.. _c2db:

==========================================
Computational 2D Materials Database (C2DB)
==========================================

If you are using data from this database in your research, please cite the
following papers:

.. container:: article

    `The Computational 2D Materials Database: High-Throughput Modeling
    and Discovery of Atomically Thin Crystals`__

    Sten Haastrup, Mikkel Strange, Mohnish Pandey, Thorsten Deilmann,
    Per S. Schmidt, Nicki F. Hinsche, Morten N. Gjerding, Daniele Torelli,
    Peter M. Larsen, Anders C. Riis-Jensen, Jakob Gath, Karsten W. Jacobsen,
    Jens Jørgen Mortensen, Thomas Olsen, Kristian S. Thygesen

    2D Materials 5, 042002 (2018)

    __ http://iopscience.iop.org/article/10.1088/2053-1583/aacfc1

.. container:: article

    `Recent Progress of the Computational 2D Materials Database (C2DB)`__

    M. N. Gjerding, A. Taghizadeh, A. Rasmussen, S. Ali, F. Bertoldo, T.
    Deilmann, U. P. Holguin, N. R. Knøsgaard, M. Kruse, A. H. Larsen,
    S. Manti, T. G. Pedersen, T. Skovhus, M. K. Svendsen, J. J. Mortensen,
    T. Olsen, K. S. Thygesen

    2D Materials 8, 044002 (2021)

    __ https://doi.org/10.1088/2053-1583/ac1059

* `Browse data <https://c2db.fysik.dtu.dk/>`_
* Download data: :download:`c2db.tar.gz`, :download:`c2db.db`
  (see :ref:`using the data`).
* The full dataset is provided upon request.


.. contents::

Brief description
=================

The database contains structural, thermodynamic, elastic, electronic,
magnetic, and optical properties of around 4000 two-dimensional (2D)
materials distributed over more than 40 different crystal structures. The
properties are calculated by density functional theory (DFT) and many-body
perturbation theory (`G_0W_0` and the Bethe- Salpeter Equation for around 300
materials) as implemented in the GPAW_ electronic structure code. The workflow
was constructed using the Atomic Simulation Recipes (ASR_) and executed with
the MyQueue_ task manager. The workflow script and a table with the numerical
settings employed for the calculation of the different properties are
provided below.

.. image:: workflow.png


Overview of methods and parameters used
=======================================

If a parameter is not specified at a given step, its value equals that
of the last step where it was specified:

.. list-table::
  :widths: 10 25
  :header-rows: 1

  * - Workflow step(s)
    - Parameters

  * - Structure and energetics(*) (1-4)
    - vacuum = 15 Å; `k`-point density = 6.0/Å\ `^{-1}`;
      Fermi smearing = 0.05 eV; PW cutoff = 800 eV;
      xc functional = PBE; maximum force = 0.01 eV/Å;
      maximum stress = 0.002 eV/Å\ `^3`; phonon displacement = 0.01Å

  * - Elastic constants (5)
    - `k`-point density = `12.0/\mathrm{Å}^{-1}`; strain = `\pm`\ 1%

  * - Magnetic anisotropy (6)
    - `k`-point density = `20.0/\mathrm{Å}^{-1}`;
      spin-orbit coupling = True

  * - PBE electronic properties (7-10 and 12)
    - `k`-point density = `12.0/\mathrm{Å}^{-1}`
      (`36.0/\mathrm{Å}^{-1}` for DOS)

  * - Effective masses (11)
    - `k`-point density = `45.0/\mathrm{Å}^{-1}`; finite difference

  * - Deformation potential (13)
    - `k`-point density = 12.0/Å\ `^{-1}`; strain = `\pm`\ 1%

  * - Plasma frequency (14)
    - `k`-point density = 20.0/Å\ `^{-1}`; tetrahedral interpolation

  * - HSE band structure (8-12)
    - HSE06\ @PBE; `k`-point density = 12.0/Å\ `^{-1}`

  * - `G_0W_0` band structure (8, 9)
    - `G_0W_0`\ @PBE; `k`-point density = `5.0/\mathrm{Å}^{-1}`;
      PW cutoff = `\infty` (extrapolated from 170, 185 and 200 eV);
      full frequency integration;
      analytical treatment of `W({q})` for small `q`;
      truncated Coulomb interaction

  * - RPA polarisability (15)
    - RPA\ @PBE; `k`-point density = `20.0/\mathrm{Å}^{-1}`;
      PW cutoff = 50 eV; truncated Coulomb interaction;
      tetrahedral interpolation

  * - BSE absorbance (16)
    - BSE\ @PBE with `G_0W_0` scissors operator;
      `k`-point density = `20.0/\mathrm{Å}^{-1}`;
      PW cutoff = 50 eV; truncated Coulomb interaction;
      at least 4 occupied and 4 empty bands

(*) *For the cases with convergence issues, we set a \(k\)-point density of
9.0 and a smearing of 0.02 eV*.


Funding acknowledgements
========================

The C2DB project has received funding from the Danish National Research
Foundation’s  Center for Nanostructured Graphene (CNG) and the European
Research Council (ERC) under the European Union’s Horizon 2020 research and
innovation program Grant No. 773122 (LIMA) and Grant agreement No. 951786
(NOMAD CoE).


Versions
========


.. list-table::
    :widths: 3 1 10

    * - Version
      - rows
      - comment

    * - 2018-06-01
      - 1888
      - Initial release

    * - 2018-08-01
      - 2391
      - Two new prototypes added

    * - 2018-09-25
      - 3084
      - New prototypes

    * - 2018-12-10
      - 3331
      - Some BSE spectra recalculated due to small bug affecting
        absorption strength of materials with large spin-orbit couplings

    * - 2021-04-22
      - 4049
      - Major update including several hundreds new monolayers
        from experimentally known crystals (from the ICSD/COD
        databases), new crystal prototype characterization scheme,
        improved properties (stiffness tensor, effective masses, optical
        absorbance), and new properties (exfoliation energies, Bader
        charges, spontaneous polarisations, Born charges, infrared
        polarisabilities, piezoelectric tensors,  band topology
        invariants, exchange couplings, Raman- and second harmonic
        generation spectra). See Gjerding et al. for a full description.

