Re M imaginary frequencies

From: A J Turner <chpajt # - at - # bath.ac.uk>
Subject: Re M imaginary frequencies
Date: Wed, 10 Jul 1996 19:14:20 +0100 (BST)

 Hi!
 There are a few points that are maybe worth thinking about.
 1: Mopac is not stable under restarts - in my experiance.  Ie if you have
 and EF job that ran half way and was restarted , then  compare to a
 complete run - the answers will not be the same. This sometime erroniously
 results in to jobs giveing different answers - and you think - err why did
 that happen!
 2: The hessian update does seem to fail sometime on EF.  I often use
 hess=3 recalc=3 or recalc=1.  Most people say this is excessive - but I
 find it kills off these imaginary frequencies.
 3:  I once accedantally started EF on a transition state - absolutelyy
 100% on the torsion barrier - eay for a z-matrix.  I said that this was a
 minimum!  IE - it cn fail in this way.
 4: My version of mopac 6.0 has ddmin  - something odd here.
 Anyhow - hess=3 recalc=3 or recalc=1 has never ever failed me yet.
 Good luck
 Alex
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 |Alexander J Turner         |A.J.Turner # - at - # bath.ac.uk                  |
 |Post Graduate              |http://www.bath.ac.uk/~chpajt/home.html|
 |School of Chemistry        |+144 1225 8262826 ext 5137             |
 |University of Bath         |                                       |
 |Bath, Avon, U.K.           |Field: QM/MM modeling                  |
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