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Alex Granovsky
gran@classic.chem.msu.su

>I am indeed interested in the couple of useful tricks on how to reuse already computed information to reduce the cost of $force nvib=2 $end computations. Please post it.

In the simplest case, just copy the computed $vib group from ircdata
file to your input file, add $force nvib=2 $end and use exactly the
same molecular geometry, orientation, and vibsiz as before. This will
resume calculations for missed displacements. Things are more
complicated in the case you have deleted ircdata files.
In this case, you need to recreate this information by gathering
$vib groups found in output files, then proceed as above.


>>This can be indeed caused by the nvib=1 numerical inaccuracies. Are there exact six additional modes corresponding rotations + translations? Did you check all these modes using e.g. ChemCraft  (the trial version is free)?

>Yes, there are six rotations + translations. The head of the frequency output reads:
>* * * WARNING, MODE 1 HAS BEEN CHOSEN AS A VIBRATION
>          WHILE MODE 7 IS ASSUMED TO BE A TRANSLATION/ROTATION.
> PLEASE VERIFY THE PROGRAM'S DECISION MANUALLY!
> MODES 2 TO 7 ARE TAKEN AS ROTATIONS AND TRANSLATIONS

>                            1       2    3      4       5        6     7      8        9       10
>       FREQUENCY:        34.48 I  23.71  20.09  10.79   11.33  19.88  20.87  106.28  153.77   160.01

>Does it implies that the problem is trivial and the suggested geometry can still be taking as stationary and not transition?.

No, unfortunately it does not. It could imply this provided there
are five close-to-zero frequencies. Then the sixth one (imaginary)
could be just an artifact. This output only supports two things:

1. You need to visualize vibrations and look at them carefully
2. You may need to use nvib=2 or better basis set

> Also what do I look for if I use ChemCraft and what can ChemCraft do to correct the problem?.

You can look at animated displacement corresponding to these
one + six modes and decide whether the first one is true vibration, or not.

Kind regards,
Alex Granovsky