The ASA Green's function program lmgf

Table of Contents


Purpose


This package implements the ASA local spin-density approximation using Green’s functions. The Green’s functions are contructed by approximating KKR multiple-scattering theory with an analytic potential function. The approximation to KKR is essentially similar to the linear approximation employed in band methods such as LMTO and LAPW. It can be shown that this approximation is nearly equivalent to the LMTO hamiltonian without the “combined correction” term. With this package a new program, lmgf is added to the suite of executables. lmgf plays approximately the same role as the LMTO-ASA band program lm: you can use lmgf to make a self-consistent density as you can do with lm. A potential is generated from energy moments Q0Q_0, Q1Q_1, and Q2Q_2, in the same way as the lm code. lmgf is a Green’s function method: Green’s functions have less information than wave functions, so in one sense the things you can do with lmgf are more limited: you cannot make the bands directly, for example. However, lmgf enables you to do things you cannot with lm. The most imprortant ones are:

  • Calculate magnetic exchange interactions
  • Calculate magnetic susceptibility (spin-spin, spin-orbit, orbit-orbit parts)
  • Calculate properties of disordered materials, either chemically disordered or spin disorder from finite temperature, within the Coherent Potential Approximation, or CPA.
  • Calculate the ASA static susceptibility at q=0q=0 to help converge calculations to self-consistency.

Structure of Green’s function program


lmgf runs in much the same way as lm. The band pass routine of lm, bndasa.f, generates the eigenvalues and eigenvectors, which can in turn generate the quantities of interest. bndasa is replaced by a Green’s function routine, gfasa. gfasa can generate output moments, DOS, density-matrix, etc., in the same way as bndasa does.

In contrast to band methods (implemented in lm) where the Hamiltonian HH is energy independent and all the bands are generated by diagonalizing H, Green’s functions are calculated for a specific energy; information is extracted from GG for a particular energy.

This fact highlights the strengths and weaknesses of a Green’s function approach. Energy-integrated properties such as the moments, must be obtained by integrating over energy. Calculating GG explicitly at a family of energies is more cumbersome than diaonalizing a hamiltonian. On the other hand, Green’s function methods are naturally suited to contexts where the energy-dependence is needed anyway. CPA theory yields an energy-dependent potential; Green’s functions are a natural way to implement it. Similarly, noninteracting susceptibilities can be expressed as G×G (`×’ implies either convolution or product, depending on the space you are working in).

lmgf always loops over some energy contour; what contour you use depends on the context as described below. gfasa accumulates various kinds of data for each mesh point, such as the point’s contribution from energy moments used in an ASA self-consistent cycle. Finally, an estimate for the Fermi level EFE_F is determined using a from Pade approximation. If the original guess for EFE_F is sufficiently close, the cycle is finished as in lm. If the estimate is too far off, a new energy mesh is taken and the process is repeated.

Energy Contours, Potential Shifts and the Determination of the Fermi Level


For energy-integrated properties, a very fine energy mesh would be required if the energy integration was carried out close to the real axis. It is much more efficient to deform the integration contour into an elliptical path in the complex plane, approaching the real axis only at the lower and upper integration limits.

To integrate quantities over occupied states, integration to the Fermi level EF is required. EF is not known but must be fixed by charge neutrality. Thus EFE_F must be guessed at and iteratively refined until the charge neutrality condition is satisfied. lmgf does not vary EF; the user specifies it at the outset. Instead lmgf looks for a constant potential shift that satisfies charge neutrality; this must be searched for iteratively. Both the potential shift and EFE_F are maintained in a file vshft.ext. Inspection of vshft.ext may look unecessarily complicated; it’s because you can use the file to add site-dependent shifts. vshft.ext is also used by the layer Green’s function code lmpg, which requires extra information about shifts on the left and right leads.

Metals and nonmetals are distinguished in that in the latter case, there is no DOS in the gap and therefore the Fermi level (or potential shift) cannot be specified precisely.

