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ZPE correction
Posted: Tue Jul 24, 2012 10:04 am
by lgxyz
hi,
I use ZPE=1/2 sum(hVi) to calculate the ZPE energy. There are always some small imaginary frequency for stable geometries. Should I include the imaginry frequency in the ZPE equation? Thanks for suggestions!
Re: ZPE correction
Posted: Tue Jul 24, 2012 2:06 pm
by graeme
No, you need to accurately calculate your frequencies. You can look for posts about this (there are quite a few). Make sure to have a well-converged stationary point, and then use a small ediff=1e-8 and test for convergence using displacements in the range of 0.01-0.001 Ang.
Re: ZPE correction
Posted: Wed Jul 25, 2012 3:42 am
by lgxyz
Thanks!
In the INCAR of optimization, I set EDIFF=1e-4, EDIFFG=-0.02 and it is well converged.
Then I use EDIFF=1e-8, POTIM=0.002 to calculate the frequency,but it still has two small imaginary frequency (43 and 44 cm-1).
What should I do to get all positive frequency? Pls give some suggestions! Thanks!
Re: ZPE correction
Posted: Wed Jul 25, 2012 3:58 am
by graeme
If your system has no frozen atoms, you will have zero modes associated with translation and/or rotation. These, you can not include in a harmonic approximation of ZPE.
But if you have frozen atoms, there should be no zero or negative modes. In this case, try going to a lower ediffg, and make sure that your modes are converged with respect to the finite difference displacement.