Convergence problems with ONIOM in G09

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For convergence problems (of the optimization) when using ONIOM in gaussian09 you can try 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. Also, sometimes for low frequency transition structures, it might be useful, but sometimes it is not useful.

From gaussian manual:

• 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.