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[交流] 【分享】SCF收敛问题的一些解决办法

If you have an SCF calculation that failed to converge, which of these tricks should you try first? Here are my suggestions with number one being the first thing I try and etc.
1. Try a different initial guess (using the "guess" keyword in Gaussian). For an open shell system, try converging the closed shell ion of the same molecule then using that as an initial guess for the open shell calculation. Adding electrons may give more reasonable virtual orbitals but as a general rule of thumb cations are easier to converge than anions.
2. Another initial guess method is to first run the calculation with a small basis set then get the initial guess for a larger basis set calculation from the previous wave function.
3. Try level shifting ( "SCF=Vshift" in Gaussian).
4. If the SCF is approaching, but not reaching, the convergence criteria relax or ignore the convergence criteria. This is usually done for geometry optimizations which do not converge at the initial geometry then often converge as they approach the equilibrium geometry.
5. Some programs use reduced accuracy integrals to speed the SCF. Using full integral accuracy may be necessary for systems with diffuse functions, long range interactions or low energy excited states.
6. Try changing the geometry. First slightly shortening a bond length then slightly lengthening a bond length then shifting the conformation a bit.
7. Consider trying a different basis set.
8. Consider doing the calculation at a different level of theory. This isn't always practical, but beyond this point the increased number of iterations may make the computation time as much as using a higher level of theory anyway.
9. Turn off the DIIS extrapolation ( "SCF=NoDIIS" in Gaussian ). You should probably give the calculation more iterations along with this.
10. Give the calculation more SCF iterations ( "SCF(MaxCyc=N)" where N is the number of iterations in Gaussian ). This seldom helps but the next option often uses so many iterations that it's worth a try.
11. Use a forced convergence method. (In Gaussian "SCF=QC" is usually the best but on rare occasions "SCF=DM" will be faster). Don't forget to give the calculation an extra thousand iterations or so. The wave function obtained by these methods should be tested to make sure it is a minimum and not just a stable point ( see the "stable" keyword in Gaussian ).
12. See if the software documentation suggests any other ways to change the DIIS method. You may well have to run hundreds of calculations to get enough experience with this to know what works when and how much to change it by.

[ Last edited by cadick on 2009-12-14 at 02:50 ]

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