How can I overcome negative frequencies that occur during Gaussian calculations?.
There are several ways to achieve this. As a negative vibrational frequency (I guess this is what you mean with imaginary frequency) indicates a transition state, it should connect to local minima on the potential-energy surface of your system. The easiest way to overcome this is to elongate your atomic positions along the normal coordinates of your imaginary frequency. Do not elongate too far, as it is sufficient to get off the transition state within numerical accuracy (0.1 Angstroms is typically sufficient). You should go into both directions of the normal coordinate vector so that you can evaluate both minima. I would also recommend to reduce the trust radius for the optimizer, i.e. MaxStep=1.
Another way would be a random distortion of the geometry, i.e. changing the coordinates of all atoms in your molecule randomly by a maximum of 0.1 Angstroms and re-optimization of the resulting structure.
What could also work is to pick only the atom with the maximum displacement in the imaginary normal coordinates and disturb only that atom's position, and subsequent re-optimization.
Finally, if you have a symmetry constraint in your calculation (any symmetry except C1) you should check if the imaginary frequency is within a irreducible representation which, if removed, would lead to the next lesser symmetry. For example, if the frequency would run orthogonal to a mirror plane, which point group would result if you remove this mirror plane? Then, apply the less symmetric point group and run the optimization again. Sometimes this helps to solve the problem as well.
If none of the aforementioned advice helps, it would be quite helpful if you could give some more information on your problem, especially on the point group of your molecule and the irreducible representation of the imaginary frequency, unless you have only C1 symmetry.
Dear Andreas Funk,
Thank you very much for your reply. Your answer gives alot information regarding the negative frequency solvation. The initial assumption of my problem was Cs symmetry and the first frequency alone was negative.
Thank you...
I'm curious to how you performed your geometry optimisation. I find that when I'm working purely with organics e.g., the centre of a protein enzymatic active site, that I never run into negative frequencies. But if I include a transition metal ion cofactor then optimising becomes a bit trickier. I find that a gradual stage of optimisation really helps, for example in steps I would:
opt=(calcfc,loose) wb97xd/3-21+g
opt=(readfc,tight) int=ultrafine wb97xd/3-21+g
opt=(readfc,loose) wb97xd/6-31+g*
opt=(readfc,tight) int=ultrafine wb97xd/6-31+g*
etc... etc...
Dear Dr. Venkatesan Ragavendran,
In short, 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.
Negative frequency or imaginary frequency suggests that your molecule is not minima on PES (Potential Energy Surface); instead, it's a transition state structure (one negative freq. refers to the first-order saddle point). You need to follow up on the negative frequency to get the equilibrium geometry. There are two methods to do so
1. manually (by changing the respective coordinate w.r.t the negative freq.)
2. use gauss view to do so...
step 1. Open the .out file/.log in gauss view
step 2. right-click on the molecule and select Vibrations in the Result menu
step 3. Now select the respective negative frequency and tick the manual displacement menu
step 4. increase the manual displacement value from 0 to 1
step 5. save this structure and use this as an input for further calculations
these are the steps to follow up the negative frequency in Gauss view.
*Generally, we avoid negative frequency below 50cm-1 (mainly with C1 symmetry), but sometimes we consider the negative frequency in the case of highly symmetric molecules with high symmetry like D3h, etc.
I hope it may help you.
Regards
G. J
Hi everyone, I am having a similar problem. I am interested in theormodynamic property of an Fe(III) complex and I have tried Opt=verytight,calcfc keywords but six negative frequencies are coming in the log file (-70, -62, -59, -58, -52, -51). I tried the manual displacement method via gaussview with -70 freq. but still no improvement. Do I have to do this displacement for all the negative frequencies to solve the problem ? if there is anything I am missing kindly let me know.
Thanks in advance
Bijoy Dey Sometime your structure is not stable in the singlet state. It means that your structure is in a triplet or quintet state. So once you are solving the TD-DFT calculations, there are some negative frequencies.
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