# Keywords¶

## Summary of all keywords¶

DispersionCorrection
Type: Multiple Choice None [None, Auto, UFF, ULG, D2, D3-BJ] This key is used to specify a 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.
Model
Type: Multiple Choice SCC-DFTB [DFTB, SCC-DFTB, DFTB3] 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. The choice has to be supported by the selected parameter set.
Occupation
Type: Block Controls the convergence criteria of the SCC cycle.
Strategy
Type: Multiple Choice Auto [Auto, Aufbau, Fermi] This optional key allows to specify the fill strategy to use for the molecular orbitals. Can either be ‘Aufbau’ for simply filling the energertically 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 5.0 Kelvin The Fermi temperature used for the Fermi-Dirac distribution. Ignored in case of aufbau occupations.
Periodic
Type: Block Block that sets various details of the calculation only relevant for periodic systems.
BZPath
Type: Block If [BandStructure%Automatic] is disabled, DFTB will compute the band structure for the user-defined path in the [BZPath] block. You should define the vertices of your path in fractional coordinates (with respect to the reciprocal lattice vectors) in the [Path] sub-block. If you want to make a jump in your path, you need to specify a new [Path] sub-block.
Path
Type: Non-standard block True A section of a k space path.
BandStructure
Type: Block Options for band structure plotting. This has no effect on the calculated energy. [Warning: The band structure is only computed in case of k-space sampling, i.e. it is not computed for Gamma-only calculations (see: Periodic%KSpace).]
Automatic
Type: Bool True Generate and use the standard path through the Brillouin zone. If not, use the user defined path (set via Custom path in the GUI, or with the Periodic%BZPath keyword in the run script).
DeltaK
Type: Float 0.1 1/Bohr Step size in reciprocal space for band structure interpolation. Using a smaller number will produce smoother band curves at an increased computational time.
Enabled
Type: Bool True Whether or not to calculate the band structure.
FatBands
Type: Bool True Control the computation of the fat bands (only when the bandstructure is calculated). The fat bands are the periodic equivalent of the Mulliken population analysis. The definition of the fat bands can be found in the Band Documentation.
UseSymmetry
Type: Bool True If set, only the irreducible wedge of the Wigner-Seitz cell is sampled. If not, the whole (inversion-unique) Wigner-Seitz cell is sampled.
DOS
Type: Block The subkeys of [DOS] allow to customize the calculation of the density of states.
EMax
Type: Float 0.75 Hartree Upper end of the energy interval in which the density of states is calculated.
EMin
Type: Float -0.75 Hartree Lower end of the energy interval in which the density of states is calculated.
NSteps
Type: Integer 300 The number of energy intervals between [EMin] and [EMax] for which the density of states is calculated.
EffectiveMass
Type: Block In a semi-conductor, the mobility of electrons and holes is related to the curvature of the bands at the top of the valence band and the bottom of the conduction band. With the effective mass option, this curvature is obtained by numerical differentiation. The estimation is done with the specified step size, and twice the specified step size, and both results are printed to give a hint on the accuracy. By far the most convenient way to use this key is without specifying any options.
Enabled
Type: Bool False In a semi-conductor, the mobility of electrons and holes is related to the curvature of the bands at the top of the valence band and the bottom of the conduction band. With the effective mass option, this curvature is obtained by numerical differentiation. The estimation is done with the specified step size, and twice the specified step size, and both results are printed to give a hint on the accuracy. By far the most convenient way to use this key is without specifying any options.
KPointCoord
Type: Float List 1/Bohr True Coordinate of the k-points for which you would like to compute the effective mass.
NumAbove
Type: Integer 1 Number of bands to take into account above the Fermi level.
NumBelow
Type: Integer 1 Number of bands to take into account below the Fermi level.
StepSize
Type: Float 0.001 Size of the step taken in reciprocal space to perform the numerical differentiation
KSpace
Type: Integer 1 This parameter controls the number of k-points used in the calculation. By default DFTB does not do any k-space sampling and uses only the Gamma-point as the only k-point. This should be sufficient for systems with large unit cells For smaller systems, k-space sampling can be enabled explicitly using this keyword. For very small unit cells (one atom wide) a value of 5 is advised. For medium sized unit cells 3 is adequate. The k-space sampling is relatively new in DFTB and as of the ADF2017 release still has some incompatibilities with other features: At the moment it is not possible to use k-space sampling in combination with DFTB3, spin-polarization, l-dependent SCC cycles or density matrix purification. Furthermore, if KSpace is not 1 (Gamma-only in GUI) DFTB can only run in serial mode. If not running via the GUI you need to do this yourself (use NSCM=1).
Screening
Type: Block For SCC-DFTB in periodic systems the Coulomb interaction is screened with a Fermi-Dirac like function defined as TODO S(r)=1/(exp((r-r_madel)/d_madel)+1). Screening is always enable, even if this section is absent. This section allows to change some details of the screening procedure.
dMadel
Type: Float Bohr Sets the smoothness of the screening function. The default is 1/10 of [rMadel].
rMadel
Type: Float Bohr Sets the range of the screening function. The default is 2x the norm of the longest lattice vector.
