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(June 30, 2021)

Tkatchenko and Scheffler
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1092
}
Phys. Rev. Lett.

(2009),
102,
pp. 073005.
Link
have developed a pairwise method
for van der Waals (vdW, *i.e.*, dispersion) interactions, based on a scaling
approach that yields *in situ* atomic polarizabilities ($\alpha $),
dispersion coefficients (${C}_{6}$), and vdW radii (${R}_{\text{vdW}}$) that reflect
the local electronic environment. These are based on scaling the free-atom
values of these parameters in order to account for how the volume of a given
atom is modified by its molecular environment. The size of an atom in a
molecule is determined using the Hirshfeld partition of the electron density.
(Hirshfeld or “stockholder” partitioning, which also affords one measure of
atomic charges in a molecule, is described in Section 10.2.1).
In the resulting “TS-vdW” approach, only a single empirical range-separation
parameter (${s}_{R}$) is required, which depends upon the underlying
exchange-correlation functional.

Note: The parameter ${s}_{R}$ is currently implemented only for the PBE, PBE0, BLYP, B3LYP, revPBE, M06L, and M06 functionals.

The TS-vdW energy expression is based on a pairwise-additive model for the dispersion energy,

$${E}_{\text{vdW}}^{\text{TS}}=-\frac{1}{2}\sum _{A}^{\text{atoms}}\sum _{B\ne A}^{\text{A}}\left(\frac{{C}_{6,AB}^{\text{eff}}}{{R}_{AB}^{6}}\right){f}_{\text{damp}}({R}_{AB}).$$ | (5.44) |

As in DFT-D the ${R}^{-6}$ potentials in Eq. (5.44) must be damped at short range, and the TS-vdW model uses the damping function

$${f}_{\text{damp}}({R}_{AB})=\frac{1}{1+\mathrm{exp}\left[-d({R}_{AB}/{s}_{R}{R}_{\mathrm{vdW},AB}^{\text{eff}}-1)\right]}$$ | (5.45) |

with $d=20$ and an empirical parameter ${s}_{R}$ that is optimized in a
functional-specific way to reproduce intermolecular interaction
energies.
^{
1092
}
Phys. Rev. Lett.

(2009),
102,
pp. 073005.
Link
Optimized values for several different
functionals are listed in Table 5.4.

PBE | PBE0 | BLYP | B3LYP | revPBE | M06L | M06 | |

${s}_{R}$ | 0.94 | 0.96 | 0.62 | 0.84 | 0.60 | 1.26 | 1.16 |

The pairwise coefficients ${C}_{6,AB}^{\text{eff}}$ in Eq. (5.44) are constructed from the corresponding atomic parameters ${C}_{6,A}^{\text{eff}}$ via

$${C}_{6,AB}^{\text{eff}}=\frac{2{C}_{6,A}^{\text{eff}}{C}_{6,B}^{\text{eff}}}{\left({\alpha}_{B}^{\text{0,eff}}/{\alpha}_{A}^{\text{0,eff}}\right){C}_{6,A}^{\text{eff}}+\left({\alpha}_{A}^{\text{0,eff}}/{\alpha}_{B}^{\text{0,eff}}\right){C}_{6,B}^{\text{eff}}},$$ | (5.46) |

as opposed to the simple geometric mean that is used for ${C}_{6,AB}$ parameters
in the empirical DFT-D methods [Eq. (5.25)]. These are “effective”
${C}_{6}$ coefficients in the sense that they account for the local electronic
environment. As indicated above, this is accomplished by scaling the
corresponding free-atom values, *i.e.*,

$${C}_{6,A}^{\text{eff}}={C}_{6,A}^{\text{free}}{\left(\frac{{V}_{A,\text{eff}}}{{V}_{A,\text{free}}}\right)}^{2}$$ | (5.47) |

where ${V}_{A,\text{eff}}$ is the effective volume of atom $A$ in the molecule, as determined using Hirshfeld partitioning. Effective atomic polarizabilities and vdW radii are obtained analogously:

$${\alpha}_{A}^{\text{0,eff}}={\alpha}_{A}^{\text{0,free}}\left(\frac{{V}_{A,\text{eff}}}{{V}_{A,\text{free}}}\right)$$ | (5.48) |

$${R}_{\text{vdW,}A}^{\text{eff}}={R}_{\text{vdW,}A}^{\text{free}}{\left(\frac{{V}_{A,\text{eff}}}{{V}_{A,\text{free}}}\right)}^{1/3}.$$ | (5.49) |

All three of these atom-specific parameters are therefore functionals of the electron density.

