Subject: DFT Programs

From: Cedric Merlot <[email-masked]>
Date: Fri, 17 Jan 1997 10:48:06 +0100

Howard Alper wrote:
> 
>         Hi,
> 
>         I am looking for information on the behavior of the combined
> Quantum/Classical forcefield (QM/MM), especially as implemented in
> CHARMM.
> 
> 1)      What types of systems, processes, or conditions, would necessitate
> the use of an ab-initio QM/MM forcefield?  For what situations would a
> semi-empirical version suffice?
> 
> 2)      What are the changes in computational speed attendant to using
> the ab-initio vs. the semi-empirical versions of the QM/MM forcefield?
> 
Hi all

I'm searching for programs to perform DFT calculations (for free or not
too expensive).

Could someone show me any site ? or postal address ?

Thanks

Bye bye
-- 
Cedric Merlot
[email-masked]

---

From: A J Turner <[email-masked]>
Date: Fri, 17 Jan 1997 11:21:09 +0000 (GMT)

> semi-empirical version suffice?
  The AM1 force field in CHARMM is only paramterised for a sub-set of the
now availble AM! paramters.  For organic and biological systems where this
subset covers all the atoms, the am1 potential can be used where ever it
would be sufficiant if you were in the gas phase.  

F seems to be a problem, and the O-H force constant is up the spout - but
it is a very good starting point.

> the ab-initio vs. the semi-empirical versions of the QM/MM forcefield?
I have written my own ab-initio qm/mm force field using charmm molecular
mechanics.  This follows about the same method as Charmm does but works
with Cadpac6 giving DFT.  Using this code the ab-initio is vcery very very
much slower than AM1.  Howver - for systems where you can get a guess from
AM1 this is not a problem as the ab-initio is just a refinement at the end
of a longer production process.

I don't know what the latest refinments to TN-PAC in Charmm are like.  It
may be posible to inherit their 'sparse' hessian,  I use explicit hessian
inheritance for non-dynamical systems, and that seems to make MP2 and DFT
triple zeta stuff in reach for systems for which the qm bit would be
tractable using gas phase technology (remeber that you tend to loos
symmetery).

> 
> 	Thanks in advance.  I will summarize to the list after all the
> responses are in.

Best wishes

Alex

> 
> 	Howard
> 
> -- 
> 
>   Dr. Howard Alper
>   Moldyn Inc.
>   955 Massachusetts Avenue
>   Fifth Floor
>   Cambridge, MA 02139-3180
>   617-354-3124 x19
>   email: [email-masked]
> 
> - helping molecules find happiness for over a 10th of a century.
> 
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|Alexander J Turner         [email-masked]                  |
|Post Graduate              |http://www.bath.ac.uk/~chpajt/home.html|
|School of Chemistry        |+144 1225 8262826 ext 5137             |
|University of Bath         |                                       |
|Bath, Avon, U.K.           |Field: QM/MM modeling                  |
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