Spectral function calculations with lmgf

Table of Contents

Preliminaries


Spectral function was implemented in lmgf v7.10 by Bhalchandra Pujari and Kirill Belashchenko (belashchenko@unl.edu). The details of the theory in the CPA case can be found in

I. Turek, V. Drchal, J. Kudrnovský, M. Šob, P. Weinberger. Electronic structure of disordered alloys, surfaces and interfaces. Kluwer academic publishers (1997), see Eqs. (4.25)-(4.29)

How to calculate spectral functions?


The spectral function can be calculated both with and without CPA. The calculation is performed in 3 steps:

  • Charge self consistency. (The spectral function can be calculated for any potential, but it is usual to work with the self-consistent one).
  • CPA self-consistency in the coherent interactor Ω\Omega (CPA only). Since Ω\Omega is energy dependent, it has to be calculated for the energy points where the spectral function is needed. For drawing spectral functions this is usually a uniform mesh of points close to the real axis.
  • Calculation of the spectral function on some contour, usually a uniform mesh close to the real axis.

Charge self consistency is performed in the usual manner, for example with the following options:

BZ       EMESH=31 10 -.9 0 .5 .0
GF       MODE=1 DLM=12 GFOPTS=p3;omgmix=1.0;padtol=1d-3;omgtol=1d-5;lotf;nitmax=50

Note the EMESH mode (contour type) is elliptical (type 10). If CPA is used, the coherent interactors Ω\Omega for all CPA sites are also iterated to self-consistency during this calculation, but this is done for the complex energy points on the elliptical contour. The following additional step is needed in this case to obtain self-consistent Ω\Omega at those points where the spectral function will be calculated.

Omega self-consistency is turned on by setting DLM=32. In this mode only Ω for each CPA site is converged, while the atomic charges are left unchanged. It is important to converge Ω to high precision. Typically omgtol=1d-6 is a good criterion. The contour type should be set to 2. Example input for this step is

BZ       EMESH=150 2 -.25 .25  .0005  0
GF       MODE=1 DLM=32  GFOPTS=p3;omgmix=1.0;padtol=1d-3;omgtol=1d-6;lotf;nitmax=50

The highlighted parameters are of particular importance. lotf is required to iterate Ω\Omega for convergence (and it is recommended to keep it enabled in all CPA calculations, including charge self-consistency). It is also necessary to monitor the output file (set --pr41) and make sure that the required precision has been achieved for all energy points. If convergence appears to be problematic, try to start with a larger imaginary part for the complex energy or reduce the mixing parameter omgmix.

Calculation of the spectral function should be done with EMESH set to the same mesh as used for Ω\Omega self-consistency, e.g.

BZ       EMESH=150 2 -.25 .25  .0005  0
GF       MODE=1 DLM=12 GFOPTS=p3;omgmix=1.0;padtol=1d-3;specfun

Important: If there are sites treated in CPA, the contour specified by EMESH should be kept exactly as in the previous step when CPA self-consistency was performed.

In order to start the calculation, invoke lmgf with the --band flag referring to the symmetry-line file (same format as used for band structure calculation with lm):

lmgf «sys» --band:fn=syml 

where «sys» is the extension of the ctrl file. Once completed, the program will generate a spf.«sys» file containing the complete spectral function along the lines given in the syml.«sys» file. Other options included with --band are currently not used.

Plotting of the spectral function


Upon successful completion of calculation spf.«sys» file will be generated. The file has the following format:

ZP        KP         SF_UP         SF_DN
.
.
-0.250   0.137      5.708839      4.742153
-0.250   0.160     10.658114      4.844647
.
.

where ZP are the points on the energy contour, KP are the points of the K-mesh and SF_UP, SF_DN are the spectral functions for Up and Down channel respectively. The very first line of the file indicate the location of the high symmetry points of the Brillouin zone. User can utilize this information to visualize the spectral function using any desired graphics package. A small bash utility, SpectralFunction.sh, is given for the sake of convenience. This bash script uses Gnuplot to view and save the spectral function. Users with standard Linux/unix distro should be able to use it without special prerequisites. Typical output of the script is shown in the figure. Please see SpectralFunction.sh -h for usage. (The file spf.«sys» should be renamed specfun.«sys» to be read by this script. See below why.)

Site-resolved spectral functions


Site-resolved spectral functions can be obtained by including the option specfr in GFOPTS. (It supersedes the option specfun.) A series of output files will then be generated, which contain the spectral function on each basis site . The file names are spfN.«sys», where N is the index of the site. The format of this file is the same as for spf.«sys», and it can be also plotted using the SpectralFunction.sh script (first rename to specfun.«sys»).

Notes


  • Spectral function calculations can be run with MPI parallelization.

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