The Fleur input generator

The basic input of Fleur is the inp.xml file. As it does not only contain switches to control the calculation but also a detailed setup of the system including e.g. its symmetries or the atomic parameters it is hard to set it up by hand. Hence, an input-file generator is provided.

The inpgen executable takes a simplified input file and generates defaults for:

• the symmetry information
• the atom species and the symmetry equivalent atoms (atom groups)
• muffin-tin radii, l-cutoffs, and radial grid parameters for the atom species
• plane-wave cutoffs ($K_\text{max}$, $G_\text{max}$, $G_\text{max}^\text{XC}$).
• (in film calculations) the vacuum distance ($D$) and d-tilda ($\tilde{D}$)
• many more specialized parameters ...

In general the input generator does not know:

• is your system magnetic? If some elements (Fe,Co,Ni...) are in the unit cell the program sets jspins=2 and puts magnetic moments. You might like to change jspins and specify different magnetic moments of our atom types.
• how many $\vec{k}$ points will you need? For metallic systems it might be more than for semiconductors or insulators. In the latter cases, also the mixing parameters might be chosen larger.
• is the specified energy range for the valence band ok? Normally, it should, but it's better to check, especially if LO's are used.

You have to modify your inp.xml file accordingly. Depending on your demands, you might want to change something else, e.g. the XC-functional, the switches for relaxation, use LDA+U etc.

Running inpgen

To call the input generator you tyically do

inpgen -f simple_file

The inpgen executable accepts a few command-line options. In particular you might find usefull

Basic input

Your input should contain (in this order):

1. A title
2. Optionally: input switches (e.g. whether your atomic positions are in internal or scaled Cartesian units)
3. Lattice information (either a Bravais-matrix or a lattice type and the required constants (see below); in units of Bohr radii)
4. Atom information (an identifier (maybe the nuclear number) and the positions (in internal or scaled Cartesian coordinates) ).
5. Optionally: for spin-spiral calculations or in case of spin-orbit coupling we need the $\vec{q}$ vector or the spin-quantization axis to determine the symmetry.
6. Optionally: Preset parameters (atoms/general)

Title

Your title cannot begin with an & and should not contain an !. Apart from that, you can specify any 80-character string you like.

Input switches

The namelist input should start with &input and end with /. Possible switches are:

An example (including the title):

 3 layer Fe film, p(2x2) unit cell, p4mg reconstruction

 &input symor=t oldfleur=t / 

Lattice information

There are two possibilities to specify the lattice information: either you specify the Bravais matrix (plus scaling information) or the Bravais lattice and the required information (axis lengths and angles).

First variant:

The first 3 lines give the 3 lattice vectors; they are in scaled Cartesian coordinates. Then an overall scaling factor (aa) is given in a single line and independent (x,y,z) scaling is specified by scale(3) in a following line. For film calculations, the vacuum distance dvac is given in one line together with a3.

Example: tetragonal lattice for a film calculation:

  1.0  0.0  0.0        ! a1
  0.0  1.0  0.0        ! a2
  0.0  0.0  1.0  0.9   ! a3 and dvac
  4.89                 ! aa (lattice constant)
  1.0  1.0  1.5        ! scale(1),scale(2),scale(3)

The overall scale is set by aa and scale(:) as follows: assume that we want the lattice vectors to be given by

 a_i = ( a_i(1) xa , a_i(2) xb , a_i(3) xc ) 

then choose aa, scale such that: $\text{xa} = \text{aa} \cdot \text{scale(1)}$, etc. To make it easy to input sqrts, if $\text{scale}(i) < 0$, then $\text{scale} = \sqrt{|\text{scale}|}$ Example: hexagonal lattice

      a1 = ( sqrt(3)/2 a , -1/2 a , 0.      )
      a2 = ( sqrt(3)/2 a ,  1/2 a , 0.      )
      a3 = ( 0.          , 0.     , c=1.62a ) 

You could specify the following:

       0.5  -0.5  0.0     ! a1
       0.5   0.5  0.0     ! a2
       0.0   0.0  1.0     ! a3
       6.2                ! lattice constant
      -3.0   0.0  1.62    ! scale(2) is 1 by default

Second variant:

Alternatively, you may specify the lattice name and its parameters in a namelist input, e.g.

 &lattice latsys='tP' a=4.89 c=6.9155 /

The following arguments are implemented: latsys, a0 (default: 1.0), a, b (default: a), c (default: a), alpha (90 degree), beta (90 degree), gamma (90 degree). Hereby, latsys can be chosen from the following table (intended to work for all entries, up to now not all lattices work). a0 is the overall scaling factor.

You should give the independent lattice parameters a,b,c and angles alpha,beta,gamma as far as required.

Atom information

First you give the number of atoms in a single line. If this number is negative, then we assume that only the representative atoms are given; this requires that the space group symmetry be given as input (see below).

