Tip

All input files can be downloaded: Files.

grimmedisp

This keyword defines how to apply Grimme dispersion correction version 3, i.e. DFT-D3.

Options

Hint

Always use DFT-D3 in your DFT calculations, especially for weak interactions. Actually, it is recommended to use it in all cases.

When there is no grimmedisp in the input file, no DFT-D3 will be applied.

type

Value	bj Becke-Johnson DFT-D3 form
	zero Zero-damp DFT-D3 form
Default	None

Define form of DFT-D3. There is no default value and you must assign bj or zero.

In most cases, bj is better, but for some functionals, like M06, M062X, and M06HF, only zero is available.

three_body

Add three body corrections in DFT-D3. This is useful for large systems.

tz

If triple zeta basis sets are used, this keyword may bring some improvement.

Theoretical Background

The DFT-D3 method is a dispersion correction to the DFT energy. It is based on the pair-wise summation of the damping functions. In modern DFT calculations, it is almost always beneficial to include DFT-D3 correction. So, it is recommended to use it in your all calculations.

Input Examples

Examples: Weak Interactions in CH₄-C₂H₆

This example shows the importance of the DFT-D3 correction in the calculation of weak interactions. For CH₄ and C₂H₆, both are nonpolar, and the interaction between them is very weak. To calculate the interactions, we simply use EDA method at B3LYP+D3BJ/def2-svp level of theory:

disp-1.inp

mol
    C     -0.99275967      0.12491197      0.13574919
    H     -1.00662206     -0.95659619      0.18835421
    H     -0.34007185      0.51620582      0.90571860
    H     -0.63121516      0.43418579     -0.83650623
    H     -1.99486438      0.50482451      0.28603834
    C      2.95955795     -0.58352084     -0.35430634
    H      3.53618084     -1.49857448     -0.47007336
    H      3.33520181     -0.05007297      0.51638813
    C      3.07143559      0.28355041     -1.59942759
    H      1.92316063     -0.85399112     -0.16804950
    H      4.10875722      0.55492780     -1.78655076
    H      2.69486480     -0.24574724     -2.47236370
    H      2.49506428      1.19877654     -1.48025100
end

basis
    def2-svp
end

scf
    charge  0
    spin2p1 1
    type    U  # For EDA, this must be set explicitly.
end

grimmedisp
    type bj
end

eda
    type tso
    frag 0 1 1-5   # Define CH4.
    frag 0 1 6-13  # Define C2H6.
end

task
    eda b3lyp
end

The output is:

disp-1.out

WITH BSSE correction:
Electrostatic interaction energy:                  -0.13 kcal/mol
Exchange-correlation interaction energy:            0.48 kcal/mol
Polarization interaction energy:                   -0.00 kcal/mol
Charge transfer interaction energy:                -0.11 kcal/mol
Grimme s dispersion interaction:                   -0.87 kcal/mol
----------------------------------------------------------------
Total interaction energy:                          -0.62 kcal/mol

Thus, the interaction energy between CH₄ and C₂H₆is -0.62 kcal/mol. The dispersion energy is -0.87 kcal/mol, which is the most important part of the interaction energy. Without DFT-D3, the interaction energy would be -0.62--0.87 = +0.25 kcal/mol! Therefore, the complex is unbound without DFT-D3! This leads to a quantitative error in the interaction energy. Therefore, always use DFT-D3 in your DFT calculations.