Metal case (set by BZ_METAL=1): once the kk- and energy-points are summed over and the deviation from charge neutrality is determined, the code will attempt to find the potential shift that fixes charge neutrality.

It finds the Fermi level in one of two ways:

  • Using a Pade approximant, lmgf interpolates the diagonal elements of G. The interpolation is used to evaluate the GF on the starting elliptical contour shifted rigidly by a constant, and the shift is iterated until the charge-neutrality condition is satisfied. At this stage, there are two possibilities:

    1. repeat the integration of GG over kk and the energy contour with the constant shift added to the potential.

    2. Assume that the Pade-approximant to the diagonal GG is a sufficiently good estimate for the actual GG.

    If the potential shift is larger than a user-specifed tolerance (see padtol in GF_GFOPTS below), option 1 is taken and the Pade shift re-evaluated. A new Pade estimate is made for the potential shift requiring charge neutrality, and it is tested once against the user-specified tolerance.

    When the shift falls below the tolerance, option 2 is taken and lmgf proceeds to the next step. The user is advised to monitor these shifts and the deviation from charge neutrality.

  • The charge is integrated in a contour near the real axis subsequent to the elliptical contour. In this mode, the determination of the potential shift is accomplished by continuing the integration contour on the real axis starting from the originally estimated Fermi level. A trapezoidal rule is used (or Simpson’s rule using a Pade approximate for the midpoint), and new energy points are computed and integrals accumulated until charge neutrality is found. There is no iterative scheme as with the Pade approximation. This option tends to be a little less accurate than the Pade, but somewhat more stable as it is less susceptible to interpolation errors.

One last comment about the METAL case: by default the program will save the potential shift to use in the next iteration. You can suppress this save (see frzvc below), which again can be less accurate, but more stable. In particular, if you are working with an insulator where stability can be an issue (determination of the Fermi level is somewhat ill conditioned), a stable procedure is to use this option together with second energy integration scheme described above (the integration contour on the real axis).

Nonmetal case (set by BZ METAL=0): lmgf will not attempt to shift the potential, or ensure charge neutrality. The user is cautioned to pay rather closer attention to deviations from charge neutrality. It can happen because of numerical integration errors, or because your assumed Fermi level does not fall within the gap. You can use METAL=1 even if the material is a nonmetal.

Some details concerning how lmgf works internally

For each energy point, the BZ integration is accomplished by routine in gf/gfibz.f, which loops over all irreducible points, generating the “scattering path operator” gg and the corresponding gg for all the points in the star of kk to generate a properly symmetrized gg. Within the ASA, second-generation LMTO, gg is converted to proper Green’s function GG, corresponding to the orthogonal gamma representation by an energy scaling. The scaling is carried out in routine gf/gfg2g.f. Next the various integrated quantities sought are assembled (done by gf/gfidos.f). The potential shift to satisfy charge neutrality is found, and stored in vshft.ext. The I/O is handled by routine subs/iovshf.f.

GF specific input


Energy integration


Green’s functions are always performed on some energy contour, which is discretized into a mesh of points in the complex energy plane. (A description of the various kinds of contours this code uses is documented in the comments to gf/emesh.f.) GG is “spikey” for energies on the real axis (it has poles where there are eigenstates).

The BZMESH token

To compute energy-integrated properties such as magnetic moments or the static susceptibility, the calculation is most efficiently done by deforming the contour in an ellipse in the complex plane.

At other times you want properties on the real axis, e.g. density-of-states or spectral functions. You specify the contour in category BZ as:

EMESH= nz mode emin emax [other args, depending mode]

where

    nz         number of energy points
    mode       specifies the kind of contour; see below
    emin,emax  are the energy window (emax is usually the Fermi level)

Right now there are the following contours:

mode=10: a Gaussian quadrature on an ellipse. This is the standard mode for integrating over the occupied states.

EMESH= nz 10 emin emax ecc eps

       ecc is the eccentricity of the ellipse, ranging from 0 (circle) to 1 (line)
       eps is a 'bunching' parameter that, as made larger, tends to bunch points near emax.
           As a rule, e2=0 is good, or maybe e2=.5 to emphasize points near the Fermi level.

