Subject: Negative freq. for ground state???

From: "Victor M. Rosas Garcia" <[email-masked]>
Date: Wed, 9 Aug 1995 20:55:56 -0400

Hi CCL'rs,

	It's me again :)  I've got a weird problem with MOPAC 93.  I trying to
calculate free energies of solvation for several sets of conformers (no TS's
involved).  This involves calculating the frequencies and principal moments of
inertia for each conformer, both in gas phase and in solution, to account for
the entropic contribution to the free energy.  I'm using FORCE for this, but
every now and then I get a *negative* frequency even though there are no
negative force constants in the force matrix.  The problem is more frequent
when I use FORCE and EPS=78.3, but it does appear for the gas phase
calculations too.  Usually there will appear one negative frequency with an
absolute value less than 40 cm^-1 (sometimes two).  FORCE found the gradients
to be acceptable (<2) in all cases so no additional minimization was done by
FORCE.
	According to the manual (regarding TS optimization) there is a way to
get rid of spurious negative frequencies which is to run a DRC along the
spurious mode.  However, when I use Cerius2 to animate the negative frequency
mode I see that it is quite complex.  Basically all the atoms move, so it is
not obvious to me how I can define a DRC along so many coordinates.
	BTW, the molecules I'm working on are either zwitterions or
 tetraalkylammonium cations (carnitine and choline).

	To make it brief my main questions are:
1) Is it possible to have a negative freq. and no negative force constant?
2) what meaning can be attached to a negative frequency (if any)?
3) Does it make sense to run FORCE and EPS=78.3 together?
4) Am I missing something really simple here?

Thanks in advance.

Victor

-- 
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Victor M. Rosas Garcia                   * "How can we contrive to be 
[email-masked]                *  at once astonished at the  
Virginia Tech doesn't necessarily share  *  world and yet at home in it?"
the opinions you just read.	         *  G. K. Chesterton
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