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- Thread starter nisler.1
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In summary, a graduate student is having trouble getting a QM/MM simulation to converge, and has tried a few methods with little success. He is using Orca, but is considering trying a different method if the current one does not converge. The system is multi-configuration and the QM/MM simulation is failing to converge; however, if the simulation is started with a low level of theory and continues to grow more accurate, it may be possible to use the new MOs from the earlier calculation as an initial guess for a production run using a larger basis set and/or a higher level of theory.f

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Of course, all this assumes that your system is reasonably well-behaved. If you have a system which is inherently multi-configuration and you're running an HF-type calculation, your SCF runs will fail to converge because your model isn't a reasonable approximation of the system.

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I just realized this made no sense when I read it to myself. What I meant was: 1) get the system to converge with an obscenely low level of theory/small basis set like HF/STO-3G or 3-21G. Then 2) use the MOs from this calculation as an initial guess for a production run with a bigger basis set and/or a higher level of theory.But if you can get it to converge once, you can usually use these new MOs for a better initial guess.

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