Following are, for each atom in a line, the atomic identification number and the position. The identification number is used later as default for the nuclear charge (Z) of the atom. (When all atoms are specified and the symmetry has to be found, the program will try to relate all atoms of the same identifier by symmetry. If you want to manipulate specific atoms later (e.g. change the spin-quantization axis) you have to give these atoms different identifiers. Since they can be non-integer, you can e.g. specify 26.01 and 26.02 for two inequivalent Fe atoms, only the integer part will be used as Z of the atom.)

The input of the atomic positions can be either in scaled Cartesian or lattice vector units, as determined by the logical cartesian (see above). For supercells, it is sometimes more natural to specify positions in scaled Cartesian units.

A possible input (for CsCl) would be:

  2
 55 0.0 0.0 0.0
 17 0.5 0.5 0.5 

or, for a p4g reconstructed Fe trilayer specifying the symmetry:

 -2
 26 0.00 0.00 0.0 
 26 0.18 0.32 2.5 

 &gen         3

   -1    0    0        0.00000
    0   -1    0        0.00000
    0    0   -1        0.00000

    0   -1    0        0.00000
    1    0    0        0.00000
    0    0    1        0.00000

   -1    0    0        0.50000
    0    1    0        0.50000
    0    0    1        0.00000 /

Here, &gen indicates, that just the generators are listed (the 3×3 block is the rotation matrix [only integers], the floating numbers denote the shift); if you like to specify all symmetry elements, you should start with &sym. You have furthermore the possibility to specify a global shift of coordinates with e.g.

 &shift 0.5 0.5 0.5 /

or, to introduce additional scaling factors

 &factor 3.0 3.0 1.0 /

by which your atomic positions will be divided (the name "factor" is thus slightly counterintuitive).

Setting up magnetic systems

The list of atoms as discussed in the previous section can also be used to configure and initalize the magnetic configuration of the system. To achieve this, after each atomic position the magnetization of this atom can be specified. For example, bcc Fe with two atoms per unit cell might be specified as this:

2
26 0.0 0.0 0.0 : 2.4
26 0.5 0.5 0.5 : 2.4

Here, the 2.4 separated by a colon from the position information indicates the initial magnetic moment in $\mu B$ this atom should have. Besides giving the moment directly, one can also rely on the default moments FLEUR will use and provide the special keywords up and/or down here. So

2
26 0.0 0.0 0.0 : up
26 0.5 0.5 0.5 : down

will setup an antiferromagnetic configuration with default moments.

This syntax can be extended to create non-collinear magnetic systems. E.g.

2
26 0.0 0.0 0.0 : 0.0 2.4 0.0
26 0.5 0.5 0.5 : 0.0 0.0 down

will create a non-collinear setup, in which the moment of the first atom will point in "y" direction with an initial moment of 2.4 $\mu B$, while the second atom will have the default moment pointing in -z direction. Inpgen will create a corresponding inp.xml with the needed switches for non-collinear calculations in this case and calculate the angles theta and phi for the atoms.

Ending an input file

If inpgen should stop reading the file earlier (e.g. you have some comments below in the file) or if inpgen fails to recognize the end of the input file (which happens with some compilers), one can use the following line:

 &end /

Special cases

Film calculations

In the case of a film calculation, the surface normal is always chosen in z-direction. A two-dimensional Bravais lattice then corresponds to the three-dimensional one according to the following table:

The z-coordinates of all atoms have to be specified in Cartesian units (a.u.), since there is no lattice in the third dimension, to which these values could be referred. Since the vacuum boundaries will be chosen symmetrically around the $z = 0$ plane (i.e. -dvac/2 and dvac/2), the atoms should also be placed symmetrically around this plane.

The initial values specified for a3 and dvac (i.e. the third dimension, see above) will be adjusted automatically so that all atoms fit in the unit cell. This only works if the atoms have been placed symmetrically around the $z = 0$ plane.

Spin-spiral or SOC

If you intend a spin-spiral calculation, or to include spin-orbit coupling, this can affect the symmetry of your system:

• a spin spiral in the direction of some vector $\vec{q}$ is only consistent with symmetry elements that operate on a plane perpendicular to $\vec{q}$, while
• (self-consistent) inclusion of spin-orbit coupling is incompatible with any symmetry element that changes the polar vector of orbital momentum $L$ that is supposed to be parallel to the spin-quantization axis (SQA)

Therefore, we need to specify either $\vec{q}$ or the SQA, e.g.:

 &qss 0.0 0.0 0.1 /

(the 3 numbers are the x,y,z components of $\vec{q}$) to specify a spin-spiral in z-direction, or

 &soc 1.5708 0.0 /

(the 2 numbers are $\theta$ and $\phi$ of the SQA) to indicate that the SQA is in x-direction.

Be careful if symmetry operations that are compatible with the chosen $\vec{q}$ vector relate two atoms in your structure, they also will have the same SQA in the muffin-tins!

Preset parameters

Atoms

After you have given general information on your setup, you can specify a number of parameters for one or several atoms that the input-file generator will use while generating the inp file instead of determining the values by itself.

