Exchange coupling parameter

To analyze an effective interaction between spins located on two atomic sites, an exchange coupling parameter between two localized spins can be evaluated based on Green's function method [16,17]. In OpenMX Ver. 3.8 the evaluation is supported for only the collinear calculations of cluster and bulk systems. Also the MPI parallelization of 'jx' is supported only when the eigenvalue solver is 'band', while the parallelization is not supported for 'cluster'. If you want to calculate the exchange coupling parameter between two spins which are localized to different atomic sites, you can calculate it by the following two steps:

(1) SCF calculation

First, you would perform a collinear DFT calculation using an input file 'Fe2.dat' in the directory 'work' as an example. Then, you have to set the following keyword 'HS.fileout' as follows:

    HS.fileout                   on       # on|off, default=off

When the execution is completed normally, then you can find a file 'fe2.scfout' in the directory 'work'.

(2) Calculation of exchange coupling parameter

Let us compile a program code for calculating the exchange coupling parameter. Move the directory 'source' and then compile as follows:

    % make jx
  

When the compile is completed normally, then you can find an executable file 'jx' in the directory 'work'. The exchange coupling parameter can be calculated from the file 'System.Name.scfout' using the program 'jx' as follows:

    % ./jx fe2.scfout
  

where an iron dimer is considered as an example. Then, you are interactively asked from the program as follow:

********************************************************************
********************************************************************
 jx: code for calculating an effective exchange coupling constant J 
 Copyright (C), 2003, Myung Joon Han, Jaejun Yu, and Taisuke Ozaki 
 This is free software, and you are welcome to         
 redistribute it under the constitution of the GNU-GPL.
********************************************************************
********************************************************************

Read the scfout file (fe2.scfout)
 Previous eigenvalue solver = Cluster
 atomnum                    = 2
 ChemP                      = -0.108015991530 (Hartree)
 E_Temp                     = 600.000000000000 (K)
 
Evaluation of J based on cluster calculation
 Diagonalize the overlap matrix
 Diagonalize the Hamiltonian for spin= 0
 Diagonalize the Hamiltonian for spin= 1

 Specify two atoms (e.g 1 2, quit: 0 0)  1 2
 J_ij between 1th atom and 2th atom is 848.136902053845 cm^{-1}
 Specify two atoms (e.g 1 2, quit: 0 0)  2 1
 J_ij between 2th atom and 1th atom is 848.136902053844 cm^{-1}
 Specify two atoms (e.g 1 2, quit: 0 0)  0 0

Please specify two atoms you want to calculate the exchange coupling parameter until typing '0 0'.