    * - 2021-06-24
      - 4056
      - Updated MARE for effective masses.

    * - 2022-09-08
      - 15686
      - Added 11.630 materials from lattice decoration and a
        deep generative model
        (see `arXiv:2206.12159 <https://arxiv.org/abs/2206.12159>`__).
        Corrected symmetry threshold used to define band structure
        path to be consistent with the threshold used to define the
        space group.

    * - 2022-11-30
      - 15733
      - Shift currents for selected materials.
        Improved deformation potentials.

    * - 2024-05-01
      - 16789
      - New webpages.
        Restructuring of the search page (removed HIGH,
        MEDIUM, LOW characterization of thermodynamic stability). Added
        various properties for the 3.330 most stable of the 11.630
        structures that were added in version 2022-09-08. Added ca. 100
        self-intercalated bilayers. Spontaneous polarisations for 63
        ferroelectrics. Symmetry classification according to layer groups
        in addition to space groups of the AA-stacked bulk.


Key-value pairs
===============

.. list-table::
    :header-rows: 1
    :widths: 3 5 2

    * - name
      - description
      - unit
    * - ``A``
      - Single-ion anisotropy (out-of-plane)
      - meV
    * - ``J``
      - Nearest neighbor exchange coupling
      - meV
    * - ``N_nn``
      - Number of nearest neighbors
      -
    * - ``alphax``
      - Static polarizability (phonons + electrons) (x)
      - Å
    * - ``alphax_el``
      - Static interband polarizability at (x)
      - Å
    * - ``alphax_lat``
      - Static polarizability (phonons) (x)
      - Å
    * - ``alphay``
      - Static polarizability (phonons + electrons) (y)
      - Å
    * - ``alphay_el``
      - Static interband polarizability at (y)
      - Å
    * - ``alphay_lat``
      - Static polarizability (phonons) (y)
      - Å
    * - ``alphaz``
      - Static polarizability (phonons + electrons) (z)
      - Å
    * - ``alphaz_el``
      - Static interband polarizability at (z)
      - Å
    * - ``alphaz_lat``
      - Static polarizability (phonons) (z)
      - Å
    * - ``area``
      - Unit cell area
      - Å\ `^{2}`
    * - ``cod_id``
      - COD id of parent bulk structure
      -
    * - ``dE_zx``
      - Magnetic anisotropy energy, xz
      - meV/unit cell
    * - ``dE_zy``
      - Magnetic anisotropy energy, yz
      - meV/unit cell
    * - ``doi``
      - Mono/few-layer report(s)
      -
    * - ``dyn_stab``
      - Dynamically stable
      -
    * - ``efermi``
      - Fermi level
      - eV
    * - ``ehull``
      - Energy above convex hull
      - eV/atom
    * - ``energy``
      - Energy
      - eV
    * - ``evac``
      - Vacuum level
      - eV
    * - ``folder``
      - Original file-system folder
      -
    * - ``formula``
      - Formula
      -
    * - ``gap``
      - Band gap
      - eV
    * - ``gap_gw``
      - Band gap (G₀W₀)
      - eV
    * - ``gap_hse``
      - Band gap (HSE06)
      - eV
    * - ``halfmetal_gap``
      - Half-metal gap (PBE)
      - eV
    * - ``halfmetal_gap_hse``
      - Half-metal gap (HSE06)
      - eV
    * - ``has_inversion_symmetry``
      - Inversion symmetry
      -
    * - ``hform``
      - Heat of formation
      - eV/atom
    * - ``icsd_id``
      - ICSD id of parent bulk structure
      -
    * - ``label``
      - Structure origin
      -
    * - ``lam``
      - Anisotropic exchange (out-of-plane)
      - meV
    * - ``layergroup``
      - Layer group
      -
    * - ``lgnum``
      - Layer group number
      -
    * - ``magmom``
      - Total magnetic moment
      - μ\ `_{B}`
    * - ``magstate``
      - Magnetic state
      -
    * - ``natoms``
      - Number of atoms
      -
    * - ``nspecies``
      - Number of species
      -
    * - ``olduid``
      - Old uid
      -
    * - ``plasmafrequency_x``
      - Plasma frequency (x)
      - Å\ `^{0.5}`
    * - ``plasmafrequency_y``
      - Plasma frequency (y)
      - Å\ `^{0.5}`
    * - ``reduced``
      - Reduced formula
      -
    * - ``spin``
      - Maximum value of S\ `_{z}` at magnetic sites
      -
    * - ``spin_axis``
      - Spin axis
      -
    * - ``stoichiometry``
      - Stoichiometry
      -
    * - ``uid``
      - Unique ID
      -


.. _using the data:

Using the data
==============

Here is an example using the ``c2db.db`` ASE-DB file:

.. literalinclude:: gaps.py

.. image:: gaps.png

Alternatively, one can unpack the ``c2db.tar.gz`` file with
``tar -xf c2db.tar.gz`` and extract the gaps like this:

.. literalinclude:: gaps2.py


.. image:: c2db-db.png

.. _Python: https://www.python.org/
.. _ASE: https://wiki.fysik.dtu.dk/ase/
.. _GPAW: https://wiki.fysik.dtu.dk/gpaw/
.. _ASR: https://asr.readthedocs.io/en/latest/
.. _MyQueue: https://myqueue.readthedocs.io/