G09
go back to Main Page, Computational Resources, Chemistry & More, Computational Codes, GAUSSIAN
Tricks
ONIOM in Gaussian09
In gaussian09 it is highly recommended to use the QuadMac option with the opt keyword (or Opt=QuadMacro). This option takes into account the coupling between atoms in the model system and the atoms only in the MM layer in order to produce more accurate steps than the regular microiterations algorithm.
- For problem convergence cases, the main alternative is Opt=QuadMac, which does a quadratic step in the coordinates of all the atoms. Such an optimization can use either an updated approximate Hessian for the internal coordinates or an analytically computed Hessian. It computes products of the low-level (MM) Hessian with vectors as needed. If there are still convergence problems, then try Opt=(QuadMac,CalcFC) or Opt=(QuadMac,CalcAll).
- For transition structures, the QuadMac option helps to ensure that you move in the direction of a proper transition structure, so Opt=(QuadMac,TS,CalcFC) is usually a good choice.
Solvent Effects
As with Gaussian03, the SCRF keyword requests that a calculation be performed in the presence of a solvent by placing the solute in a cavity within the solvent reaction field.
- The integral equation formalism variant IEFPCM, is the default SCRF method. It has not changed from Gaussian03, BUT the formalism used and its implementation has changed. That is: you will get different results in G03 and G09, using the same method.
- The default RADII used in G03 was UAO , whilst now is UFF with spheres placed by default on all Hydrogen atoms. No need to use the SPHEREONH= keyword, except if you use UAO or another radii model that does not have them explicitly.
- It seems to give no "convergence failure" problems, but maybe its too early to say so.
- It is able to perform frequency calculations in solvent, giving enthalpies, free energies, ZPE corrections...
- The G09 output for PCM gives only one energy which corresponds, by default, to the electrostic energy. The non-electroestatic components of the energy of solvation are not included. If you wish to include them you will need to include "scrf=(read,PCM,...)" in the command line, and Cav Dis Rep (in separate lines) at the end of the input.
- The SMD method includes automatically the non-electrostatic terms.
The RI-DFT method
From the Gaussian manual:
Gaussian 09 provides the density fitting approximation for pure DFT calculations. This approach expands the density in a set of atom-centered functions when computing the Coulomb interaction instead of computing all of the two-electron integrals. It provides significant performance gains for pure DFT calculations on medium sized systems too small to take advantage of the linear scaling algorithms without a significant degradation in the accuracy of predicted structures, relative energies and molecular properties. Gaussian 09 can generate an appropriate fitting basis automatically from the AO basis, or you may select one of the built-in fitting sets.
What all this means is that for all calculations that have more than about 1000 basis functions, it will speed up the calculation greatly if you use the RI-DFT method. Generally, the greater the number of basis functions, the better it compares to normal DFT. The accuracy of this approximation is very good, and generally you can afford to use a much higher basis set than you would with a normal DFT calcualtion (so you get rid of BSSE errors etc.).
How to set up an RI-DFT calculation.
You have to use a pure DFT functional... no hybrids allowed...
You also have to specify TWO basis sets... The first one is the normal basis set specification (GENECP is supported for complicated setups with transition metals and ECPs), then the second one is the "density fitting basis", which you can specify yourself or Gaussian can generate automatically.
For example:
# PBEPBE/TZVP/TZVPFit
would specify a calculation with the PBEPBE functional, TZVP "normal" basis set, and TZVPFit "density fitting" basis set. The / sign between each section is essential.
For an example of an input file, and more information on how to set up the calculation, see the following page... RI-DFT input