999999998 0.0000000000000000 -4.0780000000000000E-003 7.3099999999999999E-004 7.3099999999999999E-004 2.8814299999999999 8.6442899999999998 5.7628599999999999 -4.0619999999999996E-003 -7.2599999999999997E-004 7.2599999999999997E-004 8.6442899999999998 2.8814299999999999 0.0000000000000000 1.0700000000000000E-004 6.0000000000000002E-005 -6.0000000000000002E-005 8.6442899999999998 2.8814299999999999 5.7628599999999999 1.0300000000000000E-004 -5.7000000000000003E-005 -5.7000000000000003E-005 8.6442899999999998 8.6442899999999998 0.0000000000000000 1.0300000000000000E-004 5.7000000000000003E-005 5.7000000000000003E-005 8.6442899999999998 8.6442899999999998 5.7628599999999999 1.0700000000000000E-004 -6.0000000000000002E-005 6.0000000000000002E-005 1.4407200000000000 4.3221499999999997 4.3221499999999997 -4.8099999999999998E-004 3.2800000000000000E-004 -7.1400000000000001E-004 1.4407200000000000 4.3221499999999997 10.085010000000000 2.4350000000000001E-003 4.0000000000000003E-005 -4.0000000000000003E-005 1.4407200000000000 10.085010000000000 4.3221499999999997 2.8677000000000001E-002 1.9474999999999999E-002 -1.9474999999999999E-002 1.4407200000000000 10.085010000000000 10.085010000000000 -4.8099999999999998E-004 7.1400000000000001E-004 -3.2800000000000000E-004 7.2035799999999997 4.3221499999999997 4.3221499999999997 8.3999999999999995E-005 3.4000000000000000E-005 -4.3999999999999999E-005 7.2035799999999997 4.3221499999999997 10.085010000000000 -2.1599999999999999E-004 3.4400000000000001E-004 -3.4400000000000001E-004 7.2035799999999997 10.085010000000000 4.3221499999999997 -1.8400000000000000E-004 3.3000000000000000E-004 -3.3000000000000000E-004 7.2035799999999997 10.085010000000000 10.085010000000000 8.3999999999999995E-005 4.3999999999999999E-005 -3.4000000000000000E-005 0.0000000000000000 0.0000000000000000 0.0000000000000000 1.6290000000000000E-003 -7.2499999999999995E-004 1.4820000000000000E-003 0.0000000000000000 0.0000000000000000 5.7628599999999999 1.6290000000000000E-003 7.2499999999999995E-004 -1.4820000000000000E-003 0.0000000000000000 5.7628599999999999 0.0000000000000000 4.2000000000000002E-004 -5.8000000000000000E-005 -8.8500000000000004E-004 0.0000000000000000 5.7628599999999999 5.7628599999999999 4.2000000000000002E-004 5.8000000000000000E-005 8.8500000000000004E-004 5.7628599999999999 0.0000000000000000 0.0000000000000000 1.6550000000000000E-003 -7.3399999999999995E-004 -1.5030000000000000E-003 5.7628599999999999 0.0000000000000000 5.7628599999999999 1.6550000000000000E-003 7.3399999999999995E-004 1.5030000000000000E-003 5.7628599999999999 5.7628599999999999 0.0000000000000000 4.2000000000000002E-004 -5.7000000000000003E-005 8.8000000000000003E-004 5.7628599999999999 5.7628599999999999 5.7628599999999999 4.2000000000000002E-004 5.7000000000000003E-005 -8.8000000000000003E-004 4.3221499999999997 1.4407200000000000 4.3221499999999997 2.8958999999999999E-002 2.0014000000000001E-002 2.0014000000000001E-002 4.3221499999999997 1.4407200000000000 10.085010000000000 -4.8400000000000000E-004 7.0799999999999997E-004 3.3000000000000000E-004 4.3221499999999997 7.2035799999999997 4.3221499999999997 -4.8400000000000000E-004 3.3000000000000000E-004 7.0799999999999997E-004 4.3221499999999997 7.2035799999999997 10.085010000000000 2.4480000000000001E-003 4.0000000000000003E-005 4.0000000000000003E-005 10.085010000000000 1.4407200000000000 4.3221499999999997 -1.8000000000000001E-004 3.2899999999999997E-004 3.2899999999999997E-004 10.085010000000000 1.4407200000000000 10.085010000000000 7.7999999999999999E-005 3.4999999999999997E-005 