From owner-chemistry@ccl.net Fri Jun 28 11:06:00 2013 From: "Scott McKechnie jsm78__cam.ac.uk" To: CCL Subject: CCL: Spikes in PM6 optimization energies Message-Id: <-48870-130628110419-23031-FEyjc+/TggjDV3qDFEtT0Q]~[server.ccl.net> X-Original-From: Scott McKechnie Content-Type: multipart/alternative; boundary=047d7b6dd040ca8cc404e0383096 Date: Fri, 28 Jun 2013 16:04:08 +0100 MIME-Version: 1.0 Sent to CCL by: Scott McKechnie [jsm78(-)cam.ac.uk] --047d7b6dd040ca8cc404e0383096 Content-Type: text/plain; charset=ISO-8859-1 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> > > > --047d7b6dd040ca8cc404e0383096 Content-Type: text/html; charset=ISO-8859-1 Content-Transfer-Encoding: quoted-printable
Dear Alexander, John, Ted,

Many t= hanks for your replies and apologies for my delayed response. To answer som= e points, the geometry seems fine in the end and the SCF converges relative= ly smoothly. As Alexander mentioned, it looks to just be an effect of the l= ine 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 =A0 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 th= e
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).

=A0 =A0 If the geometry finally optimizes, it should not be a problem. Howe= ver,
this sort of situation emphasizes the need to check the optimized geometry = to
make sure you still have the molecule you want.

=A0 Good Luck!
=A0 =A0Ted Dibble


Theodore S. Dibble
Chemistry Department
SUNY-Environmental Science and Forestry
1 Forestry Drive
Syracuse, NY 13210
(315) 470-6596<= /a>
(315) 470-6856<= /a>=A0 (fax)
http://www= .esf.edu/faculty/dibble/


> "Scott Mckechnie jsm78#%#cam.ac.uk" =A0wrote:
>
> Sent to CCL by: "Scott =A0Mckechnie" [jsm78-*-cam.ac.uk]
> Dear all,
>
> I ran a PM6 optimization of an organic molecule and found spikes in th= e completed SCF cycle energies. For example:
>
> =A0SCF Done: =A0E(RPM6) =3D -0.739823088031E-01 A.U. after =A0 19 cycl= es
> =A0SCF Done: =A0E(RPM6) =3D -0.559900784544E-01 A.U. after =A0 25 cycl= es
> =A0SCF Done: =A0E(RPM6) =3D -0.742045066741E-01 A.U. after =A0 25 cycl= es
>
> - 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 o= ptimization search will start at the previous point?
> - Is this type of behaviour also commonly seen in a DFT optimization?<= br> >
> Any good references for discussions on this would be greatly appreciat= ed.
>
> Best wishes,
>
> Scott
>
>



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