After the integration is completed, there will be some deviation from charge neutrality, because emax will not exactly correspond to the Fermi level. This deviation is ignored if METAL=0; otherwise, the mesh is rigidly shifted by a constant amount, and the diagonal GF interpolated using a Pade approximant to the shifted mesh. The shifting+interpolation is iterated until charge neutrality is found, as described in section 2. If the rigid shift exceeds a specified tolerance, the Pade interpolation may be suspect. Thus, the entire cycle is repeated from scratch, on the shifted mesh where the shift is estimated by Pade.

mode=0: a uniform mesh of points between emin and emax, with a constant imaginary component.

EMESH= nz 0 emin emax Im-z [... + possible args for layer geometry.]
       Im-z is the (constant) imaginary component.

This mode is generally not recommended for self-consistent cycles because the GF has a lot of structure close to the real axis (small Imz{\mathrm{Im}}\, z), while shifting off the real axis introduces errors. It is used, however, in other contexts, e.g. transport.

mode=110: is a contour input specific to nonequilibrium Green’s function.

The nonequilibrium Green’s function requires additional information for the energy window between the left and right leads. (The nonequilibrium Green’s function is implemented for the layer geometry in lmpg.) Thus the integration proceeds in two parts: first an integration on an elliptical path is taken to the left Fermi level (as in mode=10). Then an integration over is performed on the nonequilibrium contour, i.e. the energy window from the left to the right Fermi level. This integration is performed on a uniform mesh close to the real axis, as in mode=0. For the nonequilibrium contour, three additional pieces of information must be supplied:

  nzne  number of (uniformly spaced energy points on the nonequilibrium contour
  vne   difference in fermi energies of right and left leads, ef(R)-ef(L)
  delne Im-z on the nonequilibrium contour

The mesh is specified as

EMESH= nz 110 emin ef(L) ecc eps nzne vne delne [del00]

The last argument plays the role of delne specifically for computing the self-energy of the left and right leads. There is an incompatibility in the requirements for Imz\mathrm{Im}\,z in the central and end regions. The same incompatibility applies to transport and is discussed in the following section.

mode=310: Alternative Pade approximant in finding Fermi level.

a Gaussian quadrature on an ellipse to a trial emax, as in mode 2. However, the search for the Fermi level is not done by Pade approximant, as in mode 10. Instead, a second integration proceeds along a uniform mesh from emax to some (Fermi) energy which satisfies charge neutrality. This procedure is not iterative.

EMESH= nz 310 emin emax e1 e2 delz

       e1 and e2 are just as in mode 10
       delz   is the spacing between energy points for the second integration on the uniform mesh.

mode=2: is the same contour as mode=0. However, it is designed for cases when you want to resolve the energy dependence of some quantity, such as the DOS or magnetic exchange coupling. These are discussed in the GF category below.

Modifications of energy contour for layer geometry

When computing transmission coefficients via the Landauer-Buttiker formalism, one chooses a contour as in mode=0. However, there is a problem in how to choose Imz{\mathrm{Im}}\, z. A small Imz{\mathrm{Im}}\, z is needed for a reliable calculation of the transmission coefficient, but using a small Imz{\mathrm{Im}}\, z to determine the surface Green’s function may not succeed because the GF can become long range and the iterative cycle used to generate it may not be stable. To accommodate these conflicting requirements, a surface-specific Imz{\mathrm{Im}}\, z should be used, called del00. The mode=0 mesh is specified as

EMESH= nz 0 emin emax delta xx xx xx xx del00

delta is Im z for the central region; del00 is Imz{\mathrm{Im}}\, z for the surfaces.

Entries xx have no meaning but are put there for compatibility with the contour used in nonequilibrium calculations. (A similar situation applies to the nonequilibrium part of the contour).