2.8000000000000000E-005 10.085010000000000 7.2035799999999997 4.3221499999999997 7.7999999999999999E-005 2.8000000000000000E-005 3.4999999999999997E-005 10.085010000000000 7.2035799999999997 10.085010000000000 -2.1200000000000000E-004 3.4699999999999998E-004 3.4699999999999998E-004 0.0000000000000000 2.8814299999999999 2.8814299999999999 1.6290000000000000E-003 -1.4820000000000000E-003 7.2499999999999995E-004 0.0000000000000000 2.8814299999999999 8.6442899999999998 4.2000000000000002E-004 8.8500000000000004E-004 5.8000000000000000E-005 0.0000000000000000 8.6442899999999998 2.8814299999999999 1.6290000000000000E-003 1.4820000000000000E-003 -7.2499999999999995E-004 0.0000000000000000 8.6442899999999998 8.6442899999999998 4.2000000000000002E-004 -8.8500000000000004E-004 -5.8000000000000000E-005 5.7628599999999999 2.8814299999999999 2.8814299999999999 1.6550000000000000E-003 1.5030000000000000E-003 7.3399999999999995E-004 5.7628599999999999 2.8814299999999999 8.6442899999999998 4.2000000000000002E-004 -8.8000000000000003E-004 5.7000000000000003E-005 5.7628599999999999 8.6442899999999998 2.8814299999999999 1.6550000000000000E-003 -1.5030000000000000E-003 -7.3399999999999995E-004 5.7628599999999999 8.6442899999999998 8.6442899999999998 4.2000000000000002E-004 8.8000000000000003E-004 -5.7000000000000003E-005 4.3221499999999997 4.3221499999999997 1.4407200000000000 -4.8400000000000000E-004 -3.3000000000000000E-004 -7.0799999999999997E-004 4.3221499999999997 4.3221499999999997 7.2035799999999997 2.4480000000000001E-003 -4.0000000000000003E-005 -4.0000000000000003E-005 4.3221499999999997 10.085010000000000 1.4407200000000000 2.8958999999999999E-002 -2.0014000000000001E-002 -2.0014000000000001E-002 4.3221499999999997 10.085010000000000 7.2035799999999997 -4.8400000000000000E-004 -7.0799999999999997E-004 -3.3000000000000000E-004 10.085010000000000 4.3221499999999997 1.4407200000000000 7.7999999999999999E-005 -2.8000000000000000E-005 -3.4999999999999997E-005 10.085010000000000 4.3221499999999997 7.2035799999999997 -2.1200000000000000E-004 -3.4699999999999998E-004 -3.4699999999999998E-004 10.085010000000000 10.085010000000000 1.4407200000000000 -1.8000000000000001E-004 -3.2899999999999997E-004 -3.2899999999999997E-004 10.085010000000000 10.085010000000000 7.2035799999999997 7.7999999999999999E-005 -3.4999999999999997E-005 -2.8000000000000000E-005 # POSITION (ang) TOTAL FORCE (eV/Ang) 1 -289.18629538 =t, Etot(eV) # snapshot #2 2.8929600000000000 0.0000000000000000 2.8814299999999999 -0.11758299999999999 -0.0000000000000000 -0.0000000000000000 2.8814299999999999 0.0000000000000000 8.6442899999999998 4.9600000000000002E-004 -0.0000000000000000 -0.0000000000000000 ... The first line in ``FORCEDISP1`` is the header and should contain the word POSITION. The second line consists of the energy of structure in electron volt and the lines after second are positions (first three columns, :math:`x`, :math:`y` and :math:`z`) and forces (the last three columns :math:`F_x`, :math:`F_y` and :math:`F_z` are in :math:`eV/{\\A}`) respectively. If there are many force-displacement snapshots of the structure, then the other snapshots follow these lines in the same format. There is a tool and a shell script for converting the VASP outcar or QE outputfile into the FORCEDISP1 format for the FOCEX code. It is available in **utility** folder inside FOCEX. To convert the VASP outcar into FORCEDISP1 user need to execute ``./process_dft.sh name_of_vasp_directory(s)`` or ``./process_dft.sh name_of_vasp_file`` shell script within utility folder. Here, ``name_of_vasp_directory(s)`` is