What should we do when we get two imaginary frequencies in the transition state obtained using CI-NEB? For example, I looked into the CO+O -> CO2 step on 3X3 Pt(111) with sufficient vacuum slab. The initial and final states are optimized and the CI-NEB calculation is converged but the frequency analysis gave two imaginary values of about 340 cm-1 and 150 cm-1. Should I consider my transition state to incorrect? What should be done if that is the case? I have attached the files of both CI-NEB and Frequency calculations in the .zip format. Please check and shed some light on this issue.
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$\begingroup$ For the reaction, is this meant to be a carbon monoxide combustion? I don't use VASP, but have you also optimized the transition state? $\endgroup$– WALOWLACommented Jul 29 at 3:35
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$\begingroup$ You can say so. Yes the transition state is obtained through CI-NEB method in VASP, which involves the TS optimization as well. $\endgroup$– Nidhi TiwariCommented Jul 30 at 10:23
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$\begingroup$ I'm still confused on the reaction. Are you sure it's atomic oxygen and not CO + O2? Anyway, from experience I also had an extra imaginary frequency (~60 cm-1) that disappeared after reoptimization with tighter tolerance values. Also, were the platinum atoms held fixed across the NEB? $\endgroup$– WALOWLACommented Jul 30 at 16:39
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$\begingroup$ Yes, it is the rate determining step in the langmuir-hinshelwood mechanism. In case the other imaginary frequency has a smaller value, we consider it insignificant. The Pt atoms were fixed. What should I do for this case? $\endgroup$– Nidhi TiwariCommented Jul 31 at 17:57
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1$\begingroup$ My suggestions would be to either reoptimize at a tighter tolerance threshold, or displace your geometry along the imaginary vibrational mode you want to remove and then reoptimize at that geometry. $\endgroup$– WALOWLACommented Jul 31 at 18:52
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