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.