the multiple directory containing VASP runs or QE runs or ``name_of_vasp_file`` is the OUTCAR file(s) for VASP or the outputfile for QE runs. This shell script will call the ``readoutcar.x`` or ``readpfpwscf.x`` taking as input the DFT output from VASP or QE. The shell script can be tailored as per your needs. As for the other inputs file of FOCEX, they are given below ``structure.params`` .. code-block:: python 1 1 1 90 90 90 # a, b, c, alpha, beta, gamma of the conventional cell 0 .5 .5 .5 0 .5 .5 .5 0 # reduced coordinates of primitive lattice (in this case FCC) in terms of conventional lattice (in this case cubic) 5.7628622055 # scale factor for lattice size 1 1 1 1 # include FC1234, 1st, 2nd, third and fourth order harmonic force constant(s) in the fitting process. 1 is to include and 0 is to not include 1 1 0 0 # invariances to impose, (translational, rotational, Huang) last is enforce inv using elimination 0 300 # temperature and whether or not implement it (do not implement if 0,2, or ..) 1 .true. # number of FORCEDISPi files, verbosity 1 # type of atoms 72.64 # masses of each type of atoms Ge # names of atoms 2 # number of atoms of each type in the primitive cell 1 # flag for setting the range of FC2s (if 0 take default; else use below) 5 5 # number of shells for rank 2 (harmonic) for each atom if not default 1 1 # number of shells for rank 3 (cubic) for each atom (there is no default value) 1 1 # number of shells for rank 4 (quartic) for each atom (there is no default value) 1 1 0 0 0 # atom index, type of atom, position x, position y, position z in units of a,b,c of the conventional lattice 2 1 0.25 0.25 0.25 # atom index, type of atom, position x, position y, position z in units of a,b,c The fitting is done using singular value decomposition based on the requested symmetry constraints and ``POSCAR1`` and ``FORCEDISP1`` i.e. by creating the force displacement matrix. The next input file is ``dielectric.params``. It is required to get the phonon dispersion (and eventually thermal conductivity using ``THERMACOND``). It consists of a flag which is zero if the Born charges are to be excluded. The second, third and fourth lines contain the dielectric constant tensor values which is written as follows in the example folder inside ``FOCEX`` ``dielectric.params`` .. code-block:: python 0 # do not include Born charges in the fitting 2.5078 0.0 0.0 # for example, dielectric constant (necessary but not used if flag is zero) 0.0 2.5078 0.0 0.0 0.0 2.5078 0 0 0 # Born charge tensor of atom 1 in order it appears in the ``structure.params`` 0 0 0 0 0 0 0 0 0 # Born charge tensor of atom 2 in order it appears in the ``structure.params`` 0 0 0 0 0 0 Now, put the ``POSCAR1``, ``FORCEDISP1`` , ``structure.params``, ``dielectric.params`` and ``kpbs.params`` in same directory, simply run ``focex.x``. After successful run ``fc2.dat``, ``fc2_irr.dat``, ``fc3.dat``, ``fc3_irr.dat``, ``fc4.dat`` and ``fc4_irr.dat`` along with other output files and log file should be available. ``fc2.dat``, ``fc2_irr.dat`` are the fitted second order force constants in eV/Ang^2. The former contains all the pairs regardless of symmetry, and the latter contains the irreducible ones from which all the rest is contructed using crystal symmetry. Likewise ``fc3.dat``, ``fc3_irr.dat`` and ``fc4.dat``, ``fc4_irr.dat`` contain the third order and fourth order full and irreducible force constants in eV/Ang^3 and eV/Ang^4 respectively. Users are advised to look for more information in the log file ``log***.dat`` to know more details about the run.