Mod:Hunt Research Group/dispersion
DFT and dispersion
A little bit of theory
Adding dispersion correction to B3LYP is shown in the equation below
File:Dispersion equation 1.pdf
where Cij is the dispersion coefficient, Rij is the interatomic distance between the ith and jth atoms and fdamp is included to ensure that dispersion is negligible at small Rij.
S6 is a scaling factor. This scales the dispersion coefficients to account for the difference in the intermolecular potential between two atoms when using different density functionals.
Including dispersion in your calculation
Adding Grimme’s DFT-D dispersion correction (equation above) is dependent on the version of Gaussian being used. DFT-D2 is correctly implemented in G09 Rev A for the following functionals:
B97D, B2PLYPD and wB97XD
No additional keywords are required to use the functionals in Gaussian.
In G09 Revs B.01 and C.01 the dispersion-corrected functionals above can be used. DFT-D2 can also be implemented with the following functionals:
B3LYP, TPSS, PBE and BP86
However the addition of the following keyword is required:
IOP(3/124=N)
where N is:
| N | Meaning |
|---|---|
| 0 | Default (same as 2) |
| 1 | Add it regardless |
| 2 | Add it for the DFT functionals for which it has been defined and parameterized and for which a specific name has been defined in Link1 |
| 3 | Add it for the DFT functionals for which it has been defined and parameterized |
| 4 | Do not add it regardless |
In order to ensure the correct scaling factor is used with the corresponding functional the keyword should read:
IOP(3/124=3)
You can check to see whether the scaling factor has been implemented correctly in the calculation by including the keyword:
IOP33(3=3)
This will provide a standard debug printout, an example of which is given below:
--
R6Disp: E=T FX=F FFX=F UseDF=T IUseCN= 2
R6DPar: UseDF=T IExCor= 402 ScaHFX= 0.200000 S6= 1.000000 D= 20.000000
--
The text in bold shows that the scaling factor is 1.00.
Scaling Factors
The table below lists the scaling factors for the corresponding functionals which are correctly implemented in Gaussian.
| Functional | Scaling Factor (S6) |
|---|---|
| B97D | 1.25 |
| B2PLYPD | 0.55 |
| mPW2PLYPD | 0.40 |
| wB97XD | 1.00 |
| PBE | 0.75 |
| BLYP | 1.2 |
| B3LYP | 1.05 |
| BP86 | 1.05 |
| TPSS | 1.00 |
Use of IOP(3/124=3) with a functional in Gaussian Rev A would use s6 = 1.00 as default ( the same occurs by using IOP(3/124=1) in Gaussian Rev B.01 and C.01).