# Tag Info

18

As you mention, there are many empirical dispersion corrections for density functional theory. Generally, the term "DFT-D" refers to a generic dispersion-corrected density functional calculation, regardless of the specific method used for the dispersion correction used. The D3 dispersion model is a specific dispersion correction method and is now something ...

14

Yes, vdW interaction affects energies but, also, geometries. This effect is the most pronounced if a DFT method predicts an interaction in a dimer to be repulsive: a geometry optimization will then simply dissociate the dimer. Normally, any vdW correction term does not only contribute energies, but als gradients, which in turn affect geometry optimizations (...

13

Dispersion correction DOES affect the final geometry. The dispersion interaction is a function of geometry. A crude approximation is to write the dispersion interaction between two neutral molecules as $$\Delta E_{\rm disp}(R)=-\frac{C_6}{R^6},$$ where $R$ the distance between the two entities. In some special case, like the system in which two rare gas ...

10

Optimization routines typically use the negative gradient of the energy in some form to determine the displacement during an optimization step and the dispersion correction changes the energy, which changes the gradient, and thus the optimization. You may get the same result in some cases but those are special cases, in general dispersion correction does ...

9

Imagine you have a (hypothetical) method that can exactly capture the van der Waals interaction. Then when you do a geometry optimization, you are neglecting the quantum and thermal fluctuations of the atoms, and you end up with what is called the static lattice approximation. As quantum and thermal fluctuations tend to expand the lattice, then you would ...

9

In computational chemistry, the term "van der Waals interaction" tend to refer to the London term only; the Keesom term (electrostatic interactions between two freely-rotating permanent dipoles) is generally included in the electrostatic interaction, while the Debye term (interaction between a permanent dipole and an induced dipole) is usually ...

9

2007 (Becke & Johnson): XDM XDM stands for "exchange-hole dipole moment" which is a model introduced by Becke and Johnson in 2007 for calculating dispersion constants. The formulas are as follows: \begin{align} \!\!\!\!\!\!\!\!C_6 &= \frac{\alpha_i\alpha_j}{\mathcal{M}_i\alpha_j + \mathcal{M}_j\alpha_i} \mathcal{M}_i\mathcal{M}_j ...

8

Additive dispersion correction methods such as D3, D3(BJ), D3(BJ)+ATM, TS, MBD@rsSCS, etc. are good for geometry optimizations. These methods find the London dispersion interactions governed interlayer distance lower than the mean relative error of 2%. On the other hand, it's a good idea to avoid using these methods for energy calculations. For example, ...

7

tldr; Use dispersion correction for optimization I asked a very similar question a few years ago. Many indications for dispersion corrections in density functional methods are for intermolecular interactions. Clearly these corrections are important in such intermolecular or long-range interactions (e.g., inter-layer spacing in a material.. far from the ...

7

Van der Waal's radii for all atoms in the molecule are required to compute the dispersion for $\omega$B97X-D (this is also true for Grimme's dispersion). Unfortunately, Gaussian09 doesn't seem to have the radius for $\ce{Au}$ and it is also not possible to manually enter it through the input file. The radii for the elements up through $\ce{Rn}$ (atomic ...

6

It doesn't matter. The self-consistent calculation will use the Hamiltonian constructed from the relaxed structure, in which the D3 has been applied and should not be considered again. Take bilayer MoSe2 as an example: This structure is optimized with PBE+D3 using the VASP package. With this relaxed structure, I have calculated the band structure with and ...

5

2009 (Tkatchenko−Scheffler) TS The Tkatchenko−Scheffler model for van der Waals interactions (vdW) defines the $C_6^{AB}$ parameters in an ab-initio fashion. In TS model the vdW energy $E_{vdw}$ is defined as, E_{\text{vdW}} = -\frac{1}{2}\sum_{A,B}f_{\text{damp}}\left(R_{AB},R^{0}_{A},R^{0}_{B}\right)C_{6}^{AB}R^{-6}_{AB} \tag{1} \end{...

5

A PBE vs PBE+D3 calculation will have the same band structure if the same geometry is used. If the geometry is optimized, however, they can differ but the D3 correction does not directly influence the band structure just the geometry. You can validate this method by considering that these corrections can often be done by an external step such as combining ...

3

@Xivi76 is correct. Dispersion corrections have no direct influence on the band structure. As such, there is no need to add the D3 correction on your HSE06 static calculation if you're only interested in the band structure. Nonetheless, some people include the D3 correction because the dispersion correction is basically free, and it's consistent with the use ...

3

DFT-D2 is simple and reliable. I've used DFT-D2 for most of my worked on 2D materials but I do I have to caution you that I have not specifically work on TMGs. You can test out the results you obtain from both DFT-D2 and DFT-D3 and compare them. Another idea is to look at literature. There is a labyrinth of data on TMGs and you can look under the 'methods' ...

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