Subject: Summary Correlation Electronic

From: "Dr. Gloria I. Cardenas-Jiron" <[email-masked]>
Date: Mon, 17 May 1999 14:48:21 -0400

Thank you very much to all who replied.

The question was:

Are there some way to fix the spin in MP or DFT calculations using for
example the ROHF command? 

Answers:
1.- You are correctly concerned about UHF.
It is possible to project the UHF wavefunction to get rid of
contamination by higher multiplicities, but this changes the
wavefunction, and is something I would not recommend.
Although Gaussian does not do UHF/MP2 calculations, there
are other software packages that do.  I would suggest that
you investigate these.  Good luck.  Janet Del Bene 

2.- the syntax is ROB3LYP / UB3LYP for example in case of the B3LYP
functional. Omitting of RO / U defaults to the unrestricted
wavefunction.

Cheers,

Dominik 

3.- Just say ROMP2 or ROB3LYP. 
Artem

4.- Try ROMP2 or ROBLYP or whatever the functional you want preceded by "RO"

Kazuo Teraishi; Dr.
Nippon Zeon Co. Ltd.

5.-  From: [email-masked] (Cust. Service Doug)

 Gloria,

   Gaussian 98 does not implement traditional correlation methods for
ROHF references with the exception of ROMP2 energies.  There are certainly
approaches for doing this but the theory is subtantially more complicated.
We have plans for development of the Brueckner Doubles method which does
a very good job of removing spin contamination in UHF reference wavefunctions
but this will not be available until a future major release of Gaussian.

   DFT methods can be done with RO- constraints.  In place of U you can
specify RO to get methods like ROB3LYP for open shell solutions.  For most
pure DFT functionals, i.e. BLYP or BPW91, spin contamination is small and
application of HF style methods to remove it cause more problems than they
solve.  The hybrid style functionals, B3LYP, do inherit contamination from
the the HF exchange contribution.

   With G98 the method which does best at correcting spin contamination is
QCISD and adds the option of gradients.  CCSD and the triples corrections
are good for cases where you can get by with just energies.  

6.- Hola Gloria,

La mejor forma de tratar el problema de la correlacion electronica cuando
uno tiene metales de transicion es usar una funcion CASSCF como punto de
partida. La contaminacion de spin que tiene en una funcion UHF (RHF o DFT)
no la puedes eliminar completamente (se puede proyectar el primer
contaminante, pero siempre queda bastante) y ademas, se amplifica por el
MP2. Un RHOF/MP2 puede no tener contaminacion de spin, pero tu funcion de
onda de partida puede participar con un 80% o menos en la expansion MP2,
lo que provoca grandes fluctuaciones en la estimacion de la energia de
correlacion electronica y grandes errores en propiedades como energias de
reaccion, geometrias, frecuencias de vibracion, ... todo lo que dependa de
tener siempre la misma cantidad de energia de correlacion incorporada en
tu expansion MP2.

Volviendo al principio, si tu problema es de metales de transicion (y
sobre todo con capas abiertas) usa una funcion CASSCF, al menos.

Si tienes alguna duda escribe, intentare contestar pronto.

Saludos, Antonio Marquez

7.- Hi Gloria,
Gamess (http://www.msg.ameslab.gov/GAMESS/GAMESS.html) does do MP2 on
a ROHF wavefunction. I used it, it works great.

Greetings,   Gert

8.- Hie,

You could use ROMP2. It is based on a restricted wave function.

Cheers,

  Theis Soelling

9.- Hola Gloria,
 
Respecto a spin-contamination es un problema mayor ya que los tratamientos
UHF siempre sobreestiman el spin.  En el caso relativista (Dirac) para
open-shell systems este no es problema.
Debes recordar que spin  esta definido solamente en la Teoria de Dirac y no
en la de Schrodinger.

Saludos cordiales,

Ramiro Arratia Perez, Ph.D.

Dr. Gloria I. Cardenas-Jiron 
Facultad de Quimica y Biologia,
Universidad de Santiago de Chile, 
Santiago, CHILE.