Properties
Type: Block DFTB can calculate various properties of the simulated system. This block configures which properties will be calculated.
BondOrders
Type: Bool False Whether or not Mayer bond orders are calculated based on the final molecular orbitals.
DipoleMoment
Type: Bool True Whether of not the electric dipole moment is calculated from the calculated Mulliken charges. While it is technically possible to calculate the dipole moment with the DFTB0 model, it is not recommended and the SCC-DFTB or DFTB3 model should be used instead. For periodic systems the dipole moment is ill-defined and should not be interpreted.
Excitations
Type: Block Contains all options related to the calculation of excited states, either as simple single orbitals transitions or from a TD-DFTB calculation.
SingleOrbTrans
Type: Block The simplest approximation to the true excitations are the single orbital transitions (sometimes called Kohn-Sham transitions), that is transitions where a single electron is excited from an occupied Kohn-Sham orbital into a virtual orbital. The calculation of these transitions is configured in this section. Note that the SingleOrbTrans section is optional even though the single orbital transitions are also needed for TD-DFTB calculations. If the section is not present all single orbital transitions will still be calculated and used in a subsequent TD-DFTB calculation, but no output will be produced.
Enabled
Type: Bool False Calculate the single orbital transitions.
Filter
Type: Block This section allows to remove single orbital transitions based on certain criteria. All filters are disabled by default.
OSMin
Type: Float Removes single orbital transitions with an oscillator strength smaller than this threshold. A typical value to start (if used at all) would be 1.0e-3.
dEMax
Type: Float Hartree Removes single orbital transitions with an orbital energy difference larger than this threshold.
dEMin
Type: Float Hartree Removes single orbital transitions with an orbital energy difference smaller than this threshold.
PrintLowest
Type: Integer 10 The number of single orbital transitions that are printed to the screen and written to disk. If not a TD-DFTB calculation, the default is to print the 10 lowest single orbital transitions. In case of a TD-DFTB calculation it is assumed that the single orbital transitions are only used as an input for TD-DFTB and nothing will be printed unless PrintLowest is specified explicitly.
TDDFTB
Type: Block Calculations with time-dependent DFTB can be configured in the TDDFTB section and should in general give better results than the raw single orbital transitions. TD-DFTB calculates the excitations in the basis of the single orbital transitions, whose calculation is configured in the SingleOrbTrans section. Using a filter in SingleOrbTrans can therefore be used to reduce the size of the basis for TD-DFTB. One possible application of this is to accelerate the calculation of electronic absorption spectra by removing single orbital transitions with small oscillator strengths from the basis. Note that the entire TDDFTB section is optional. If no TDDFTB section is found, the behavior depends on the existence of the SingleOrbTrans section: If no SingleOrbTrans section is found (the Excitations section is completely empty then) a TD-DFTB calculation with default parameters will be performed. If only the SingleOrbTrans section is present no TD-DFTB calculation will be done.
Calc
Type: Multiple Choice None [None, Singlet, Triplet] Specifies the multiplicity of the excitations to be calculated.
DavidsonConfig
Type: Block This section contains a number of keywords that can be used to override various internals of the Davidson eigensolver. The default values should generally be fine.
ATCharges
Type: Multiple Choice Precalc [Precalc, OnTheFly] Select whether the atomic transition charges are precalculated in advance or reevaluated during the iterations of the Davidson solver. Precalculating the charges will improve the performance, but requires additional storage. The default is to precalculate the atomic transition charges, but the precalculation may be disabled if not not enough memory is available.
SafetyMargin
Type: Integer 4 The number of eigenvectors the Davidson method will calculate in addition to the ones requested by the user. With the Davidson eigensolver it is generally a good idea to calculate a few more eigenvectors than needed, as depending on the initial guess for the eigenvectors it can happen that the found ones are not exactly the lowest ones. This problem is especially prominent if one wants to calculate only a small number of excitations for a symmetric molecule, where the initial guesses for the eigenvectors might have the wrong symmetry. Note that the additionally calculated excitations will neither be written to the result file nor be visible in the output.
Tolerance
Type: Float 1e-09 Convergence criterion for the norm of the residual.
Diagonalization
Type: Multiple Choice Auto [Auto, Davidson, Exact] Select the method used to solve the TD-DFTB eigenvalue equation. The most straightforward procedure is a direct diagonalization of the matrix from which the excitation energies and oscillator strengths are obtained. Since the matrix grows quickly with system size (number of used single orbital transitions squared), this option is possible only for small molecules. The alternative is the iterative Davidson method, which finds a few of the lowest excitations within an error tolerance without ever storing the full matrix. The default is to make this decision automatically based on the system size and the requested number of excitations.
Lowest
Type: Integer 10 Specifies the number of excitations that are calculated. Note that in case of the exact diagonalization all excitations are calculated, but only the lowest ones are printed to screen and written to the output file. Also note that if limited both by number and by energy, (lowest and upto), DFTB will always use whatever results in the smaller number of calculated excitations.