As with DFT-D, the cost to evaluate the dispersion correction in
Eq. (5.44) is essentially zero in comparison to the cost of a DFT
calculation. A recent review
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449
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Chem. Rev.

(2017),
117,
pp. 4714.
Link
shows that the performance of
the TS-vdW model is on par with that of other pairwise dispersion corrections.
For example, for intermolecular interaction energies in the S66 data
set,
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J. Chem. Theory Comput.

(2011),
7,
pp. 2427.
Link
the TS-vdW correction added to PBE affords a mean absolute error of
0.4 kcal/mol and a maximum error of 1.5 kcal/mol, whereas the corresponding
errors for PBE alone are 2.2 kcal/mol (mean) and 7.2 kcal/mol (maximum).

During the implementation of the TS-vdW scheme in Q-Chem, it was noted that
evaluation of the free-atom volumes affords substantially different results as
compared to the implementations in the FHI-aims and Quantum Espresso codes, *e.g.*,
${V}_{\text{H,free}}$ = 8.68 a.u. (Q-Chem), 10.32 a.u. (FHI-aims), and
10.39 a.u. (Quantum Espresso) for hydrogen atom using the PBE
functional.
These discrepancies were traced to different
implementations of Hirshfeld partitioning. In Q-Chem, the free-atom volumes
are computed from an unrestricted atomic SCF calculation and then spherically
averaged to obtain spherically-symmetric atomic densities. In FHI-aims and
Quantum Espresso they are obtained by solving a one-dimensional radial
Schrödinger equation, which automatically affords spherically-symmetric
atomic densities but must be used with fractional occupation numbers for open-shell atoms.
These differences could likely be ameliorated by reparameterizing the
damping function in Eq. (5.45) for use with atomic volumes calculated
self-consistently using Q-Chem, wherein the representation of the electronic structure is quite
different as compared to that in either FHI-aims or Quantum Espresso.
This has not been done, however, and the parameters were simply taken from a
previous implementation.
^{
1092
}
Phys. Rev. Lett.

(2009),
102,
pp. 073005.
Link
In order to reproduced TS-vdW dispersion energies obtained with FHI-aims or Quantum Espresso,
it is possible to use this code in Q-Chem with scaling factors for the atomic Hirshfeld volumes,
recommended values for which are obtained by linear regression, comparing Q-Chem atomic volumes to those
obtained in FHI-aims. For full self-consistency, however, these scaling factors should not be used.

The TS-vdW dispersion energy is requested by setting TSVDW = TRUE. Energies and analytic gradients are available.

TSVDW

Flag to switch on the TS-vdW method

TYPE:

INTEGER

DEFAULT:

0

OPTIONS:

0
Do not apply TS-vdW.
1
Apply the TS-vdW method to obtain the TS-vdW energy.
2
Apply the TS-vdW method to obtain the TS-vdW energy and corresponding gradients.

RECOMMENDATION:

Since TS-vdW is itself a form of dispersion correction, it should *not* be used in conjunction with any of the
dispersion corrections described in Section 5.7.2.

TSVDW_SR

Set custom value of the ${s}_{R}$ damping parameter

TYPE:

INTEGER

DEFAULT:

no default value defined

OPTIONS:

$n$
Corresponding to $n\cdot {10}^{-4}$

RECOMMENDATION:

Use predefined values for supported functionals, otherwise consult
Ref. 1092 and other relevant literature.

HIRSHFELD_CONV

Set different SCF convergence criterion for the calculation of the single-atom
Hirshfeld calculations

TYPE:

INTEGER

DEFAULT:

same as SCF_CONVERGENCE

OPTIONS:

$n$
Corresponding to ${10}^{-n}$

RECOMMENDATION:

5

HIRSHMOD

Apply modifiers to the free-atom volumes used in the calculation of the scaled
TS-vdW parameters

TYPE:

INTEGER

DEFAULT:

4

OPTIONS:

0
Do not apply modifiers to the Hirshfeld volumes.
1
Apply built-in modifier to H.
2
Apply built-in modifier to H and C.
3
Apply built-in modifier to H, C and N.
4
Apply built-in modifier to H, C, N and O

RECOMMENDATION:

Use the default

$molecule 0 1 O H 1 0.95 H 1 0.95 2 104.5 $end $rem BASIS 6-31G* METHOD PBE TSVDW TRUE !vdw settings HIRSHFELD_CONV 6 ! sets SCF_CONVERGENCE for single atom calculations HIRSHMOD 4 ! Apply modifiers to the free-atom volumes for H, C, N, and O $end