CCL: Is CASSCF appropriate for study of PES?

[Yingbin Ge <yingbin.ge|gmail.com>]

Thanks for your kind reply!

I need to study all possible reaction pathways of a small molecular system for example:

A --> B + C

A --> D + E

D + E --> F + G

F + G --> H

H --> A

...

And then I need to get the relative energies of local minima and transition states (TS)

of the above reactions.

Different sigma bonds are broken and/or formed in the above reactions, which

makes it hard to choose the active space for every reaction consistently.

However I need to get the relative energies right, at least within 2 kcal/mol accuracy.

CCSD(T) or QCISD(T) does not do well for TS structures with streching bonds.

That's why I resort to the CAS methods.  I plan to use the full valence active space,

but still choosing the "correct" virtual orbitals is not easy especially for transition states.

What do you think?

 

Thanks again,

Yingbin

 

On 11/20/05, errol lewars elewars%x%trentu.ca <owner-chemistry^ccl.net> wrote:

Sent to CCL by: errol lewars [elewars(!)trentu.ca]
2005 Nov 20

If your reaction is A--> B size-consistency (SC) should not be a
problem. If it is A--> B + C, e.g. the energy of a dimer compared to the
separated monomers, you have to worry about  SC.

"No good way... virtual orbitals". CASSCF is not a model chemistry (see
e.g. Foresman and Frisch, "Exploring Chemistry with Electronic
Structutre Methods", so there in no _automatic_ way to choose the MOs of
the active space. But people still use CASSCF and get useful results.

E. Lewars
===


Yingbin Ge yingbin.ge^gmail.com wrote:

>Sent to CCL by: Yingbin Ge [yingbin.ge]![gmail.com]
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>Content-Type: text/plain; charset=ISO-8859-1
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>Hi CCLers,
> I am studying the PES of a small molecule and wondering
>whether CASSCF or CASPT2//CASSCF is an appropriate
>method for the PES study since these methods are not
>size consistent. Another downside of using CAS is that there
>is no good way of choosing virtual orbitals into the active space.
> Any comments?
> Thanks a lot!
>Yingbin
>
>------=_Part_4657_175608.1132432456569
>Content-Type: text/html; charset=ISO-8859-1
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><div>Hi CCLers,</div>
><div>&nbsp;</div>
><div>I am studying the PES of a small molecule and wondering</div>
><div>whether CASSCF or CASPT2//CASSCF is an appropriate</div>
><div>method for the PES study since these methods are not</div>
><div>size consistent. Another downside of using CAS&nbsp;is that there</div=
>
>
><div>is no good way of choosing virtual orbitals into the active space.</di=
>v>
><div>&nbsp;</div>
><div>Any comments?</div>
><div>&nbsp;</div>
><div>Thanks a lot!</div>
><div>Yingbin</div>
>
>------=_Part_4657_175608.1132432456569-->
>
>
>
>



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