I am doing the Optimization and frequency calculations of some porphyrin metal( zinc and palladium) complexes in Gaussian at B3LYP and GENECP level. I got some imaginary frequencies in most of the complexes. How can i fix them?
Dear Bhat!
Imaginary frequencies are generally structural problems please check your structures carefully more importantly for bond distances and angles. If problem continues please let me know.
if everything is right then use keywords for strict convergence like opt=tight
You could try to impose more strict convergence criteria for the geometry optimizations.
I suggest the keyword "opt=verytight".
If you still had negative frequencies, it could be helpful to visualize which normal mode s are associated with those frequencies, and manually move the atoms.
First of all find out which mode is giving imaginary frequency. If you are using Gaussview you can very easily change the geometry of the molecule along any particular mode. Change it for the one giving imaginary frequency and re-optimize using that altered geometry as input. If more than one mode gives you imaginary frequency, try with the one having largest value.
All the above answers are correct. However, from my experience, if you are good at guessing the initial geometry then it may be the level of theory particularly the basis set could cause such problems. So, play with the basis set.
Dear Dr. Haamid Rasool Bhat,
In general, there are the three popular methods to fix the undesired imaginary frequencies as follows:
1) Using the keywords: opt=tight or opt=verytight
2) Changing the different function/basis set.
3) Using a method known as "Screwing": Open the output file of that molecule using GaussView software --> Results item --> Vibrations tab --> Manual Displacement tab (adjust the values of +1 or -1) --> Save a new input file.
Best regards.
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