The mesh for self-consistent nonequilibrium calculations is

EMESH= nz 110 emin ef(L) ecc eps nzne vne delne del00

Green’s function category


lmgf requires a GF-specific category.

GF  MODE=1 GFOPTS=options
The GF\MODE token

MODE=n controls what lmgf calculates. Options are MODE=1, MODE=10, MODE=11, MODE=26, described below.

MODE=1 goes through the self-consistency cycle, calling gfasa. It performs a function analogous to bndasa in the band program, generating output density, moments, and other quantities such as density-of-states.

Taken with the special integration contour mode=2 (see EMESH above), the density-of-states D(E)D(E) and its integral are computed and tabulated over the window specified.

With the following sample input segment:

% const ef=-0.025725
BZ      EMESH=5 2 {ef} {ef+.002*4} .001 0

The integration will be tabulated for five points ef, ef+.002, ef+.004, ef+.006, ef+.008 like so (spin-polarized case)

       Re z       Im z   spin       dos           idos
    -0.025725   0.001000   1      13.55272       0.00000
    -0.025725   0.001000   2      10.38435       0.00000
    -0.025725   0.001000   t      23.93706       0.00000
    -0.023725   0.001000   1       9.17407       0.02273
    -0.023725   0.001000   2       4.13694       0.01452
    -0.023725   0.001000   t      13.31101       0.03725
    -0.021725   0.001000   1      15.33776       0.04724
    -0.021725   0.001000   2       7.42200       0.02608
    -0.021725   0.001000   t      22.75976       0.07332
    -0.019725   0.001000   1      19.58433       0.08216
    -0.019725   0.001000   2       7.52708       0.04103
    -0.019725   0.001000   t      27.11141       0.12319
    -0.017725   0.001000   1      20.83078       0.12258
    -0.017725   0.001000   2       9.31350       0.05787
    -0.017725   0.001000   t      30.14428       0.18045

If the partial DOS is generated, the usual tokens in the BZ category specifying the window (DOS=) and number of points (NPTS=) are overridden by the parameters in EMESH.

MODE=10 invokes a special branch that computes magnetic exchange interactions using a linear response technique. (The source code has its entry point in gf/exasa.f.)

In particular, JijJ_{ij} is computed for pairs of sites (i,j)(i,j), where the J’s are the parameters in the Heisenberg hamiltonian

E(si,sj)=ijJijsisjE(s_i, s_j) = \sum_{ij} J_{ij} s_i s_j

Thus, the J’s are coefficients to energy changes for small rotations of the spins. They can be computed from a change in the band energy; changes from small rotations are done analytically.

Taken with the usual elliptical integration contour, the J’s are computed by energy integration to the Fermi level. Taken with the special integration contour mode=2 (see EMESH above), dJ/dE is computed instead. There is a shell script

$ gf/test/getJq0z

(invoke with no arguments to see usage) that will collect some of the ouput for you into tables. The data are collected into file dj0dz. For an example illustrating this mode, invoke

$ gf/test/test.gf co 5

This test computes the exchange coupling both for the usual elliptical contour and resolves the energy-dependence of J in a small window near the Fermi level.

Often only some atoms are magnetic, and all that is desired are the exchange parameters J connecting a partial list of sites to its neighbors. This can be useful, even essential, for large systems because it can be very expensive both in time and memory to compute exchange interactions for all pairs. To compute exchanges only for a list of sites, use command-line argument

--sites:pair:site-list

For more details, see command-line arguments invoked with lmgf.

Caution. lmgf reads and writes a potential shift file vshft.ext which shifts site potential by a constant to cause the Fermi level to match what is specified by the input. This shift also gets added into the atom file; potential VES in line PPAR is adjusted. When calculating exchange interactions, vshft.ext is not read. However, the shift is preserved because they are held in the potential parameters section of the atom file. But if you run the atom part lm or lmgf and remake the PP’s from the moments (START BEGMOM=1), this causes estat potential to be remade, but the sphere program does not add the contents vshft (it is done at the start of the Green’s function calculation). The exchange parameters should be evaluated with the potential parameters generated by lmgf. If they are alternatively evaluated from the atom files generated by lm, the Fermi level needs to be aligned to the Fermi level of lm (or close to it; there are slight differences between Fermi levels generated by lm and by lmgf).

