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 | D4]

DispersionCorrection

Type: Multiple Choice
Default value: None
Options: [None, Auto, UFF, ULG, D2, D3-BJ, D4]
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.

The newest and most accurate dispersion correction is D4. We recommend both the D3-BJ and D4 dispersion corrections as good defaults, depending on their availability for the specific combination of the model Hamiltonian and parameterization. Note that the D4 dispersion corrections is computationally more expensive than D3-BJ for bulk periodic systems (it scales as O(N³) with the number of atoms and is not parallelized), thus the user may first want to evaluate if the increased accuracy justifies the increased computational cost.

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 [None | Acetone | Acetonitrile | CHCl3 | CS2 | DMSO | Ether | H2O | Methanol | 
            THF | Toluene]
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¶

With SCC DFTB the parametrized Hamiltonian depends on partial atomic charges, that need to be determined self consistently. These charges are usually atomic charges, but they may be shell and/or spin resolved. The self consistency requirement

$\vec{q}^\text{in}=\vec{q}^\text{in}$

is numerically expressed as

$\frac{1}{\sqrt{N_\text{atoms}}} | \vec{q}^\text{in}-\vec{q}^\text{in} | < \epsilon$

The vector norm is by default the so-called L-infinity norm, being the maximum absolute value of the vector elements. The underlying algorithm, however, will minimize the L-2 norm. Based upon the history of past input and ouput charge vectors a next one is guessed

$\vec{q}^\text{guess}=\sum_i c_{i-1}^N (\vec{q}^\text{in}_i + \sigma(\vec{q}^\text{out}_i-\vec{q}^\text{in}_i))$

How many past vectors (N) are used and the value of the coefficients depends on the algorithm, as is the mix factor $\sigma$. The default method is the MultiStepper, which is explained separately. The older DIIS method is more simple to tweak in case the SCC does not converge.

SCC
   AlwaysClaimConvergence Yes/No
   Converge
      Charge float
      Norm [L2 | L-Infinity]
   End
   HXDamping Yes/No
   InheritMixFromPreviousResult Yes/No
   Iterations integer
   Method [DIIS | MultiStepper]
   MinimumAdaptiveMixingFactor float
   OrbitalDependent Yes/No
   Unrestricted Yes/No
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.

AlwaysClaimConvergence

Type: Bool
Default value: No
Description: Even if the SCC does not converge, claim convergence.

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.

Norm

Type: Multiple Choice
Default value: L-Infinity
Options: [L2, L-Infinity]
Description: The LInfinity norm is the more stringent choice. The L2 norm is directly what is optimized by the DIIS procedure, it is scaled by the extra constant factor 2/sqrt(nAtoms).

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.

InheritMixFromPreviousResult

Type: Bool
Default value: No
Description: For some run types, such as GeometryOptimization, a previous result is available. By using the charges from the previous geometry a better initial guess for the SCC procedure may be obtained. Also the last mix factor from the previous result can be loaded, possibly speeding up the SCC.

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.

Method

Type: Multiple Choice
Default value: MultiStepper
Options: [DIIS, MultiStepper]
Description: The DIIS option is the old method. The MultiStepper is much more flexible and is controlled by the SCFMultiSolver block

MinimumAdaptiveMixingFactor

Type: Float
Default value: 0.003
Description: In case of AdaptiveMixing the lower bound for the MixingFactor.

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.

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
Unit: Hartree
Description: (KT) Boltmann constant times temperature, used for electronic temperature with strategy is auto. The default value is the default value for Temperature*3.166815423e-6. This key and Temperature are mutually exlusive.

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.

MultiStepper¶

The MultiStepper introduces the concept of alternating between different steppers (methods). Methods are not switched at every SCF cycle, but rather after a sequence of them, called a stint. At the end of a stint it is considered whether it makes sense to try another stepper.

The key component is the Stepper. This wraps the type of the Stepper, say DIIS or SimpleMixing. Another important component is the MixAdapter. A step is controlled by a mix factor $\sigma$, also often called greed. The next guess charge vector is a linear combination of previous input and output charges

$\vec{q}^\text{guess}=\sum_i c_{i-1}^N (\vec{q}^\text{in}_i + \sigma(\vec{q}^\text{out}_i-\vec{q}^\text{in}_i))$

The larger the mix factor the more aggressive the algorithm. Choosing it too small may simply stall the progress and choosing it too large can cause the error to grow. That is why using a MixAdapter is useful. It tries to predict a reasonable mix value, based on the progress of the error and also based on the number of previous iterations $N$ that can be used without running into numerical problems.

A whole SCFMultiStepper block can be loaded from a file as a preset, and many reside in $AMSHOME/data/presets/multi_stepper. Normal users are not recommended to try to improve the standard preset. Which preset to loaded is controlled by the SCF%MultiStepperPresetPath key, and this may be an absolute path to your own preset.

The the log file (ams.log) shows the active stepper and mix factor.