The list of parameters for one atom must contain a leading &atom flag and end with a /. You have to specify the atom for which you set the parameters by using the parameter element. If there are more atoms of the same element, you can specify the atom you wish to modify by using the id tag.

You might encounter the situation that different atoms of the same element need to be treated differently. For example, atoms might be in different chemical surounding or have different magnetic properties. You might also be in a situation in which you ant to break the symmetry between different atoms on purpose. In this cases an additional id tag should be supplied to the element. For example you could have not only Silicon as &atom element=14 / but also &atom element=14 id=14.1/ in your list of atoms.

All parameters available are

General

You might also want to set more general parameters like the choice of the exchange-correlation potential or the desired reciprocal grid in the Brillouin zone beforehand. Those parameters can be given as a namelist using the &comp, &exco, &film and/or &kpt flag. The corresponding line in the input-file for the input-file generator has to end with a /. All parameters available are, sorted by their affiliation

&comp:

&exco:

&film:

&kpt:

Here is an example of an inpgen input file for Europium Titanate in which local orbitals are used for the $5s$ and $5p$ states of Europium and the muffin-tin radius of one Oxygen atom is manually set. Also, the exchange-correlation potential is chosen to be that of Vosko, Wilk, and Nusair and a $\vec{k}$-point mesh is defined for the Brillouin-zone.

Europium Titanate Perovskite Structure

 &input cartesian=t inistop=t oldfleur=f /

 &lattice latsys='sc' a= 7.38 a0= 1.0 /

   5 
   63     0.000  0.000  0.000
   08.01  0.000  0.500  0.500
   08.02  0.500  0.000  0.500
   08.03  0.500  0.500  0.000
   22     0.500  0.500  0.500

 &atom element="eu" lo="5s 5p" econfig="[Kr] 4d10|4f7 5s2 5p6 6s2" /
 &atom element="o" id=08.03 rmt=1.17 /
 &exco xctyp='vwn' /
 &kpt div1=5 div2=5 div3=5 /

Modifying inp.xml

The input generator can also read an existing inp.xml file. This is achieved by using inpgen -inp.xml as the command. At the moment two use cases exist for this feature.

Some old versions of an inp.xml can be read and converted to the current version. We are trying to improve this feature to make sure that at least inp.xml files of older releases can be converted. However, not all features might be included and not all switches or settings will be transfered. So please check the resulting file carefully.

An existing inp.xml can also be read to modify the k-points as discussed in the next section.

K-Point Generator

The generation of K-Points is now always performed by inpgen. The fleur executable no longer contains code for k-point generation and a list of K-Points now has to be provided in the inp.xml file.

Beside specifying k-points in the simple input for inpgen as outlined above. You can explicitely specify options for generating K-Points on the command line of 'inpgen'. For example you might want to call

inpgen -inp.xml -kpt testset#gamma@grid=10,10,10

Here we use an already existing inp.xml file (by specifying -inp.xml) and invoke the k-point generator with the -k option. This option expects a string specifying the k-point set to generate. The general form of this string is:

name#modifier@mode=details

• name: specifies a name for the set. (Optional)
• modifier: some modes of k-point generation can be modified with options (see below)
• mode: inpgen supports several different modes of k-point generation (see below)
• details: each mode needs parameters that are described below.

Possible modifier currently include gamma to indicate that the Gamma-Point should be included in the k-point set. The modifiers gauss, tria, tetra change the default integration method from the standard method. And the 'soc' modifier indicated that a k-point set compatible with spin-orbit coupling should be generated.

Grid mode

This mode is choosen by using grid as the mode in the k-point string. It expects the dimension of the grid as three integers separated by ',' in the details part of the string. The grid can be modified by all modifiers. For example you will use this mode as

inpgen -inp.xml -kpt gamma@grid=10,10,10

Density mode

This mode is activated by the den keyword for the mode. The requested density of k-points must be given as real number. Again, all modifiers are supported. Example:

inpgen -inp.xml -kpt testname#den=0.1

Number mode

This mode is activated by the nk keyword. It expects the approximate number of k-points to be used and is compatible with all modifiers.

inpgen -inp.xml -kpt nk=10

Band structure mode

This is a more complex mode generated by the using band as a mode. It expects the total number of k-points to generate and does not support any of the modifiers. In the most simple form you can use it as

inpgen -inp.xml -kpt band=100

to generate a bandstructure with 100 k-points.

In this most simple form, the code will try to determine a 'standard path' for your structure connecting a set of special k-points. However, you might not find this path appropriate or your structure might not be recognized. Hence, there is an additional option to overwrite the default set of special points. For example you can specify

-kpt band=100 -kptsPath "gamma=0,0,0;x=0.5,0.5,0.5"

Please note:

• The string to specify the special points should contain a list of such points (at least two to have a single line).
• The different points are given in the form name=kx,ky,kz.
• As the different points must be separated by ; you have to set the string in quotation marks to avoid shell confusion.