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The following references are related to the imaginary frequencies of a geometry
1.In an article on BC3 honeycomb published in J. Phy.Chem.C in 2011
"Our fragment structure II is in fact a ninth-order saddle point. Geometry optimization following the imaginary frequencies led us to structure I, which is the most stable isomer found in our calculations. "
"However, the completely planar structure of the 1,2,3-C6H4(BH2)2 molecule has two imaginary frequencies. Thus, we proved that the repulsion between hydrogen atoms is responsible for the instability of isomer II in the C6(BH2)6 case. "
2.Prof. Gordon commented on an article published in JACS in 1991
"...The twisted group IVB carbenes (2b) have two imaginary frequencies-one which corresponds largely to rotation about the M-C bond leading to the planar structure (2a) and one which entails bending of the MH2 fragment such that the coordination about the metal goes from trigonal planar to pyramidal. ..."
"If all else(meaning the methods to eliminate the imaginary frequencies resulting from a flat PES and numerical errors) fails, it is probably better to pretend that the imaginary frequency is real and add the corresponding vibrational free energy contribution. However, this needs to be systematically tested".
4. In an article published in Journal of Organometallic Chemistry in 1995 on tetrafluorocyclodisilazanes on References and notes 18
"In the case of 8c a very small imaginary frequency ( ~ 7 cm- ’) corresponding to rotation of the exocyclic silyl groups was present. Because the effect of such a small imaginary frequency on the energy or the structure is neglegible, we have not reoptimized this structure."
5. In an article pubished in Chemical Physics in 1992
"However the C, keto form contains a planar secondary amine moiety, and it is therefore not surprising that it represents a transition state with an a” transition vector (imaginary frequency 96i cm- ’ )."(Singlet excited-state intramolecular proton transfer in 2- ( 2 ’ -hydroxyphenyl ) benzoxazole: spectroscopy at low temperatures, femtosecond transient absorption, and MNDO calculations)
6. In an article published on Computational Materials Science in 2015:
" The results, shown in Fig. 4, demonstrate that if the frequency is very low (<39.0 cm-1), a high-accuracy frequency calculation can remove up to 42% of the imaginary frequencies. However, high accuracy re-optimization is less helpful for larger imaginary frequencies. In contrast, the two molecular geometry perturbation strategies work extremely well: they remove at least 21 out of the 24 imaginary frequencies for the whole range of imaginary frequencies."
7. In an article published in J. Chem. Edu. in 2002
"Often, as in the case of (PH3)2Ir(H)2(H2)Cl discussed later in this paper, optimizations with a symmetry plane present (Cs in this example) result in a converged structure with one or more imaginary frequencies that correspond to motions that would break the symmetry plane. To converge a structure to an energy minimum (no imaginary frequencies), the computation must be carried out again in a lower symmetry group (here C1). "
8. In an article published in J, Phy. Chem. A in 2012
"the u-B3LYP/6-31G(d)-optimized 1B1 structure had one imaginary frequency, corresponding to rotation of the perpendicular CH2. "
9. In an article published in JACS in 1987 entitled A Theoretical Study of Thermal Reactions of Bicyclo[ 2.1.0 ]pent-2-ene.
"A similar analysis for 4 gave one imaginary frequency (134i cm"1). This vibrational mode of a" symmetry is mainly a twist of the CH2 group showing that 4 is a transition state for the conversion of 3 into itself."
10. In an article published on pyrimidine in JACS in 1992
"The coordinate associated with this imaginary frequency is an out-of-plane one of bl symmetry which lowers the symmetry of the molecule from C2v to Clh, the reflection plane being the zx plane. The subsequent optimization of the geometry within Clh symmetry leads to a minimum corresponding to a nonplanar geometry for pyrimidine in the 3Bl(n7r*) state at an energy 68 cm-I below that of the corresponding state in the C, geometry."
I think most of these comments are suitable.
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