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Article ID: 1412 - Last Modified: August 9, 2011

I am having difficulty converging a QSite calculation. Is there anything I can try that might improve the convergence?

For difficult-to-converge QSite calculations, you can try the following:

  • Turn off the use of non-bonded cutoffs. You can turn these of by clearing the Use non-bonded cutoffs option in the Potentials tab of the QSite panel. Using non-bonded cutoffs leads to energy jumps when the number of non-bonded interactions change, which can happen when atoms cross the cutoff boundary. This makes it very difficult for QSite to achieve convergence. Using non-bonded cutoffs also means that the energies obtained will probably not be comparable between different jobs, unless the number of non-bonded interactions happens to be the same.
  • Use tighter MM convergence criteria. In the MM Minimization tab, the default gradient criterion is 0.01, so you should not use values that are larger than this value. Even if you are trying to do a "quick and dirty" job, using loose convergence criteria for the MM region will probably be counterproductive, as the MM region will not be able to move enough to be stable with respect to the QM region, and consequently the adiabatic approximation will not be valid.

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