# CCL:G: Gaussian03 Error: Density matrix is not changing but DIIS error -
Suggested solutions

*From*: Eric Breynaert <eric.breynaert,biw.kuleuven.be>
*Subject*: CCL:G: Gaussian03 Error: Density matrix is not changing
but DIIS error - Suggested solutions
*Date*: Tue, 28 Nov 2006 18:02:00 +0100

Sent to CCL by: Eric Breynaert [eric.breynaert_._biw.kuleuven.be]
Thanks for the suggestions. I'm giving an overview below.
1/
- SCF=qc will probably solve the problem, albeit at a cost
- Change the SCF converger to either SD, Quadratic or Fermi
2/
- lower the symmetry of optimize with and optimize
with the "nosymm" keyword

`I solved the problem using a variation on the
``first suggestion. Normally the scf took less than
``80 cycles to converge. So i
``used scf=(Maxconventionalcycles=100,xqc) which
``resulted in a good compromise between using
``scf=qc and optimisation speed. In the case of the
``DIIS error the scf always took more than 100
``cycles before the error, so by adding
``scf=(Maxconventionalcycles=100,xqc) the scf
``switched to qc after 100 cycles in the standard DIIS mode.
` Regards,
Eric
u wrote:

Sent to CCL by: "Eric Breynaert" [Eric.breynaert]=[biw.kuleuven.be]
Hello all,

`I'm trying to optimize a transition metal
``structure using DFT ub3lyp LANL2DZ basisset for
``the metal center , 6-31G(d) for all other elements o.a. O, H, C, S,
....
`

`In most cases the optimisation succeeds. But
``sometimes g03 generates the following error
``after about 20 to 80 steps in the optimisation.
` Density matrix is not changing but DIIS error= 2.02D-05 CofLast= 3.35D-01.
The SCF is confused.

`I've already btried different approaches to get
``arround this but nothing has succeeded. For
``example, recalculating force constants on the
``geometry obtained 1 or 2 steps before the error.
``Forcing g03 to make a new guess at every step of
``the optimisation using guess=(mix,always).
`

`Has anyone encountered such a problem before, or
``do you know a solution to it ?
` Thanks,
Kind Regards,

`Eric--
`

Eric Breynaert
Labo voor Colloïdchemie
Kasteelpark Arenberg 23
B-3001 Leuven
Tel: +3216321457
Fax: +3216321998
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