DFTB+

class DFTBplus

This interface supports energy and gradient evaluations with the DFTB+ semi-empirical electronic structure package (v20 or later).

As a QM interface, the options described in QM interfaces are available, as well as the DFTB+ specific options outlined below.

Note

DFTB+ is a semi-empirical code, so the interface will ignore some QM keyword options that are not applicable e.g. basis, ecp, functional.

A DFTB+ Theory object can be specified as follows:

my_frag = Fragment(...)

my_theory = DFTBplus(frag=my_frag,
                     ...)

which would then be called as theory=my_theory in subsequent tasks (see Tasks).

General options

method

High-level method flag that will be used to set default parameters for the calculation. The value of skf_path should be set consistently.

Allowed values:

  • 'mio': (default) Slater-Koster parameters for “bio or organic molecules”. Older than the newer 3ob set.

  • '3ob': Slater-Koster parameters for third-order “bio and organic molecules”. Intended to replace mio.

  • 'matsci' : Slater-Koster parameters for “various problems in materials science”.

  • 'ob2' : Slater-Koster parameters for a “Long range corrected parameterization for bio and organic molecules”

  • 'pbc' : Slater-Koster parameters for “solids and surfaces”.

If it is unambiguous, the method option will be used to set default values for max_angular_momentum, hubbard_derivs, spin_constants, third_order and dispersion_param (although a dispersion correction will not be applied unless requested). Additional parameters may also be set on a case by case basis if the Slater-Koster parameter set instructions require them.

Any options that have been set explicitly will over-write these defaults if they conflict.

scc

(default: True) Use the Self-Consistent-Charge (SCC-DFTB) method.

skf_path

(default: '') Path to directory containing Slater-Koster files (*.skf). These files can be downloaded from https://dftb.org/parameters/download/.

waveplot

(default: True) Enable output files required by the “waveplot” post processing tool for the visualisation of molecular orbitals (namely detailed.xml and eigenvec.bin).

SCF options

hubbard_derivs

(default: {}) Hubbard derivatives for calculation of the 3rd order correction (if requested). These should be given as a dictionary, where a number is given for each elemental symbol in the calculation. The appropriate values depend on the Slater-Koster files employed, and default values will be set by the method option. For example, {‘C’:-0.1492}.

hydrogen_correction

Hydrogen correction to apply (if any).

Allowed values:

  • '': (default) No Hydrogen correction.

  • 'damping': Damping method is applied to short-range interactions between Hydrogen-like atoms and other atoms.

hydrogen_damping

(default: 0.0) Damping factor applied when a damping Hydrogen correction is employed.

max_angular_momentum

(default: {}) Dictionary containing the maximum angular momentum for each element in the calculation. Default values will be set by the method option. For example, {‘H’:’s’, ‘C’:’p’}.

maxiter

(default: 100) Maximum SCC iterations allowed if using SCC-DFTB.

smear

(default: True) Whether to apply smearing to the SCC occupation. DFTB+ refers to smearing as ‘filling’.

smear_method

Smearing method to be applied when smearing is used.

Allowed values:

  • 'Fermi': (default) Apply a Fermi-Dirac distribution to the single electron states.

  • 'MethfesselPaxton': Apply a Methfessel-Paxton distribution to the single electron states, which can be an improvement for systems that require a high electronic temperature.

smear_mp_order

(default: 2) Order of the Methessel-Paxton smearing, if it is being employed.

smear_temperature

(default: '0.0') Electronic temperature for smearing, given in Kelvin.

third_order

(defualt: None) Whether to perform 3rd order DFTB. Default value depends on the method option. Corresponds to the ThirdOrderFull option in DFTB+.

Dispersion options

dispersion

Dispersion correction to apply (if any).

Allowed values:

  • '' : (default) No dispersion correction applied.

  • 'LennardJones' : Dispersion is included via a Lennard-Jones potential between each pair of atoms. Parmeters are automatically taken from the Universal Force Field (UFF).

  • 'D3' : The Grimme D3 correction. Parameters are defaulted from the method keyword, and can be modified using dispersion_param.

  • 'MBD' : Applies the Tkatchenko many-body-dispersion model.

dispersion_param

(default: {}) A dictonary used to set parameters for dispersion corrections.

Allowed keys:

  • 'a1', 'a2', 's6', 's8' : Parameters for the D3 dispersion correction.