From owner-chemistry@ccl.net Sun Aug 31 05:14:01 2008
From: "Orlin Blajiev blajiev-*-vub.ac.be" <owner-chemistry.@.server.ccl.net>
To: CCL
Subject: CCL:G: APT And Mulliken
Message-Id: <-37683-080831051046-27743-ZDRgxfoWN6hCff7mniSEug.@.server.ccl.net>
X-Original-From: Orlin Blajiev <blajiev[*]vub.ac.be>
Date: Sun, 31 Aug 2008 11:10:23 +0200 (CEST)


Sent to CCL by: Orlin Blajiev [blajiev,,vub.ac.be]
Hi everybody,

I have a system which I charge and discharge by adding or subtracting electrons. Then when I looked at the output I saw that the total number of electrons is correctly given by the Mulliken approach and was not so by the APT.

For example two electrons out, then:
Sum of Mulliken charges=   2.00000
But 
Sum of APT charges=   2.14530

Can someone tell me why APT charge is not an integer one?

Thanks.
Orlin


>Sent to CCL by: frisch^_^gaussian.com (Michael Frisch)
>On Wed, Jun 18, 2008 at 02:17:41PM -0400, Steve Williams willsd]|[appstate.edu wrote:
>> 
>> Sent to CCL by: Steve Williams [willsd^_^appstate.edu]
>> Raman intensities are available in G03, but since they require numerical
>> derivatives for MP2 and DFT methods they are very slow to compute and by
>> default they are not calculated on a frequency job.  If you change your
>> input to include freq=raman on the route line, they will be computed.
>> Six separate calculations will be done for each atom (+/- displacements
>> in x, y, z) to compute the needed polarizability derivatives.  You can
>> estimate how long this will take by doing a single point polarizability
>> calculation then multiplying the time required for this by 6*N where N
>> is the number of atoms in your molecule (the portion you want to compute
>> with b3lyp).
>> 
>
>This is not correct.  In G03, MP2 and B3LYP Raman intensities (static
>polarizability derivatives, "Freq=Raman" on the route card) are done
>by numerically differentiating the analytic dipole derivatives with
>respect to an applied electric field.  So there are only 6 additional
>calculations regardless of the size of the molecule.  The cost of each
>of the 6 calculations in a finite electric field is somewhat more than
>a polarizability calculation but much less than the calculation of the
>second derivatives for the basic frequency calculation.  So for very
>small molecules the total cost of Freq=Raman with DFT or MP2 is around
>than 3x the cost of just doing Freq, but this drops to 1.5x for larger
>systems.
>
>Freq=Raman does work with ONIOM.  With ONIOM(MO:MM) and mechanical
>embedding the cost is about the same as for the MO calculation on the
>model system.  With ONIOM(MO:MM) and electronic embedding, the cost is
>higher than just doing the model system (with or without Raman)
>because derivatives with respect to all the nuclei have to be included
>in the QM calculation.
>
>Mike Frisch>
>
>
>


Orlin Blajiev
Department of Metallurgy, Electrochemistry 
and Materials Science
Faculty of Applied Science
Vrije Universiteit Brussel
Pleinlaan 2, B-1050 Brussels
Belgium

http://www.vub.ac.be/META/

tel.: 32-(0)2-6293538
fax : 32-(0)2-6293200