Model Hamiltonians

As of the 2020 release, the DFTB engine supports two different classes of model Hamiltonians, Grimme’s extended tight-binding, and the classic Slater-Koster based DFTB. All of these model Hamiltonians are obtained by applying tight-binding approximations to the DFT total energy expression.

Slater-Koster based DFTB

The efficiency of Slater-Koster based DFTB stems from its use of an optimized minimum valence orbital basis that reduces the linear algebra operations, and a two center-approximation for the Kohn-Sham potential that allows precalculation and storage of integrals using the Slater-Koster technique. This makes DFTB orders of magnitude faster than DFT, but requires parameter files (containing the integrals) for all pair-wise combinations of atoms in a molecule. Many elements can be handled with the parameter sets included in the distribution. Alternatively, sets of parameters in the SKF format can be downloaded and used from third party sources.

There are three flavors of Slater-Koster based DFTB available in our implementation:

  • The “plain” DFTB Hamiltonian as introduced by Porezag and Seifert without a self-consistency cycle.
  • The second order self-consistent charge extension SCC-DFTB (recently also called DFTB2), which accounts for density fluctuations and improves results on polar bonds. Note that the self-consistent calculations is about an order of magnitude slower than calculations with the “plain” DFTB Hamiltonian.
  • The third order extension known as DFTB3, which improve the description of hydrogen-bonded complexes and proton affinities. Note that DFTB3 calculations are only marginally slower than SCC-DFTB based calculations.

Note that since these methods have been respectively parametrized, it is important to specify a matching parameter set when applying one of these models.

Extended tight-binding (xTB)

The extended tight-binding (xTB) model Hamiltonian as recently been introduced by Grimme and coworkers. It makes similar approximations as Slater-Koster based DFTB, but instead of using precalculated integrals, xTB employs a (small) basis of Slater-type orbitals and uses an extended Hückel-like approximation for the Hamiltonian.

The DFTB Engine supports the GFN1-xTB parameterization of xTB, which is optimized for geometries, frequencies and non-covalent interactions and covers all elements of the periodic table up to radon.

Model Hamiltonian

The following keys allow you to select a model Hamiltonian and control different aspects of how the stationary Schroedinger equation is solved.

Model [DFTB | SCC-DFTB | DFTB3 | GFN1-xTB | NonSCC-GFN1-xTB]
Model
Type:Multiple Choice
Default value:GFN1-xTB
Options:[DFTB, SCC-DFTB, DFTB3, GFN1-xTB, NonSCC-GFN1-xTB]
Description:Selects the Hamiltonian used in the DFTB calculation: - DFTB/DFTB0/DFTB1 for classic DFTB without a self-consistent charge cycle - SCC-DFTB/DFTB2 with a self-consistency loop for the Mulliken charges - DFTB3 for additional third-order contributions. - GFN1-xTB for Grimme’s extended tight-binding model in the GFN1 version. - NonSCC-GFN1-xTB for a less accurate but faster version of GFN1-xTB without a self-consistency cycle The choice has to be supported by the selected parameter set.

Different parameters may be suitable for different model Hamiltonians. It is important to choose the appropriate parameter set for the type of calculation and molecular system under study, see parameter sets.

ResourcesDir string
ResourcesDir
Type:String
Description:The directory containing the parameter files. The path can be absolute or relative. Relative paths starting with ./ are considered relative to the directory in which the calculation is started, otherwise they are considered relative to $AMSRESOURCES/DFTB. This key is required for the Slater-Koster based DFTB models, but optional for xTB.

Examples:

ResourcesDir Dresden
Uses the resource directory $AMSRESOURCES/DFTB/Dresden.
ResourcesDir /home/myusername/myparamsdir
Uses the specified path /home/myusername/myparamsdir as the resource directory.

NOTE: Each resource directory must contain a file called metainfo.yaml, which specifies the capabilities of the parameter set. For details see metainfo.yaml.

Dispersion correction

The selected model Hamiltonian can be extended with dispersion correction:

DispersionCorrection [None | Auto | UFF | ULG | D2 | D3-BJ]
DispersionCorrection
Type:Multiple Choice
Default value:None
Options:[None, Auto, UFF, ULG, D2, D3-BJ]
GUI name:Dispersion
Description:This key is used to specify an empirical dispersion model. Please refer to the DFTB documentation for details on the different methods. By default no dispersion correction will be applied. Setting this to auto applies the dispersion correction recommended in the DFTB parameter set’s metainfo file. Note that the D3-BJ dispersion correction is enabled by default when using the GFN1-xTB model Hamiltonian, but can be disabled manually by setting this keyword to None.

