5.7 DFT Methods for van der Waals Interactions

5.7.4 Tkatchenko-Scheffler van der Waals Model (TS-vdW)

Tkatchenko and SchefflerTkatchenko:2009 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 (α), dispersion coefficients (C6), and vdW radii (RvdW) 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 (sR) is required, which depends upon the underlying exchange-correlation functional.

Note:  The parameter sR 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,

EvdWTS=-12AatomsBAA(C6,ABeffRAB6)fdamp(RAB). (5.37)

As in DFT-D the R-6 potentials in Eq. (5.37) must be damped at short range, and the TS-vdW model uses the damping function

fdamp(RAB)=11+exp[-d(RAB/sRRvdW,ABeff-1)] (5.38)

with d=20 and an empirical parameter sR that is optimized in a functional-specific way to reproduce intermolecular interaction energies.Tkatchenko:2009 Optimized values for several different functionals are listed in Table 5.4.

PBE PBE0 BLYP B3LYP revPBE M06L M06
sR 0.94 0.96 0.62 0.84 0.60 1.26 1.16
Table 5.4: Optimized damping parameters [Eq. (5.37)] for the TS-vdW model, from Ref. Tkatchenko:2009.

The pairwise coefficients C6,ABeff in Eq. (5.37) are constructed from the corresponding atomic parameters C6,Aeff via

C6,ABeff=2C6,AeffC6,Beff(αB0,eff/αA0,eff)C6,Aeff+(αA0,eff/αB0,eff)C6,Beff, (5.39)

as opposed to the simple geometric mean that is used for C6,AB parameters in the empirical DFT-D methods [Eq. (5.23)]. These are “effective” C6 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.,

C6,Aeff=C6,Afree(VA,effVA,free)2 (5.40)

where VA,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:

αA0,eff=αA0,free(VA,effVA,free) (5.41)
RvdW,Aeff=RvdW,Afree(VA,effVA,free)1/3. (5.42)

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. (5.37) is essentially zero in comparison to the cost of a DFT calculation. A recent reviewHermann:2017 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,Rezac:2011 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., VH,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.Barton:2018 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. Q-Chem’s value for the free-atom volume of hydrogen atom (7.52 a.u. at the Hartree-Fock/aug-cc-pVQZ level) is very to the analytic result (7.50 a.u.), lending credence to Q-Chem’s implementation of Hirshfeld partitioning and suggesting that it probably makes sense to re-parameterize the damping function in Eq. (5.38) for use with Q-Chem, where 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.Tkatchenko:2009 It was then noted that for S66 interaction energiesRezac:2011 the PBE+TS-vdW results obtained using FHI-aims and Quantum Espresso are slightly closer to the benchmarks as compared to results from Q-Chem’s implementation of the same method, with root-mean-square deviations of 0.55 kcal/mol (Quantum Espresso) versus 0.70 kcal/mol (Q-Chem). Comparing ratios VA,eff/VA,free between Q-Chem and FHI-aims, and performing linear regression analysis, affords scaling factors that can be applied to these atomic volume ratios, in order to obtain results from Q-Chem that are consistent with those from the other two codes using the same damping function.Barton:2018 Use of these scaling factors is controlled by the $rem variable HIRSHMOD, as described below.

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.

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

Example 5.13  Sample input illustrating a calculation of a water molecule, including the TS-vdW energy.

$molecule
   0 1
   O
   H 1 0.95
   H 1 0.95 2 104.5
$end

$rem
   BASIS               6-31G*
   METHOD              PBE
   !vdw settings
   TSVDW               TRUE
   !setting the SCF_CONVERGENCE for single
   !atom calculations to 6
   HIRSHFELD_CONV      6
   !Apply modifiers to the free-atom volumes
   !the elements H, C, N, and O
   HIRSHMOD            4

$end