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 ABOUT
=======

                  << Welcome to ElaStic_1.0 >>

This software package allows you to obtain full second-order elastic tensors (elastic constants)
for all crystal lattice types and third-order elastic constants for cubic, hexagonal, and 
rhombohedral crystals. 
This code is written in Python programming language and uses the SGROUP tool as symmetry and 
space-group calculator and utilizes Exciting, Wien2k and Quantum Espresso density-functional codes 
for energy and stress calculations.

 DESCRIPTION
=============

     In order to use "ElaStic" software package, please follow the steps below:
STEP 0.1:
     The "ElaSticROOT" shell variable must be set by the user. For instance, add the following 
     line with the correct "ElaStic" path, in the ".bashrc" file:
     export ElaSticROOT=/home/dir1/dir2/ElaStic

STEP 0.2:
     Download and install "SGROUP" program:
     This program is a utility which allows "ElaStic" to determine the space-group and symmetry 
     operations of a crystal structure. More information can be found in: 

     Programmers       : B.Z. Yanchitsky, A.N. Timoshevskii
     Download Link     : http://cpc.cs.qub.ac.uk/summaries/adon
     Journal Reference : Comput. Phys. Commun. 139(2001)235

     You can also find "SGROUP" program in the file "adon_v1_0.tar.gz" in "ElaStic"
     directory: Go to the "ElaStic" directory or where you saved this zipped file and execute the 
     following commands ($ means command line in a terminal):

     $ tar xfvp adon_v1_0.tar.gz
     $ cd SpaceGroups
     $ make
     $ cp sgroup $ElaSticROOT

     You can find more information about "SGROUP" program in "SpaceGroups/README" file.

STEP 1.1:
     In order to generate input files for a series of distorted structure, run the following script 
     from the command line:

     $ ElaStic_Setup

     This program requires a structural input file which can be the same as the input file of the 
     following DFT codes:

     exciting         (http://exciting-code.org/)
     WIEN2k           (http://www.wien2k.at/)
     Quantum ESPRESSO (http://www.quantum-espresso.org/)

     Once the input file is ready, "ElaStic" requires these values:

     -The order of the elastic constants which you are interested in calculating.
     -The absolute value of the maximum strain.
     -Number of deformed structures equally spaced between the maximum negative strain and the 
      maximum positive one.
     
     Entries must be typed on the screen when they are requested.

STEP 1.2:
     Calculate the energy or stress tensor of deformed structure using one of the mentioned DFT 
     codes.

     NOTICE to "WIEN2k" users:
     A shell script program is provided under the name "ElaStic_WIEN2k_init" which allows you 
     to initialize the deformed structures automatically.
     Before executing this program, you must specify input variables of "WIEN2k" calculations.
     It is recommended to copy this file to the directory where "Dst??" directories, e.g. Dst01, 
     exist. Then, modify line 23 of this program according to the accuracy of your calculations.

     The "WIEN2k_init_lapw_silent" script is the modified script file of the "init_lapw" program.
     You find FLAGS of "WIEN2k_init_lapw_silent" program if you write following command in terminal:

     $ WIEN2k_init_lapw_silent -H

     Once the WIEN2k calculations are finished, you can run the following scripts to save and clean
     your calculations.
    
     $ ElaStic_Save_Clean_WIEN2k

     NOTICE to "Quantum ESPRESSO" users:
     The output file must be saved under same name of the input file but with ".out" extension. 
     For instance, if a given input file is named as "Dst02_25.in", the output file must be saved
     as "Dst02_25.out".

STEP 2.1:
     In order to calculate elastic constants accurately, you have to analyze your calculations,
     first.
     The script "ElaStic_Analyze" allows you to analyze the dependence of the calculated 
     derivatives of the "energy-vs-strain" or "energy-vs-stress" curves on the two following
     parameters:

     -The range of distortions.
     -The degree of the polynomial fit.

     The script can be run giving the following command in terminal:

     $ ElaStic_Analyze
     
     At this point, "xmgrace" plots automatically appear on your screen. For each distortion,
     you can find the best distortion range and the best order of polynomial fit. You can find more
     details about these issues in "ElaStic_Paper.pdf".

STEP 2.2:
     In order to obtain the numerical values of the second- and third-order elastic constants,
     you should edit the "ElaStic_2nd.in" and "ElaStic_3rd.in", respectively.
     
     In each row of these files, you should insert a value for "eta_max" and "Fit_order". According 
     to the result of the visual analysis of the figures in the previous step, these two values are
     extracted by considering the plateau regions of the corresponding plot: "eta_max" is a value 
     of maximum distortion located in the plateau region of the curve representing the polynomial
     fit of order "Fit_order".

STEP 3.0:
     At this stage, in order to calculate elastic constants and moduli, run the following script:

     $ ElaStic_Result

     Numerical values of the elastic constants are output in "ElaStic_2nd.out" and
     "ElaStic_3rd.out" files for the second- and third-order elastic constants, respectively.

 POST-PROCESSING PROGRAMS
==========================

     1- "ElaStic_xyz2XYZ" is a program to transform elastic constants from one Cartesian coordinate
     to another Cartesian systems.
     In order to transform elastic constant tensor from an old Cartesian coordinate system to a new
     one, "ElaStic" requires two input files:

     i - The output file of "ElaStic" (ElaStic_2nd.out).
     ii- A file with "Transformation_Matrix" name. This file contains 3X3 matrix in following
     format:
                              i'.i   i'.j   i'.k
                              j'.i   j'.j   j'.k
                              k'.i   k'.j   k'.k
     where i', j' and k' are the unit vectors in the new system and i, j and k are the
     unit vectors in old Cartesian coordinate systems. Dot (.) indicates the "dot product" of two 
     vectors.

     2- "ElaStic_Analyze_B0" is a program to check the dependence of the bulk modulus (B0) and its 
     pressure-derivative (B0') on the range of distortion and the degree of the polynomial fit
     parameters.
     This program requires an input file, with an arbitrary name, which contains volume and energy 
     value in two columns. 

 REFRANCE PAPER
================

ElaStic: A tool for calculating second-order elastic constants from first principles
Computer Physics Communications 184, 1861-1873 (2013).
This article contains method, the numerical algorithm and results which are calculated with ElaStic.
In case of publication, if you used ElaStic for any purpose, please cite this reference. 

 CONTACT
=========

If you have any suggestion or you found any bug, please write to  

E-mail: [email protected]

Rostam Golesorkhtabar
Humboldt-Universität zu Berlin
Institut für Physik und IRIS Adlershof, Theoretische Festkörperphysik
Zum Grossen Windkanal 6, 12489 Berlin, Germany.