Print
Type: String Specifies whether to print details on the contribution of the individual single orbital transitions to the calculated excitations.
UpTo
Type: Float Hartree Set the maximum excitation energy. Attempts to calculate all excitations up to a given energy by calculating a number of excitations equal to the number of single orbital transitions in this window. This is only approximately correct, so one should always add some safety margin. Note that if limited both by number and by energy, (lowest and upto), DFTB will always use whatever results in the smaller number of calculated excitations.
TDDFTBGradients
Type: Block This block configures the calculation of analytical gradients for the TD-DFTB excitation energies, which allows the optimization of excited state geometries and the calculation of vibrational frequencies in excited states (see J. Comput. Chem., 28: 2589-2601). If the gradients are calculated, they will automatically be used for geometry optimizations or vibrational frequency calculations, if the corresponding Task is selected. Vibrationally resolved UV/Vis spectroscopy (Franck-Condon Factors) can be calculated in combination with the FCF program. See the ADF documentation on Vibrationally resolved electronic spectra.
Eigenfollow
Type: Bool False If this option is set, DFTB uses the transition density in atomic orbital basis to follow the initially selected excited state during a geometry optimization. This is useful if excited state potential energy surfaces cross each other and you want to follow the surface you started on.
Excitation
Type: Integer 1 Select which excited state to calculate the gradients for. Gradients can only be calculated for an excited states that has been calculated using TD-DFTB. Make sure that enough excitations are calculated.
NBOInput
Type: Bool False Whether or not an input file for the NBO program is written to disk as nboInput.FILE74. The input file follows the FILE47 format as described in the NBO6 manual available on nbo6.chem.wisc.edu. By default, only the calculation of the natural bond orbitals and the natural localized molecular orbitals is enabled, but the nboInput.FILE47 file can be edited by hand to enable other analysis models. Please refer to the NBO6 manual for details.
VCD
Type: Bool False Calculate the VCD spectrum after calculating the IR spectrum. Note: symmetry must be set to NOSYM.
Purify
Type: Block By default (when the DFTB%Purify section is not present), the next step’s density matrix is calculated from molecular orbitals obtained as eigenvectors of the charge-dependent Hamiltonian. An alternative way to obtain the density matrix is using an iterative purification procedure enabled by this keyword. Note also that molecular orbitals are not calculated when using the density matrix purification method, meaning that any subsequent calculations using molecular orbitals (e.g. TD-DFTB or Mayer bond order analysis) are incompatible with this keyword. Density matrix purification can be considerably faster than diagonalization for molecular of gamma-only periodic systems with large HOMO-LUMO gaps. Note that density matrix purification is faster for systems where the density matrix is sparse. The fill-in of the density matrix is printed to the standard output during the calculation. If it is too large (e.g. >50%), it is probably faster to use normal diagonalization.
Enabled
Type: Bool False By default (when purification is not used), the next step’s density matrix is calculated from molecular orbitals obtained as eigenvectors of the charge-dependent Hamiltonian. An alternative way to obtain the density matrix is using an iterative purification procedure. Note that molecular orbitals are not calculated when using the density matrix purification method, meaning that any subsequent calculations using molecular orbitals (e.g. TD-DFTB or Mayer bond order analysis) are incompatible with this keyword. Density matrix purification can be considerably faster than diagonalization for molecular of gamma-only periodic systems with large HOMO-LUMO gaps. Density matrix purification is faster for systems where the density matrix is sparse. The fill-in of the density matrix is printed to the standard output during the calculation. If it is too large (e.g. >50%), it is probably faster to use normal diagonalization.
Iterations
Type: Integer 50 Set the maximum number of steps in the purification cycle. If the desired tolerance is not reached by then, the calculation is aborted.
Tolerance
Type: Float 1e-08 Set the purification convergence threshold. Purification is considered converged when the trace of the density matrix becomes equal to the total number of electrons within the specified tolerance.
Repulsion
Type: Block Configures various details of the repulsive potential.
ResourcesDir
Type: String The directory containing the DFTB parameter files. Absolute starting with / or relative to \$ADFRESOURCES/DFTB otherwise.
SCC
Type: Block This optional section configures various details of the self-consistent charge cycle. If DFTB%Model is set to DFTB0/DFTB1, none of this information is used and the entire section will be ignored.
Converge
Type: Block Controls the convergence criteria of the SCC cycle.
Charge
Type: Float 1e-08 The maximum change in atomic charges between subsequent SCC iterations. If the charges change less, the SCC cycle is considered converged.
HXDamping
Type: Bool 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.
Iterations
Type: Integer 500 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 0.15 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 Activates or disables orbital resolved calculations. If this key is absent the recommended settings from the DFTB parameter file’s metainfo.
Unrestricted
Type: Bool False 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 or k-space sampling periodic calculations.
nDIIS
Type: Integer 20 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.
StoreMatrices
Type: Bool False Determines whether the Hamiltonian and overlap matrices are stored in the binary result file.
UnpairedElectrons
Type: Integer 0 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.