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.. module:: ase.calculators.dftb
=====
DFTB+
=====
Introduction
============
`DFTB+`_ is a density-functional based tight-binding code using
atom-centered orbitals. This interface makes it possible to use `DFTB+`_
as a calculator in ASE. You need Slater-Koster files for the combination
of atom types of your system. These can be obtained at dftb.org_.
.. _DFTB+: https://www.dftbplus.org/
.. _dftb.org: http://www.dftb.org/
Environment variables
=====================
.. highlight:: bash
The default command that ASE will use to start DFTB+ is
``dftb+ > PREFIX.out``. You can change this command by setting the
:envvar:`ASE_DFTB_COMMAND` environment variable, e.g.::
$ export ASE_DFTB_COMMAND="/path/to/dftb+ > PREFIX.out"
For compatibility, also the old :envvar:`DFTB_COMMAND` variable can
be used, and the resulting command will be ``$DFTB_COMMAND > PREFIX.out``.
Before each DFTB+ calculation, also make sure that the
:envvar:`DFTB_PREFIX` variable points to the directory where
the Slater-Koster files are kept, e.g.::
$ export DFTB_PREFIX=/path/to/mio-0-1/
Parameters
==========
As a FileIOCalculator, ``ase.calculators.dftb.Dftb`` writes input files,
runs DFTB+, and extracts the required information from the resulting output.
The input files are :file:`dftb_in.hsd` (the calculation settings),
:file:`geo_end.gen` (the initial geometry) and
:file:`dftb_external_charges.dat` (the external point charges
in case of electrostatic QM/MM embedding).
.. highlight:: none
All keywords for the :file:`dftb_in.hsd` input file (see the DFTB+ manual)
can be set by ASE. Consider the following input file block::
Hamiltonian = DFTB {
SCC = Yes
SCCTolerance = 1e-8
MaxAngularMomentum = {
H = s
O = p
}
}
.. highlight:: python
This can be generated by the DFTB+ calculator by using the
following settings::
calc = Dftb(Hamiltonian_='DFTB', # this line is included by default
Hamiltonian_SCC='Yes',
Hamiltonian_SCCTolerance=1e-8,
Hamiltonian_MaxAngularMomentum_='',
Hamiltonian_MaxAngularMomentum_H='s',
Hamiltonian_MaxAngularMomentum_O='p')
In addition to keywords specific to DFTB+, also the following keywords
arguments can be used:
restart: str
Prefix for restart file. May contain a directory.
Default is None: don't restart.
ignore_bad_restart_file: bool
Ignore broken or missing restart file. By default, it is an
error if the restart file is missing or broken.
label: str (default 'dftb')
Prefix used for the main output file (<label>.out).
atoms: Atoms object (default None)
Optional Atoms object to which the calculator will be
attached. When restarting, atoms will get its positions and
unit-cell updated from file.
kpts: (default None)
Brillouin zone sampling:
* ``(1,1,1)`` or ``None``: Gamma-point only
* ``(n1,n2,n3)``: Monkhorst-Pack grid
* ``dict``: Interpreted as a path in the Brillouin zone if it
contains the 'path_' keyword. Otherwise it is converted into a
Monkhorst-Pack grid using ``ase.calculators.calculator.kpts2sizeandoffsets``
* ``[(k11,k12,k13),(k21,k22,k23),...]``: Explicit (Nkpts x 3) array of k-points
in units of the reciprocal lattice vectors (each with equal weight)
.. _path: https://ase-lib.org/ase/dft/kpoints.html#ase.dft.kpoints.bandpath
Examples
========
Below are three examples of how to use the DFTB+ calculator. The required
SKF files (i.e. :file:`H-H.skf`, :file:`H-O.skf`, :file:`O-H.skf`
and :file:`O-O.skf`) can for example be chosen from the mio-1-1_
parameter set.
.. _mio-1-1: http://www.dftb.org/parameters/download/mio/mio-1-1-cc/
Geometry optimization by ASE
----------------------------
.. literalinclude:: dftb_ex1_relax.py
Geometry optimization by DFTB+
------------------------------
.. literalinclude:: dftb_ex2_relaxbyDFTB.py
NVE-MD followed by NVT-MD (both by DFTB+)
-----------------------------------------
Note that this example is unphysical for at least two reasons:
- no spin polarization for the oxygen molecule
- the Berendsen coupling is too strong (0.01 here should be 0.0001)
.. literalinclude:: dftb_ex3_make_2h2o.py
DFTB+ calculator class
======================
.. autoclass:: ase.calculators.dftb.Dftb
|
