# Wannier90

[Wannier90](http://www.wannier.org/) is a useful package to generating the maximally-localized Wannier functions (MLWFs), which can be used to compute advanced electronic properties. Some post-processing tools (such as WannierTools, etc.) will use MLWFs for further analysis and calculations. 

Currently ABACUS provides an interface to Wannier90 package. The users are assumed to be familiar with the use of Wannier90. The ABACUS-Wannier90 interface is suitable for nspin=1, 2, 4 (including lspinorb=1).

To construct the MLWFs using the wave functions of ABACUS generally requires four steps. Here we use the diamond as an example which can be found in [examples/interface_wannier90/](https://github.com/abacusmodeling/abacus-develop/tree/develop/examples/interface_wannier90).

1. Enter the `ABACUS_towannier90_pw/` folder, prepare a Wannier90 input file `diamond.win`, which is the main input file for Wannier90. Then To generate `diamond.nnkp` file by running Wannier90,  which ABACUS will read later: 

    ```
    wannier90 -pp diamond.win
    ```

    The content of `diamond.win` is as follows:

    ```
    num_wann        =  4
    num_iter        = 20
    
    wannier_plot=.true.
    wannier_plot_supercell = 3
    wvfn_formatted = .true.

    begin atoms_frac
    C   -0.12500  -0.1250    -0.125000
    C    0.12500   0.1250     0.125000
    end atoms_frac

    begin projections
    f=0.0,0.0,0.0:s
    f=0.0,0.0,0.5:s
    f=0.0,0.5,0.0:s
    f=0.5,0.0,0.0:s
    end projections

    begin unit_cell_cart
    -1.613990   0.000000   1.613990
     0.000000   1.613990   1.613990
    -1.613990   1.613990   0.000000
    end unit_cell_cart

    mp_grid : 4 4 4

    begin kpoints
    0.0000  0.0000     0.0000
    0.0000  0.2500     0.0000
    0.0000  0.5000     0.0000
    0.0000  0.7500     0.0000
    ...
    end kpoints
    ```

2. Do a self-consistent calculation and get the converged charge density:

    ```
    cp INPUT-scf INPUT
    cp KPT-scf KPT
    mpirun -np 4 abacus
    ```


3. Do a non-self-consistent calculation: 

    ```
    cp INPUT-nscf INPUT
    cp KPT-nscf KPT
    mpirun -np 4 abacus
    ```

    below are the INPUT file (nscf):

    ```
    INPUT_PARAMETERS

    pseudo_dir              ../../../tests/PP_ORB
    orbital_dir             ../../../tests/PP_ORB
    ntype                   1
    ecutwfc                 50
    nbands                  4
    smearing_method         fixed
    calculation             nscf
    scf_nmax                50
    pw_diag_thr             1.0e-12
    scf_thr                 1.0e-13
    init_chg                file
    symmetry                -1
    towannier90             1
    nnkpfile                diamond.nnkp
    basis_type              pw
    out_wannier_unk         0
    ```

    There are seven interface-related parameters in the `INPUT` file:

    - [towannier90](../input_files/input-main.md#towannier90): `1`, generate files for wannier90 code; `0`, do not generate.
    - [nnkpfile](../input_files/input-main.md#nnkpfile) : the name of the file generated by running "wannier90 -pp ...".
    - [wannier_spin](../input_files/input-main.md#wannier_spin): If you use nspin=2, `up`: calculate the Wannier functions for the spin up components ; `down`: calculate the Wannier functions spin down components.
    - [out_wannier_mmn](../input_files/input-main.md#out_wannier_mmn): control whether to output the "*.mmn" file.
    - [out_wannier_amn](../input_files/input-main.md#out_wannier_amn): control whether to output the "*.amn" file.
    - [out_wannier_eig](../input_files/input-main.md#out_wannier_eig): control whether to output the "*.eig" file.
    - [out_wannier_unk](../input_files/input-main.md#out_wannier_unk): control whether to output the "UNK.*" file.
    - [out_wannier_wvfn_formatted](../input_files/input-main.md#out_wannier_wvfn_formatted): control what format of the Wannier function file to output, `true`: output the formatted text file; `false`: output the binary file. Note that the `wvfn_formatted` option in `*.win` file (input file of Wannier90) has to be set accordingly with this option.

    **Note: You need to turn off the symmetry during the entire nscf calculation.**

    To setup the `KPT` file according to the `diamond.win` file, which is similar to "begin kpoints ..." in the `diamond.win` file: 

    ```
    K_POINTS
    64
    Direct
    0.0000 0.0000 0.0000 0.0156250
    0.0000 0.2500 0.0000 0.0156250
    0.0000 0.5000 0.0000 0.0156250
    0.0000 0.7500 0.0000 0.0156250
    ...
    ```

    After the nscf calculation, ABACUS will generate `diamond.amn`, `diamond.mmn`, `diamond.eig`, `UNK` files in the `OUT.` folder which are input files needed by Wannier90 code. 

4. Copy `.amn`, `.mmn`, `.eig`, `UNK` file to `wannier/` folder, to get the MLWFs by running Wannier90: 

    ```
    wannier90 diamond.win
    ```

Notes:

- The ABACUS-wannier90 interface can be used in both PW and LCAO basis.
- If you want to plot the Wannier function, you must set `wvfn_formatted = .true.` in `diamond.win`, otherwise Wannier90 code cannot read files generated by ABACUS because these files are not binary files. You also have to generate the `diamond.amn` and `UNK` files for the plot. Otherwise, the two types of file are not necessary.