CCL: 6 degree of freedom for linear molecule ??????

Dear ABHISHEK SHAHI

Looking at your negative frequencies, I see two that are nearly identical (-378.71 and -378.71) and another pair that are nearly identical (-215.19 and -215.19).  This is a priori extremely unlikely UNLESS your molecule possesses 4-fold or greater symmetry.  You say it is linear, so obviously it has a lot of symmetry.

You ran the optimization using the 'nosymm' keyword, but that does NOT guarantee that optimization won't get stuck in a symmetrical saddle point.  Gradients can be so small near such a saddle that optimization might not find the eigenvector that actually points downhill.

What has happened is this: the true minimum for your molecule is NOT linear, but optimization is stuck in a linear geometry because you started with a linear geometry.

To solve the problem, perturb the molecule by in the direction of the eigenvector associated with one of the 'negative' modes.  How much to perturb? -- I can't say exactly, but make the angle(s) 165 degrees or less, rather than 180 degrees.  And if there are four or more atoms, make it non-planar too, with torsional angles between 15 and 165 degrees.  Then restart optimization from this emphatically non-symmetric geometry and see what happens.

Peace,
David Anick PhD MD

 ---- Original message ----

Date: Mon, 06 Sep 2010 08:33:21 +0200
From: "Dr.Wolfgang Quapp quapp]~[rz.uni-leipzig.de" <owner-chemistry]=[ccl.net>
Subject: CCL: 6 degree of freedom for linear molecule ??????
To: "Anick, David " <david.anick]=[rcn.com>
>
>Sent to CCL by: "Dr.Wolfgang Quapp" [quapp{}rz.uni-leipzig.de]
>Zitat von "ABHISHEK SHAHI shahi.abhishek1984!A!gmail.com"
><owner-chemistry=-=ccl.net>:
>> Dear All
>> Here optimization give a linear geometry but on frequency calculation,
>> Job gives 6(translational+rotational) frequencies and they are negative
>> enough to make any sense.My route section is "# MP2=full/aug-cc-PVTZ
>> opt=(verytight,maxcycle=100) nosymm freq=noraman
>> scf=(xqc,vshift=150,tight)".And same problem is faced by me several other
>> complexes.Please help me in this problem.your suggestions are valuable for
>> me.
>>
>> Full mass-weighted force constant matrix:
>> Low frequencies -378.7133 -378.7122 -215.1945 -215.1944 0.0043 0.0046
>> Low frequencies 0.0060 714.7626 2094.2659
>> Molecule is linear but NTrRo=6.
>> Error termination via Lnk1e in /home/pkg/lic/g09/em64t/g09/l716.exe at Fri
>> Sep 3 19:32:28 2010.
>> Job cpu time: 0 days 3 hours 7 minutes 24.7 seconds.
>>
>
>Dear ABHISHEK SHAHI,
>
>first of all, there are 7 interesting frequencies.
>Three zeros for translation are o.k.
>My be that the 2 lowest are from a saddle of index 2?
>But opt searches for a minimum?
>
>Dr.Wolfgang Quapp
>
>
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