Solvation (GBSA)

Solvation effects can be included via the implicit GBSA solvation model. We gratefully acknowledge the Grimme’s group in Bonn for their contribution of the GBSA solvation method code.

To enable the GBSA method, specify the desired solvent:

Solvation
   Solvent [...]
End
Solvation
Type:Block
Description:Generalized Born solvation model with Solvent Accessible Surface Area (GBSA).
Solvent
Type:Multiple Choice
Default value:None
Options:[None, Acetone, Acetonitrile, CHCl3, CS2, DMSO, Ether, H2O, Methanol, THF, Toluene]
Description:Solvent used in the GBSA implicit solvation model.

More options can be specified in the Solvation block:

Solvation
   UseGSASA Yes/No
   GSolvState [Gas1BarSolvent | Gas1MSolvent1M | Gas1BarSolvent1M]
   Temperature float
   SurfaceGrid [230 | 974 | 2030 | 5810]
End
Solvation
UseGSASA
Type:Bool
Default value:Yes
GUI name:Solvation Free Energy
Description:Include shift term and G(SASA) terms in the energy and gradient.
GSolvState
Type:Multiple Choice
Default value:Gas1MSolvent1M
Options:[Gas1BarSolvent, Gas1MSolvent1M, Gas1BarSolvent1M]
Description:Reference state for solvation free energy shift.
Temperature
Type:Float
Default value:298.15
Unit:Kelvin
Description:The temperature used when calculating the solvation free energy shift. Only used for ‘Gas1BarSolvent’ and ‘Gas1BarSolvent1M’ GSolvState options.
SurfaceGrid
Type:Multiple Choice
Default value:230
Options:[230, 974, 2030, 5810]
Description:Number of angular grid points for the construction of the solvent accessible surface area. Usually the default number of grid point suffices, but in case of suspicious behaviors you can increase the number of points.

