Parameterizations¶

DFTB.org¶

The DFTB implementation shipped by SCM provides the most up-to-date parameter sets available on the DFTB.org website. The following sets are currently shipped:

• 3ob-3-1 (Br, C, Ca, Cl, F, H, I, K, Mg, N, Na, O, P, S, Zn): general purpose set for the DFTB3 method
• mio-1-1 (H, C, N, O, S, P): for bio and organic molecules with SCC-DFTB
• pbc-0-3 (Si, F, O, N, C, H, Fe): for solid and surfaces
• matsci-0-3 (Al, Si, Cu, Na, Ti, Ba): for various compounds in material science

In addition, we ship the following specific purpose parameter sets:

• 3ob-freq: modified 3ob parameters for a better description of vibrational frequencies
• 3ob-hhmod: modified H-H for 3ob (for a better description of H2)
• 3ob-nhmod: modified N-H for 3ob (improves sp3-N proton affinities)
• 3ob-ophyd: modified O-P for 3ob (improves description of pentavalent phosphorus species)
• auorg (Au + mio): for gold-thiolate compounds
• borg (B, H): boron systems (solids and molecules)
• chalc-0-1 (As + mio): for chalcogenide glasses
• halorg (F, Cl, Br, I + mio): for halogens
• hyb-0-1 (Ag, Ga, As, Si + mio): for organic and inorganic hybrid systems
• magsil (Mg, Si, O, H, Mg): for chrisotyle nanotubes
• miomod-hh: contains a modified parameter set for H2
• miomod-nh: contains a modified parameter set fo N-H to improve N-H binding energies
• siband (Si, O, H): electronic parameters for accurate silicon and silicon dioxide band structures
• tiorg-0-1 (Ti + mio): for Ti bulk, TiO2 bulk, TiO2 surfaces, and TiO2 with organic molecules
• trans3d-0-1 (Sc, Ti, Fe, Co, Ni + mio): transition metal elements for biological systems
• znorg-0-1 (Zn + mio): for Zn bulk, ZnO bulk, ZnO surfaces, and ZnO with organic molecules

We recommend to visit the DFTB.org web site for more detailed information about each set. We are committed to shipping all DFTB.org parameter sets in their latest version. If you miss one of the DFTB.org parameter sets in our distribution, please contact us at support@scm.com. Please note that our implementation of DFTB currently does not support parameter sets containing f-functions, such as the “rare” set.

QUASINANO¶

The QUASINANO2013.1 set of DFTB parameter files available in the AMS package is designed by Mohammad Wahiduzzaman et al. contains parameters for a large part of the periodic table (no f-elements). Note that the QUASINANO2013.1 set only contains the electronic part of the interaction, so that only the spectrum for a given geometry can be calculated, but no total energy, and thus also no forces. These parameters can be used in TD-DFTB calculations, for example.

The QUASINANO2015 parameter set extends the QUASINANO2013.1 parameter set, and includes terms that are needed to compute the total energy and its gradient.

Dresden¶

The so called Dresden set of DFTB parameter files available in the AMS package were designed by J. Frenzel, A.F. Oliveira, N. Jardillier, T. Heine, and G. Seifert, mainly at the Technische Universität in Dresden, Germany, see also some additional information about the generation of these parameter files. These parameter files are kept in the directory $AMSHOME/atomicdata/DFTB/Dresden.

Slater-Koster files¶

Most of the data constituting a DFTB parameter set is contained in the so called Slater-Koster files. These are typically text files with the file extension .skf. For legal reasons, some parameter sets that are shipped with AMS the Slater-Koster files are encrypted though, in which case their file extension is .ske.

There is generally a Slater-Koster file per pair of elements supported by the parameter set, e.g. for a set supporting the four elements C,H,O,N there will be 16 Slater-Koster files in total. The Slater-Koster file names contain the symbols of the elements, e.g. C-H.skf, H-O.skf and C-C.skf. Note that files for both element orders, e.g. C-H.skf and H-C.skf, are needed and differ in general. The Slater-Koster files contain the matrix elements of the Hamiltonian operator and the overlap between between basis functions centered on two atoms, tabulated for different distances. They also contain a description of a repulsive potential between the two atoms. Furthermore the one element Slater-Koster files (like H-H.skf and C-C.skf) contain some information about the individual atom, e.g. orbital energies of the atomic orbitals. A detailed description of the Slater-Koster file format can be found at DFTB.org.

Additional .yaml files¶

The Slater-Koster file format is relatively old and very inflexible. Over the years extensions of the DFTB method (e.g. spin-polarization, DFTB3, dispersion corrections) have been developed that require parameters which do not have a place in the Slater-Koster files. In the AMS implementation of DFTB, these parameters are stored in additional .yaml files in the resources directory of the parameter set: The additional_parameters.yaml file as well as per element .yaml files, e.g. H.yaml and C.yaml.

The additional_parameters.yaml file contains anything that applies to the entire parameter set and does not depend on the individual elements. At the moment this is:

grimme_d3bj_params: s6 s8 a1 a2

The fitting parameters for Grimme’s D3-BJ dispersion correction. This entry is mandatory if the metainfo.yaml file lists D3-BJ as a supported dispersion correction method.

grimme_d4_params: s6 s8 a1 a2

The fitting parameters for Grimme’s D4 dispersion correction. This entry is mandatory if the metainfo.yaml file lists D4 as a supported dispersion correction method.

zeta_Hcorr: zeta

A single number zeta used in the HX-damping usually applied in DFTB3 calculations.

The per element .yaml files may contain the following entries:

hubbard: U_atom

The atomic Hubbard parameter used in a normal, atomic SCC cycle is specified in the element’s .yaml file as the hubbard entry. It is quite surprising that such a commonly used parameter does not have its place in the Slater-Koster files, which only hold the shell-dependent Hubbard parameters. For atomic SCC cycles it is common practice to use the Hubbard parameter of the s-shell as the atomic Hubbard parameter, even though the two values are not strictly related. For consistency with other DFTB implementations, AMS DFTB will do the same if the atomic Hubbard parameter is not found in the element’s .yaml file. However, it will also notify the user about this potentially questionable behavior.

hubbard_derivative: dUdq

The derivative of the the atomic hubbard parameter with respect to the atomic charge. This is information is required to perform DFTB3 calculations.

magnetic_hubbard and magnetic_hubbard_ldep

The magnetic Hubbard parameters (often abbreviated W in the literature). These are required for unrestricted calculations and TD-DFTB singlet-triplet excitations. Depending on whether the parameter set allows atomic and/or shell resolved SCC cycles, the magnetic Hubbard parameter is given as a single number and/or a small matrix:

magnetic_hubbard: W_atom

magnetic_hubbard_ldep: >
    W_ss  W_sp
    W_ps  W_pp

The size of the matrix is determined by the number of basis functions on the element. Note that the > is essential to start a multiline entry (in which line breaks are ignored).

Basis function information in .rkf files¶

The QUASINANO parameter sets additionally have per element .rkf files in the resources directory, e.g. H.rkf and C.rkf. These binary files, which can be opened in the GUI with KFBrowser, contain information about the basis functions used to calculate the matrix elements in the Slater-Koster files. While this information is not needed to perform the DFTB calculation itself, it is used by the GUI in order to visualize properties like molecular orbitals or densities.