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# DFT-D3

## Introduction

The DFTD3 calculator class wraps the ‘dftd3’ command line utility by the research group of Stefan Grimme. This can be used to calculate classical vdW dispersion corrections to a large number of common DFT functionals. This calculator can be used in conjunction with other DFT calculators such as GPAW to allow seamless calculation of dispersion-corrected DFT energies, forces, and stresses.

This is a list of all supported keywords and settings:

Keyword | Default value | Description
--- | --- | ---
`xc` | `'pbe'` | Use parameters optimized for the selected XC functional.
`func` | `None` | Alternative to `xc`. Use one or the other.
`grad` | `True` | Enable or disable calculation of gradients (forces, stress tensor).
`abc` | `False` | Enable three-body ATM correction.
`cnthr` | 40 Bohr | Cutoff radius for coordination number and three-body calculations.
`cutoff` | 95 Bohr | Cutoff radius for two-body dispersion calculations.
`old` | `False` | Enable older DFT-D2 dispersion correction method.
`damping` | `'zero'` | Damping method. Valid options are `'zero'`, `'bj'`, `'zerom'`, and `'bjm'`.
`tz` | `False` | Custom parameters optimized for triple-zeta basis sets.
`s6` |  | Custom damping parameter used in all damping methods.
`sr6` |  | Custom damping parameter used in `'zero'` and `'zerom'` damping methods.
`s8` |  | Custom damping parameter used in all damping methods.
`sr8` |  | Custom damping parameter used in `'zero'` and `'zerom'` damping methods.
`alpha6` |  | Custom damping parameter used in all damping methods.
`a1` |  | Custom damping parameter used in `'bj'` and `'bjm'` damping methods.
`a2` |  | Custom damping parameter used in `'bj'` method.
`beta` |  | Custom damping parameter used in `'bjm'` method.

## Examples

DFTD3 can be used by itself to calculate only the vdW correction to a system’s energy, forces, and stress. Note that you should not use these properties alone to perform dyanmics, as DFTD3 is not a full classical potential.

    from ase.calculators.dftd3 import DFTD3
    from ase.build import bulk

    diamond = bulk('C')
    d3 = DFTD3()
    diamond.set_calculator(d3)
    diamond.get_potential_energy()

If used in conjunction with a DFT calculator, DFTD3 returns dispersion-corrected energies, forces, and stresses which can be used to perform dynamics.

    import numpy as np
    from gpaw import GPAW, PW

    from ase.calculators.dftd3 import DFTD3
    from ase.build import bulk
    from ase.constraints import UnitCellFilter

    from ase.optimize import LBFGS

    np.random.seed(0)

    diamond = bulk('C')
    diamond.rattle(stdev=0.1, seed=0)
    diamond.cell += np.random.normal(scale=0.1, size=(3,3))
    dft = GPAW(xc='PBE', kpts=(8,8,8), mode=PW(400))
    d3 = DFTD3(dft=dft)
    diamond.set_calculator(d3)

    ucf = UnitCellFilter(diamond)

    opt = LBFGS(ucf, logfile='diamond_opt.log', trajectory='diamond_opt.traj')
    opt.run(fmax=0.05)

## Additional information

This calculator works by writing either an `xyz` file (for non-periodic systems) or a `POSCAR` file (for periodic systems), calling the `dftd3` executable, and parsing the output files created. It has been written such that its interface should match that of the `dftd3` utility itself as closely as possible, while minimizing the possibility of setting redundant and contradictory options. For example, you can only select one damping method, and the interface will sanity-check any provided custom damping parameters.

Without any arguments, the DFTD3 will default to calculating the PBE-D3 dispersion correction with `'zero'` damping. If a DFT calculator is attached, DFTD3 will attempt to glean the XC functional from the DFT calculator. This will occasionally fail, as `dftd3` is very particular about how the names of XC functionals are to be formatted, so in general you should supply the XC functional to both the DFT calculator and the DFTD3 calculator.

### Caveats

The `dftd3` does not handle systems with only 1D- or 2D-periodic boundary conditions. If your system has 1D or 2D PBC, DFTD3 will calculate the dispersion correction as though it was fully 3D periodic.

If your system is very large, the dispersion calculation can take quite long, especially if you are including three-body corrections (`abc=True`). For highly parallel calculations, this may result in the dispersion correction taking longer than the DFT calculation! This is because the `dftd3` utility is not parallelized and will always run on a single core. Be sure to benchmark this calculator interface on your system before deploying large, heavily parallel calculations with it!

Documentation v: 3.19.0

Versions
  3.20.0b1 (development)
  3.19.0 (latest stable)