So I have obtained a TS during TS Berny calculation and in the results, there is only one imaginary frequency. However, when I did the IRC calc there is a warning statement in the txt file stating that there is more than one imaginary frequency which maybe is the reason it does not converge to the reactant side (the forward and reverse direction leads to the products...) what should i do regarding this problem?
Dear Karen Diez ,
Assuming you are using Gaussian, there are some times in which the program gets more than one negative force constant but, when it calculates the frequencies, only one imaginary arises. Did you use the calcfc (calculate analytic 2nd derivatives only at the first step of the optimization) or noeigentest (do not check the number of negative force constants during optimization) keywords during TS optimization? These keywords can lead to higher-order saddle points (more than one negative force constant, i.e. imaginary frequency). Try to reoptimize the structure, from the last point of the previous calculation, using the opt=(calcall,ts) keyword, so force constants are calculated analytically at each optimization step. It is more computationally demanding, but will lead to better results.
Another possible source of error is if you are trying to calculate the IRC using a different level of theory than the one used for the optimization. If this is the case, which I don't think, just change the level of theory for the original one.
Hope you find it helpful
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