ONIOM tips

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ONIOM tips:

• You can use iop(4/33=2) to print out all MM contributions.
• To get out the model system from an oniom calculation include iop(1/33=2). In the output file, search for: "ONIOM: generating point 2 high level on model system". All atoms without the label -1 belong to the model system (this list includes also the link hydrogens).
• There are two options if you want to check other force fields which are not implemented in Gaussian09;

Option1: You can do one-dimensional potential energy surface scans over a reaction coordinate, where the constrained structure optimisations at the potential energy surface can be performed with ONIOM(DFT:UFF), ONIOM(DFT:DREIDING) or ONIOM (DFT:AMBER), where the iop(1/33=2) option helps to obtain QM and MM parts of the molecule. Then you can do MM optimisations on the QM and MM parts of the molecule separately with MACROMODEL or TINKER, where they have various force filed parameters.

Option2: ´External´ keyword.

There are two examples in the G09 test suite which make use of the ´External´ keyword.

(1) Test726 uses a shell script to run g09 as an external process with UFF. The script ´extuffex´ is stored in the g09/bsd directory.

(2) Test769 uses a perl script, mygau, which is stored in the g09/bsd subdirectory. Note that you will likely need to edit mygau to change the path to perl, rather than /usr/local/bin, because most machines store it in /usr/bin.

In both cases you could use ´External´ keyword in the input file.