SCC details and spin-polarization

SCC
   AdaptiveMixing Yes/No
   Converge
      Charge float
   End
   HXDamping Yes/No
   Iterations integer
   Mixing float
   OrbitalDependent Yes/No
   Unrestricted Yes/No
   nDIIS integer
End
SCC
Type:Block
Description:This optional section configures various details of the self-consistent charge cycle. If the model Hamiltonian does not need a self-consistent solution (e.g. plain DFTB0), none of this information is used and the entire section will be ignored.
AdaptiveMixing
Type:Bool
Default value:Yes
Description:Change the mixing parameter based on the monitored energy. A significant increase of energy will strongly reduce the mixing. Then it will slowly grow back to the SCC%Mixing value.
Converge
Type:Block
Description:Controls the convergence criteria of the SCC cycle.
Charge
Type:Float
Default value:1e-08
GUI name:Charge convergence
Description:The maximum change in atomic charges between subsequent SCC iterations. If the charges change less, the SCC cycle is considered converged.
HXDamping
Type:Bool
Description:This option activates the DFTB3 style damping for H-X bonds. Note that this is always enabled if the DFTB%Model key is set to DFTB3. Not used with xTB.
Iterations
Type:Integer
Default value:500
Description:Allows to specify the maximum number of SCC iterations. The default should suffice for most standard calculations. Convergence issues may arise due to the use of the Aufbau occupations for systems with small HOMO-LUMO gaps. In this case the use of a Fermi broadening strategy may improve convergence. Choosing a smaller mixing parameter (see DFTB%SCC%Mixing) may also help with convergence issues: it often provides a more stable but slower way to converge the SCC cycle.
Mixing
Type:Float
Default value:0.15
Description:The parameter used to mix the DIIS linear combination of previously sampled atomic charge vectors with an analogous linear combination of charge vectors resulting from population analysis combination. It can assume real values between 0 and 1.
OrbitalDependent
Type:Bool
Description:Activates or disables orbital resolved calculations. If this key is absent the recommended settings from the parameter file’s metainfo.
Unrestricted
Type:Bool
Default value:No
Description:Enables spin unrestricted calculations. Only collinear spin polarization is supported, see Theor Chem Acc (2016) 135: 232, for details. Must be supported by the chosen parameter set. Not yet compatible with DFTB3, k-space sampling periodic calculations or the xTB models.
nDIIS
Type:Integer
Default value:20
Description:Specifies the maximum number of samples considered during the direct inversion of iteration of subspace (DIIS) extrapolation of the atomic charges during the SCC iterations. A smaller number of samples potentially leads to a more aggressive convergence acceleration, while a larger number often guarantees a more stable iteration. Due to often occurring linear dependencies within the set of sample vectors, the maximum number of samples is reached only in very rare cases. 
Occupation
   KT float
   NumBoltz integer
   Strategy [Auto | Aufbau | Fermi]
   Temperature float
End
Occupation
Type:Block
Description:Configures the details of how the molecular orbitals are occupied with electrons.
KT
Type:Float
Default value:0.0
Unit:Hartree
Description:(KT) Boltmann constant times temperature, used for electronic temperature with strategy is auto
NumBoltz
Type:Integer
Default value:10
Description:The electronic temperature is done with a Riemann Stieltjes numerical integration, between zero and one occupation. This defines the number of points to be used.
Strategy
Type:Multiple Choice
Default value:Auto
Options:[Auto, Aufbau, Fermi]
GUI name:Occupation
Description:This optional key allows to specify the fill strategy to use for the molecular orbitals. Can either be ‘Aufbau’ for simply filling the energetically lowest orbitals, or ‘Fermi’ for a smeared out Fermi-Dirac occupation. By default the occupation strategy is determined automatically, based on the other settings (such as the number of unpaired electrons).
Temperature
Type:Float
Default value:300.0
Unit:Kelvin
GUI name:Fermi temperature
Description:The Fermi temperature used for the Fermi-Dirac distribution. Ignored in case of aufbau occupations. 
UnpairedElectrons integer
UnpairedElectrons
Type:Integer
Default value:0
GUI name:Spin polarization
Description:This specifies the number of unpaired electrons (not the multiplicity!). This number will then be used in the orbital-filling strategy. Has to be compatible with the total number of electrons, meaning it must be an even number if the total number of electrons is even and odd if the total number is odd. Must be an integer value. Note that this does not activate spin polarization, it only affects the filling of the orbitals.

k-space integration

As of the 2019 release, the k-space integration is unified between BAND and DFTB and uses the same keys as input, and the same defaults. See the page on k-space integration in the BAND manual for details and recommendations.

KSpace
   Quality [GammaOnly | Basic | Normal | Good | VeryGood | Excellent]
   Regular
      NumberOfPoints integer_list
   End
   Symmetric
      KInteg integer
   End
   Type [Regular | Symmetric]
End
KSpace
Type:Block
Description:Options for the k-space integration (i.e. the grid used to sample the Brillouin zone).
Quality
Type:Multiple Choice
Default value:Normal
Options:[GammaOnly, Basic, Normal, Good, VeryGood, Excellent]
GUI name:K-space
Description:Select the quality of the K-space grid used to sample the Brillouin Zone. If ‘GammaOnly’, only one point (the gamma point) will be used. For the other options, the actual number of K points generated depends on the size of the unit cell. The larger the real space cell, the fewer K points will be generated. The CPU-time and accuracy strongly depend on this option.
Regular
Type:Block
Description:Options for the regular k-space integration grid.
NumberOfPoints
Type:Integer List
Description:Use a regular grid with the specified number of k-points along each reciprocal lattice vector. For 1D periodic systems you should specify only one number, for 2D systems two numbers, and for 3D systems three numbers.
Symmetric
Type:Block
Description:Options for the symmetric k-space integration grid.
KInteg
Type:Integer
GUI name:Accuracy
Description:Specify the accuracy for the Symmetric method. 1: absolutely minimal (only the G-point is used) 2: linear tetrahedron method, coarsest spacing 3: quadratic tetrahedron method, coarsest spacing 4,6,… (even): linear tetrahedron method 5,7…. (odd): quadratic method The tetrahedron method is usually by far inferior.
Type
Type:Multiple Choice
Default value:Regular
Options:[Regular, Symmetric]
GUI name:K-space grid type
Description:The type of k-space integration grid used to sample the Brillouin zone (BZ) used. ‘Regular’: simple regular grid. ‘Symmetric’: symmetric grid for the irreducible wedge of the first BZ (useful when high-symmetry points in the BZ are needed to capture the correct physics of the system, graphene being a notable example).

xTB specific keywords

A few keywords only apply to the xTB model Hamiltonian.

