Hello,

The parameters of the empirical dispersion in Gaussian09 are not selected for all DFT methods, so either you specify them manually, or use the method for which they are defined. Here is a fragment of the documentation:

EMPIRICAL DISPERSION

The EmpiricalDispersion keyword enables empirical dispersion. It takes the following options:

PFD

Add the Petersson-Frisch dispersion model from the APFD functional [Austin12].

GD2

Add the D2 version of Grimme’s dispersion [Grimme06]. The table below gives the list of functionals in Gaussian 09 for which GD2 parameters are defined. The functionals highlighted in bold include this dispersion model by default when the indicated keyword is specified (e.g., B2PLYPD). For the rest of the functionals, dispersion is requested with EmpiricalDispersion=GD2.

Functional S6 SR6

B97D 1.2500 1.1000

B2PLYPD 0.5500 1.1000

mPW2PLYPD 0.4000 1.1000

PBEPBE 0.7500 1.1000

BLYP 1.2000 1.1000

B3LYP 1.0500 1.1000

BP86 1.0500 1.1000

TPSSTPSS 1.0000 1.1000

The damping function used by this model also contains a D6 parameter with a fixed value of 6.0.

You can use this empirical dispersion method with other functionals by defining the values ​​of the SR6 and S6 parameters (the value of SR6 is always 1.1). This is done using an environment variable with the name GAUSS_DFTD3_S6. The value of the environment variable sets the corresponding parameter to value/1,000,000. For example, the command:

export GAUSS_DFTD3_S6=1200000

sets the value of S6 to 1200000/1000000=1.2000.

The wB97XD functional—specified as an independent keyword—uses a version of this dispersion model with values ​​of S6 and SR6 of 1.0 and 1.1, respectively. This functional uses a similar damping function to that used by the GD3 model, with D6 and IA6 having fixed values ​​of 6.0 and 12, respectively.

GD3

Add the D3 version of Grimme’s dispersion with the original D3 damping function [Grimme10]. The table below gives the list of functionals in Gaussian 09 for which GD3 parameters are defined. For the rest of the functionals, dispersion is requested with EmpiricalDispersion=GD3.

Functional S6 SR6 S8

B2PLYPD3 [Goerigk11] 0.6400 1.4270 1.0220

B97D3 1.0000 0.8920 0.9090

B3LYP 1.0000 1.2610 1.7030

BLYP 1.0000 1.0940 1.6820

PBE1PBE 1.0000 1.2870 0.9280

TPSSTPSS 1.0000 1.1660 1.1050

PBEPBE 1.0000 1.2170 0.7220

BP86 1.0000 1.1390 ​​1.6830

BPBE 1.0000 1.0870 2.0330

B3PW91 1.0000 1.1760 1.7750

BMK 1.0000 1.9310 2.1680

CAM-B3LYP 1.0000 1.3780 1.2170

LC-wPBE 1.0000 1.3550 1.2790

M05 1.0000 1.3730 0.5950

M052X 1.0000 1.4170 0.0000

M06L 1.0000 1.5810 0.0000

M06 1.0000 1.3250 0.0000

M062X 1.0000 1.6190 0.0000

M06HF 1.0000 1.4460 0.0000

This model also use a SR8 parameter with a fixed value of 1.0. The damping function used by this model also contains D6, IA6, D8 and IA8 parameters with fixed values ​​of 6.0, 14, 6.0, and 16, respectively.

You can use this empirical dispersion method with other functionals by defining the values ​​of the SR6 and S8 parameters (the value of S6 is always 1.0). This is done using environment variables with names of the form GAUSS_DFTD3_param, where param is one of the parameter names. The value of the environment variable sets the corresponding parameter to value/1,000,000. For example, the command:

export GAUSS_DFTD3_S8=1375000

sets the value of S8 to 1375000/1000000=1.375.

GD3BJ

Add the D3 version of Grimme’s dispersion with Becke-Johnson damping [Grimme11]. The table below gives the list of functionals in Gaussian 09 for which GD3 parameters are defined. The functionals highlighted in bold include this dispersion model by default when the indicated keyword is specified (e.g., B2PLYPD3). For the rest of the functionals, dispersion is requested with EmpiricalDispersion=GD3BJ.

