CCL: Spikes in PM6 optimization energies

[Scott McKechnie <jsm78]^[cam.ac.uk>]

Dear Alexander, John, Ted,

Many thanks for your replies and apologies for my delayed response. To answer some points, the geometry seems fine in the end and the SCF converges relatively smoothly. As Alexander mentioned, it looks to just be an effect of the line search.

Best wishes,

Scott

On 25 June 2013 17:08, Ted Dibble tsdibble-$-esf.edu <owner-chemistry++ccl.net> wrote:

Sent to CCL by: "Ted   Dibble" [tsdibble[*]esf.edu]
I see this kind of problem occasionally with DFT optimizations, mostly of
transition states. The jump in energy of 0.18 a.u. (~110 kcal/mol)
corresponds to a huge amount of energy. I suspect this error arises when the
calculation converges on the wrong wavefuntion: that of an excited state.
This occurs more readily when bonds are partly broken (transition state
searches) than geometry optimizations (unless the initial geometry is poor,
as suggested by John McKelvy).

    If the geometry finally optimizes, it should not be a problem. However,
this sort of situation emphasizes the need to check the optimized geometry to
make sure you still have the molecule you want.

  Good Luck!
   Ted Dibble


Theodore S. Dibble
Chemistry Department
SUNY-Environmental Science and Forestry
1 Forestry Drive
Syracuse, NY 13210
(315) 470-6596
(315) 470-6856  (fax)
http://www.esf.edu/faculty/dibble/

> "Scott Mckechnie jsm78#%#cam.ac.uk"  wrote:
>
> Sent to CCL by: "Scott  Mckechnie" [jsm78-*-cam.ac.uk]
> Dear all,
>
> I ran a PM6 optimization of an organic molecule and found spikes in the completed SCF cycle energies. For example:
>
>  SCF Done:  E(RPM6) = -0.739823088031E-01 A.U. after   19 cycles
>  SCF Done:  E(RPM6) = -0.559900784544E-01 A.U. after   25 cycles
>  SCF Done:  E(RPM6) = -0.742045066741E-01 A.U. after   25 cycles
>
> - Is this simply a result of an optimization step that goes uphill on the PES?
> - If an increase in the energy is found will it be discarded and the optimization search will start at the previous point?
> - Is this type of behaviour also commonly seen in a DFT optimization?
>
> Any good references for discussions on this would be greatly appreciated.
>
> Best wishes,
>
> Scott
>
>



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