<Nov22-2022> <15:24:28>  cyc=  0 err=0.00E+00 cpu=  75s ela=  76s
<Nov22-2022> <15:25:26>  cyc=  1 err=4.26E+00 meth=1 nvec= 1 mix=0.0750 cpu=  57s ela=  58s fit=7.06E-02
<Nov22-2022> <15:26:26>  cyc=  2 err=8.33E+00 meth=1 nvec= 2 mix=0.1455 cpu=  59s ela=  60s fit=6.49E-02
<Nov22-2022> <15:27:23>  cyc=  3 err=7.85E+00 meth=1 nvec= 3 mix=0.1499 cpu=  56s ela=  57s fit=6.42E-02
<Nov22-2022> <15:28:24>  cyc=  4 err=7.09E+00 meth=1 nvec= 4 mix=0.1544 cpu=  60s ela=  61s fit=6.37E-02
<Nov22-2022> <15:29:21>  cyc=  5 err=9.49E+00 meth=2 nvec= 1 mix=0.0060 cpu=  57s ela=  57s fit=7.91E-02
<Nov22-2022> <15:30:20>  cyc=  6 err=2.63E+00 meth=2 nvec= 2 mix=0.0062 cpu=  59s ela=  59s fit=7.88E-02
<Nov22-2022> <15:31:18>  cyc=  7 err=3.82E+00 meth=2 nvec= 3 mix=0.0060 cpu=  57s ela=  58s fit=7.84E-02
<Nov22-2022> <15:32:16>  cyc=  8 err=3.53E+00 meth=2 nvec= 4 mix=0.0062 cpu=  58s ela=  58s fit=7.81E-02

From cycle 5 (cyc=5) on the second stepper is tried (meth=2), in this case because the error has grown too much since the start. Furthermore it restarts from the first density, not shown in the log file, using only one older density (nvec=1). Note that the second stepper starts with using a much more conservative mix factor (mix=0.006).

SCC
   SCFMultiStepper
      AlwaysChangeStepper Yes/No
      ErrorGrowthAbortFactor float
      FractionalStepFactor float
      MinStintCyclesForAbort integer
      Stepper header
         AbortSlope float
         DIISStepper
            EDIISAlpha float
            MaxCoefficient float
            MaxVectors integer
            MinVectors integer
            Mix float
         End
         ErrorGrowthAbortFactor float
         ExpectedSlope float
         FractionalStepFactor float
         MaxInitialError float
         MaxIterationNumber integer
         MaxStintNumber integer
         MinInitialError float
         MinIterationNumber integer
         MinStintCyclesForAbort integer
         MinStintNumber integer
         MixAdapter
            ErrorGrowthPanicFactor float
            GrowthFactor float
            MaxMix float
            MinMix float
            NTrialMixFactors integer
            TrialMode [CurrentMixCentered | FullRange]
            Type [Error | Energy | UnpredictedStep | Trial]
         End
         MixStepper
            Mix float
         End
         MultiSecantStepper
            MaxCoefficient float
            MaxVectors integer
            Mix float
            Variant [MSB1 | MSB2 | MSR1 | MSR1s]
         End
         StintLength integer
      End
      StintLength integer
      UsePreviousStintForErrorGrowthAbort Yes/No
   End
   MultiStepperPresetPath string
End

SCC

SCFMultiStepper

Type: Block
Description: To solve the self-consistent problem multiple steppers can be tried during stints using the ones that give the best progress.

AlwaysChangeStepper

Type: Bool
Default value: No
Description: When the progress is fine there is no reason to change the stepper. In practice this is always set to true, because also the Stepper%ExpectedSlope can be used to achieve similar behavior.

ErrorGrowthAbortFactor

Type: Float
Default value: 1000.0
Description: Abort stint when the error grows too much, compared to the error at the start of the stint.

FractionalStepFactor

Type: Float
Default value: -1.0
Description: Multiply the step by this factor. If smaller than zero this is not used.

MinStintCyclesForAbort

Type: Integer
Default value: 0
Description: Look at ErrorGrowthAbortFactor only when a number of steps has been completed since the start of the stint. A value of 0 means always.

Stepper

Type: Block
Recurring: True
Description: ??

AbortSlope

Type: Float
Default value: 100.0
Description: If the slope (at the end of a stint) is larger than this: abort the stepper

DIISStepper

Type: Block
Description: DIIS stepper

EDIISAlpha

Type: Float
Default value: 0.01
Description: The extra energy vector is weighed by this factor. .

MaxCoefficient

Type: Float
Default value: 20.0
Description: The largest allowed value of the expansion coefficients. If exceed the number of vectors is reduces until the criterion is met.

MaxVectors

Type: Integer
Default value: 10
Description: Maximum number of previous densities to be used (size of the history).

MinVectors

Type: Integer
Default value: -1
Description: Try to prevent to make nVectors shrink below this value, by allowing for significantly larger coefficents.

