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I was looking for a reasonable explanation of the Grimme's dispersion correction methods but his papers are written in a very difficult language. Does anyone could explain me the differences between these three methods?

  • S. Grimme, "Accurate description of van der Waals complexes by density functional theory including empirical corrections", J. Comput. Chem. 25 (2004) 1463-1473.
  • S. Grimme, "Semiempirical GGA‐type density functional constructed with a long‐range dispersion correction", J. Comput. Chem. 27 (2006) 1787-1799.
  • S. Grimme, J. Antony, S. Ehrlich and H. Krieg, "A consistent and accurate ab initio parametrization of density functional dispersion correction (DFT-D) for the 94 elements H-Pu", J. Chem. Phys. 132 (2010) 154104.
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Empirical dispersion accounts for the fact that Density Functional Theory systematically underestimates Van der Waals interactions. DFT-D and DFT-D2 are very simple. DFT-D3 is a bit more complex.

DFT-D and DFT-D2 both consist of a pairwise -1/r^6 attraction, like a Lennard-Jones attraction, scaled by a parameter C6 and a damping function which prevents the energy from going to -∞ at r=0. The distance parameter r0 is just the sum of the covalent radii of the interacting pair of atoms. The energy parameter C6 is the geometric mean of the two element-wise C6 parameters. Importantly, neither C6 or r0 depend on anything but the pair of elements involved. That makes DFT-D and DFT-D2 very easy to implement.

The following code implements the DFT-D2 energy as used in the wB97X-D functional ("Long-range corrected hybrid density functionals with damped atom–atom dispersion corrections", Chai & Head-Gordon, 2008). DFT-D and variants use other choices of damping function but are otherwise the same. Parameters C6 and covalent radii are available in "Semiempirical GGA-type density functional constructed with a long-range dispersion correction", Grimme, 2006. Currently both of these papers are available as PDFs on Google Scholar. You can get the parameters c6_coefficients and covalent_radii there.

def dftd2_wB97XD(r, ai, aj):
    r6 = r**6
    c6 = sqrt(c6_coefficients[ai] * c6_coefficients[aj])
    r0 = covalent_radii[ai] + covalent_radii[aj]
    fdamp = 1.0 / (1.0 + 6.0 * (r / r0)**-12.0)
    return -fdamp * c6 / r6

Be careful about the units of the parameters - J/mol nm^6 for C6, and Å for the covalent radii. In a real use case, you probably want C6 in kcal/mol Å^6 or Hartree Å^6.

DFT-D3 is conceptually only a little more complex: -C6/r^6 * fdamp6 + C8/r^8 * fdamp8. However, the parameters C6 and C8 depend on the coordination number of each atom, estimated using the distances to other nearby atoms. This makes DFT-D3 more accurate than the earlier versions. A good description can be found at https://www.vasp.at/wiki/index.php/DFT-D3.

There is also an even newer variant, DFT-D4, which continues to add accuracy and complexity by first estimating the atomic partial charges, and then using them to estimate the dispersion parameters. In this way, each atom's dispersion parameters depend on the entire surrounding environment. The paper for the D4 correction is here: https://chemrxiv.org/articles/A_Generally_Applicable_Atomic-Charge_Dependent_London_Dispersion_Correction_Scheme/7430216

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    $\begingroup$ Thank you for the wondeful explanation! I am confused about this line in the above code though, where the C6 term is calculated - c6 = sqrt(c6_coefficients[ai] * c6_coefficients[aj])**0.5. It looks like the square root is being taken twice here (unless I'm incorrect in assuming **0.5 is an exponentiation to the 1/2 power). Is this a typo, or is there some reason a double square root would be needed? $\endgroup$ Aug 11 at 19:30
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    $\begingroup$ You're right, I took the square root twice by mistake. It should be just the geometric mean. $\endgroup$
    – MD_ftw
    Aug 11 at 23:02

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