How to properly denote imaginary magnitudes of vibrational frequencies

Vibrational frequency computations provide the magnitudes of vibrational frequencies for each vibrational mode of a molecule ($3N-6$ degrees of freedom for non-linear molecules and $3N-5$ for linear molecules). Sometimes these magnitudes can be non-real (i.e. imaginary). This is common when characterizing a transition state which contain one, and exactly one, imaginary mode of vibration.


Some popular software packages will print these magnitudes as a negative number. This syntax has unfortunately propagated itself into common, everyday practice even if technically incorrect.


My question is, what is the best way to denote these imaginary magnitudes, especially in the context of a peer-reviewed journal? Should we stick to using the minus sign convention (e.g. -15) with a note explicitly defining the meaning of the $-$ sign, or should they simply be listed with an $i$ (e.g. 15$i$)?


(I've just been told to NEVER use the minus sign convention as this will incur the wrath of the reviewers. Please feel free to verify.)




The suffix **i** to the **positive** wavenumber to denote an imaginary frequency is seemingly accepted by reviewers, see [Photochemical transformations of 5-methyltetrazole. Matrix isolation FTIR and DFT studies](http://dx.doi.org/10.1016/j.jphotochem.2013.12.011) as one example I could find in a very quick search.


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I'm completely out of my depth here, but I would, since the minus sign convention seems to be frowned upon, look for other symbols:


* As mentioned, with an $i$ denoting the imaginarity: $15i$


This is maybe not so beautiful if we're actually talking about the *absolute value* of the imaginary magnitude, because in mathematics the $i$ clearly denotes the imaginary part of an imaginary number.
* If not yet taken, I would give my vote for the asterisk (\*): $15^\*$
* Since we're talking about transition state magnitudes, maybe the double dagger ($\ddagger$): $15^\ddagger$


Of course this notation would have to be defined somewhere towards the beginning of your article, stating that this symbol denotes imaginary (transition state) magnitudes.


Using this method you could even have the option of distinguishing between normal state and transition state imaginary magnitudes: Asterisk for the former, double dagger for the latter.


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