Mix

Type: Float
Default value: 0.2
Description: Also known as greed. It determines the amount of output density to be used. May be changed by the MixAdapter.

ErrorGrowthAbortFactor

Type: Float
Default value: -1.0
Description: Abort stint when the error grows too much, compared to the error at the start of the stint. Overides global ErrorGrowthAbortFactor when set to a value > 0

ExpectedSlope

Type: Float
Default value: -100.0
Description: If the slope of the total SCF is better than this keep on going.

FractionalStepFactor

Type: Float
Default value: -1.0
Description: Multiply the step by this factor. If smaller than zero this is not used.

MaxInitialError

Type: Float
Description: Only use the stepper when error is smaller than this.

MaxIterationNumber

Type: Integer
Default value: -1
Description: Stepper will only be active for iterations smaller than this number. (Negative value means: Ignore this option)

MaxStintNumber

Type: Integer
Default value: -1
Description: Stepper will only be active for stints smaller than this number. (Negative value means: Ignore this option)

MinInitialError

Type: Float
Description: Only use the stepper when error is larger than this.

MinIterationNumber

Type: Integer
Default value: -1
Description: Stepper will only be active for iterations larger than this number.

MinStintCyclesForAbort

Type: Integer
Default value: 0
Description: Look at ErrorGrowthAbortFactor only when a number of steps has been completed since the start of the stint. A value of 0 means always. Overides global value.

MinStintNumber

Type: Integer
Default value: -1
Description: Stepper will only be active for stints larger than this number.

MixAdapter

Type: Block
Description: Generic mix adapter

ErrorGrowthPanicFactor

Type: Float
Default value: 10.0
Description: When the error increases more than this factor, this mix is reduced a lot.

GrowthFactor

Type: Float
Default value: 1.1
Description: When the mix is considered too low it is multiplied by this factor. Otherwise it is divided by it.

MaxMix

Type: Float
Default value: 0.3
Description: Do not grow the mix above this value.

MinMix

Type: Float
Default value: 0.1
Description: Do not shrink the mix below this value.

NTrialMixFactors

Type: Integer
Default value: 3
Description: Only used with Type=Trial. Must be an odd number.

TrialMode

Type: Multiple Choice
Default value: CurrentMixCentered
Options: [CurrentMixCentered, FullRange]
Description: How are the NTrialMixFactors chosen?

Type

Type: Multiple Choice
Default value: Error
Options: [Error, Energy, UnpredictedStep, Trial]
Description: Adapt the mix factor based on the observed progress (slope).

MixStepper

Type: Block
Description: Simple mixing stepper, only using the previous (in/out) denstity.

Mix

Type: Float
Default value: 0.1
Description: ???.

MultiSecantStepper

Type: Block
Description: Multi secant stepper.

MaxCoefficient

Type: Float
Default value: 20.0
Description: ???.

MaxVectors

Type: Integer
Default value: 10
Description: ???.

Mix

Type: Float
Default value: 0.2
Description: ???.

Variant

Type: Multiple Choice
Default value: MSB2
Options: [MSB1, MSB2, MSR1, MSR1s]
Description: There are several version of the Multi secant method.

StintLength

Type: Integer
Description: Override global StintLength.

StintLength

Type: Integer
Default value: 10
Description: A stepper is active during a number of SCF cycles, called a stint.

UsePreviousStintForErrorGrowthAbort

Type: Bool
Default value: No
Description: The error is normally checked against the first error of the stint. With this option that will be the one from the previous stint, if performed with the same stepper.

MultiStepperPresetPath

Type: String
Default value: DFTB/default2023.inc
Description: Name of file containing a SCFMultiStepper key block. This will be used if no Explicit SCFMultiStepper block is in the input, and Method=MultiStepper. If the path is not absolute, it is relative to $AMSHOME/data/presets/multi_stepper’

DIIS¶

When selecting the SCC method DIIS, these are the relevant options. Compared to the MultiStepper it is more straightforward to tweak.

SCC
   AdaptiveMixing Yes/No
   DIIS
      Enabled Yes/No
      MaxSamples integer
      MaximumCoefficient float
      MinSamples integer
      MixingFactor float
   End
End

SCC

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.

DIIS

Type: Block
Description: Parameters influencing the DIIS self-consistency method

Enabled

Type: Bool
Default value: Yes
Description: If not enabled simple mixing without DIIS acceleration will be used.

MaxSamples

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.

MaximumCoefficient

Type: Float
Default value: 10.0
Description: When the diis expansion coefficients exceed this threshold, the solution is rejected. The vector space is too crowded. The oldest vector is discarded, and the expansion is re-evaluated.

MinSamples

Type: Integer
Default value: -1
Description: When bigger than one, this affects the shrinking of the DIIS space on linear depence. It will not reduce to a smaller space than MinSamples unless there is extreme dependency.

MixingFactor

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.

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.

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Model Hamiltonians¶

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