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cenwanglai荣誉版主 (知名作家)
老和山猥琐派九段
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[求助]
CNEB收敛到0.05后，提高精度不收敛
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（1）q-m算法收敛到1eV/A (2) LBFGS算法熟练到0.05 （3）按照（2）的设置，修改EDIFFG=-0.03，很难收敛。 下面一段话是早前Graeme给某个帖子的回复。看他的建议，似乎是 q-m算法收敛到1,然后CG或者LBFGS熟练到0.05，这时EDIFFG=1E-4或者1E-5；如果要达到更高的精度，需要更高的力的精度，要求EDIFFG = 1E-5或者更低。这时可以退回q-m算法。 及CNEB的熟练过程的算法切换是：Q-M （<1 eV/A）->CG/LBFGS (<0.05 eV/A)->Q-M (<0.03 eV/A). 这样的话，如果（有人报道过这样的结果）力精度提高到0.001,EDIFFG岂不是要设置的很小很小？ Quickmin is a first-order optimizer which is relatively stable at any point during optimization. Near the minimum where the potential can be expanded using a harmonic approximation, it is not as efficient as second-order optimizers such as CG or LBFGS. There are a few caveats. CG and LBFGS are not particularly stable far from the minimum where forces are high. So using quick-min until the forces drop below 1 eV/Ang is a sensible way to start. You can also make sure that the path at this point makes physical sense. Then it will be more efficient to switch to a second order method. When I talk about forces being accurate enough near the minimum, I'm referring to the errors in the forces which are controlled by the electronic convergence criteria, ediff. When the forces are larger than the errors, curvatures can be determined with sufficient accuracy. Near the minimum, the errors are a higher portion of the force. At this point, you can either go back to quick-min (or fire), or decrease ediff. Your value of ediff=1e-8 is extremely tight. Using a value of 1e-5 will generally speed up your calculations without causing problems unless you are trying to get forces down to ~0.05 eV/Ang. Also, as a side note, your cutoff is very high and I have never seen a case where you need to use prec=accurate for the calculation of a barrier. [ Last edited by cenwanglai on 2011-9-27 at 20:45 ] |
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