XTBConfig
   SlaterRadialThreshold float
   useXBTerm Yes/No
End
XTBConfig
Type:Block
Description:This block allows for minor tweaking.
SlaterRadialThreshold
Type:Float
Default value:1e-05
Description:Threshold determining the range of the basis functions. Using a larger threshold will speed up the calculation, but will also make the results less accurate.
useXBTerm
Type:Bool
Default value:No
Description:Whether to use the Halogen bonding (XB) term. This is not advised as it has a non-continuous PES.

Note

The GFN1-xTB implementation in AMS currently does not implement the electronic entropy term from the article by Grimme et al. It therefore gives slightly different energies (but not gradients!) for systems with partially occupied molecular orbitals.

Technical options

Technical
   AnalyticalStressTensor Yes/No
   EwaldSummation
      CellRangeFactor float
      Enabled Yes/No
      Tolerance float
   End
   MatricesViaFullMaxSize integer
   Parallel
      nCoresPerGroup integer
      nGroups integer
      nNodesPerGroup integer
   End
   ReuseKSpaceConfig Yes/No
   Screening
      dMadel float
      rMadel float
   End
   UseGeneralizedDiagonalization Yes/No
End
Technical
Type:Block
Description:This optional section is about technical aspects of the program that should not concern the normal user.
AnalyticalStressTensor
Type:Bool
Default value:Yes
Description:Whether to compute the stress tensor analytically. Note: This can only be used together with Ewald summation as it will give (slightly) wrong results with Madelung screening.
EwaldSummation
Type:Block
Description:Configures the details of the Ewald summation of the Coulomb interaction.
CellRangeFactor
Type:Float
Default value:2.0
Description:Smaller values will make the Ewald summation less accurate but faster.
Enabled
Type:Bool
Default value:Yes
Description:Whether to use Ewald summation for the long-range part of the Coulomb interaction. Otherwise screening is used.
Tolerance
Type:Float
Default value:1e-10
Description:Larger values will make the Ewald summation less accurate but faster.
MatricesViaFullMaxSize
Type:Integer
Default value:2047
Description:Matrices smaller than this size are constructed via a full matrix. This is faster, but uses more memory in the construction.
Parallel
Type:Block
Description:Calculation of the orbitals in several k-points is trivially parallel.
nCoresPerGroup
Type:Integer
Description:Number of cores in each working group.
nGroups
Type:Integer
Description:Total number of processor groups. This is the number of tasks that will be executed in parallel.
nNodesPerGroup
Type:Integer
GUI name:Cores per task
Description:Number of nodes in each group. This option should only be used on homogeneous compute clusters, where all used compute nodes have the same number of processor cores.
ReuseKSpaceConfig
Type:Bool
Default value:Yes
Description:Keep the number of k-points constant during a lattice optimization. Otherwise the PES might display jumps, because the number of points depends on the lattice vector sizes. If this option is on it will always use the number of k-points that was used from a previous result.
Screening
Type:Block
Description:For SCC-DFTB in periodic systems the Coulomb interaction can (instead of using Ewald summation) be screened with a Fermi-Dirac like function defined as S(r)=1/(exp((r-r_madel)/d_madel)+1). This section allows to change some details of the screening procedure. Note that Coulomb screening is only used if the Ewald summation is disabled.
dMadel
Type:Float
Unit:Bohr
Description:Sets the smoothness of the screening function. The default is 1/10 of [rMadel].
rMadel
Type:Float
Unit:Bohr
Description:Sets the range of the screening function. The default is 2x the norm of the longest lattice vector.
UseGeneralizedDiagonalization
Type:Bool
Default value:Yes
Description:Whether or not to use generalized diagonalization. Does not affect the results, but might be faster or slower. 
StoreMatrices Yes/No
StoreMatrices
Type:Bool
Default value:No
Description:Determines whether the Hamiltonian and overlap matrices are stored in the binary result file.