Functional S6 S8 ABJ1 ABJ2

B2PLYPD3 [Goerigk11] 0.6400 0.9147 0.3065 5.0570

B97D3 1.0000 2.2609 0.5545 3.2297

B3LYP 1.0000 1.9889 0.3981 4.4211

BLYP 1.0000 2.6996 0.4298 4.2359

PBE1PBE 1.0000 1.2177 0.4145 4.8593

TPSSTPSS 1.0000 1.9435 0.4535 4.4752

PBEPBE 1.0000 0.7875 0.4289 4.4407

BP86 1.0000 3.2822 0.3946 4.8516

BPBE 1.0000 4.0728 0.4567 4.3908

B3PW91 1.0000 2.8524 0.4312 4.4693

BMK 1.0000 2.0860 0.1940 5.9197

CAM-B3LYP 1.0000 2.0674 0.3708 5.4743

LC-wPBE 1.0000 1.8541 0.3919 5.0897

You can use this empirical dispersion method with other functionals by defining the values ​​of the S8, ABJ1 and ABJ2 parameters (the value of S6 is always 1.0). This is done using environment variables with names of the form GAUSS_DFTD3_param, where param is one of the parameter names. The value of the environment variable sets the corresponding parameter to value/1,000,000. For example, the command:

export GAUSS_DFTD3_S8=2375000

sets the value of S8 to 2375000/1000000=2.375.

The keyword "empiricaldispersion" refers to dispersion correction methods in the context of density functional theory (DFT). These methods are used to account for dispersion interactions (also known as van der Waals interactions) that are not always adequately described by standard DFT functionals.

Key aspects of using "empiricaldispersion"

Dispersion correction: DFT approaches such as DFT-D (Dispersion-corrected DFT) use empirical potentials to improve the description of long-range interactions. This is especially important for molecules and crystals, where such interactions can significantly affect the energy levels and geometry of the systems13.

Correction methods: There are various empirical correction schemes, such as:

DFT-D2: The Grimme method, which uses a pair of atomic parameters to calculate the dispersion energy.

DFT-D3: An improved version of DFT-D2 with additional parameters and more accurate calculations12.

DFT-CHG: An alternative approach by Chai and Head-Gordon that also accounts for dispersion using a different decay function2.

Parameters and settings: Depending on the functionality used, the parameters for "empiricaldispersion" can be configured via variables in software such as Q-Chem or Gaussian. This allows users to select the appropriate methods and parameters for their specific calculations23.

Applications: Using "empiricaldispersion" allows for more accurate results in the calculation of molecular and material properties, especially in systems where weak interactions are important, such as crystals or large molecules45.

Thus, the keyword "empiricaldispersion" denotes an important tool in computational chemistry for improving the accuracy of DFT calculations.

Dear Dr. George, Gaussian 09 does not recognize the keyword

empiricaldispersion=D3

that is for later versions. Instead you should use overlap 3 in the keywords line, see IOp(3/124) here:

https://gaussian.com/overlay3/

Best, Pablo

In Gaussian software, the keyword EmpiricalDispersion (or its variants like EmpiricalDispersion=GD3) is used to incorporate empirical dispersion corrections into quantum chemical calculations. These corrections are particularly important for accurately modeling non-covalent interactions, such as van der Waals forces, which standard density functional theory (DFT) often struggles to describe.

Usage of EmpiricalDispersion

The keyword enables a specific type of dispersion correction depending on the method chosen. Here's an explanation of the main options:

Why Use Empirical Dispersion?

• Improved Accuracy: Empirical dispersion corrections improve the reliability of DFT calculations, especially for weak interactions.
• Flexibility: The keyword allows you to customize the level of correction for specific systems and requirements.
• Compatibility: Works with various functionals, including hybrid (e.g., B3LYP, PBE0) and non-hybrid (e.g., PBE, TPSS).

Example Input File

Here’s an example Gaussian input file using the keyword:

perlCopy code%NProcShared=4 %Mem=8GB # B3LYP/6-31G(d) EmpiricalDispersion=GD3 Opt Title Card Required 0 1 H 0.0 0.0 0.0 H 0.0 0.0 0.74

Notes for Gaussian Versions

• Version Support:EmpiricalDispersion is supported in Gaussian 09, but the specific options depend on the sub-version (A.02, B.01, etc.). The keyword's capabilities expand in later versions. For example, GD3 and GD3BJ are more commonly supported in Gaussian 09 revisions after A.02.
• Error Messages: If you encounter errors while using this keyword, it might be due to unsupported options in your Gaussian version. Consider upgrading to a more recent revision if necessary.