MODE=11 is an exchange branch that is run after MODE=10. It prints out the JijJ_{ij} and does several other analyses.

Switch --sites:pair:site-list applies to mode 11 as well as mode 10; see command-line arguments.

The GF\GFOPTS token

The GF category has a token GFOPTS=tag;tag;…, which causes lmgf to perform a variety of special purpose functions.

Options are entered as a sequence of tags delimited by a semicolons:  tag1;tag2;… .

Tag Purpose
emom generate the output ASA moments, needed for self-consistency
noemom suppress generation of the output ASA moments
idos make integrated properties, such as the sum of one-electron energies
noidos reverse of idos
dmat make the density-matrix GRL,R’L’
sdmat make the site-diagonal density-matrix GRL,RL’. The density matrix is written to dmat.ext.
pdos Make the partial density of states (this has not been checked recently).
p3 Use third order potential functions
shftef Find charge neutrality point by shifting the Fermi level, rather than adding a constant potential shift
frzvc Suppress saving the constant potential shift used to determine charge neutrality
padtol Set the tolerance for maximum potential shift permissible by Pade interpolation, as described above

The following are specific to the CPA:

omgtol Tolerance in the Omega potential, CPA self-consistency
omgmix How much of prior iterations to mix, CPA self-consistency
nitmax Maximum number of iterations for CPA self-consistency
lotf Learn on-the-fly
specfun Make spectral function
specfr Make spectral function resolved by site
specfrl Make spectral function resolved by site and l
dmsv Record density matrix to file
dz Shift Omega potential by dz
sfrot Rotate the GF in spin space by this angle around the y axis

See Table of Contents

lmgf-specific command-line arguments


-ef=# overrides upper limit of energy integration (Fermi level) and assigns to #

The following are specific to the exchange calculation modes 10 and 11:

--sites[:pair]:site-list  Make the exchange parameters J_ij only for sites i in the site list.  
In mode 11, option :pair means that only parameters J_ij where both i and j are printed.

Example: running lmgf using MODE=10 with this command line argument

--sites:pair:1,3,5,7

generates J connecting sites 1, 3, 5 and 7 to all neighbors. See Syntax of Integer Lists for the syntax of site-list.

Running lmgf using MODE=11 with the same --sites argument will print out the exchanges just between pairs of these sites.

Running lmgf using MODE=11 without any --sites argument will print out the exchanges between these sites and all neighbors.

--wrsj[:fn=name][:scl=#][:tol=#]  (mode 11 only)
  Writes the Heisenberg exchange parameters in a standard format, suitable for use
  in spin dynamics simulations.
  fn=name writes to file 'name'  (default name is rsj)
  scl=#   scales the parameters by #
  tol=#   writes only parameters with energy > tol

--rcut=#
  Truncates the range of the R.S exchange parameters ...
  useful to assist in the determination of the effect distant neighbors.

--2xmsh
  When integrating over the BZ to estimate Tc from Tablikov formula, this option doubles the k-mesh.
  Can be helpful in testing k-convergence of the singular q->0 limit entering into the formula.

--amoms=mom1,mom2,...
--amom=mom1,mom2,...
  This switch overrides ASA moments (which are automatically generated).
  The first switch reads a vector of nbas moments, one for each site.
  The first switch reads a vector of nclass moments, one for each class.

Sphere magnetic moments are tabulated in the printout at the end of mode 10, and the start of mode 11. If you are importing exchange parameters (file jr.ext , e.g. from the full-potential code, you will want to supply the moments calculated from that program.)

See Table of Contents

Test cases and examples


This script:

$ gf/test/test.gf --all

carries out a number of tests, which also demonstrate various branches of the code. To see the materials and corresponding tests try

$ gf/test/test.gf --list

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