Subject: summary: compound methods / G2 theory

From: Jeremy R Greenwood <[email-masked]>
Date: Mon, 2 Sep 1996 19:10:48 +1000 (EST)

Greetings all,

I recently asked a question about cheaper compound methods than G2(MP2)
and was inundated with excellent advice. Thanks to all who replied; a
short trip to the library and I'd found a good solution within a day.

Suggestions were generally of the following nature:

* Try DFT or hybrid DFT methods instead
* Try CBS methods instead
* Try simplifying the problem by only performing high level correlation
  calculations on the most important part of the molecules
* Try the G2(MP2,SVP) method of Smith, Radom, et al.

The solution I've chosen is a modification of the latter; G2 energies
include an empirical, basis-set-specific deltaE-HLC term, which is a linear 
combination of the number of alpha and beta electrons. Since all my systems 
have the same number of these, and since I'm most interested in relative 
energies, this term cancels, and I'm free to use the best basis sets 
I can afford.

G2(MP2) = ZPE*.8929 + HLC(G2) + QCISD(T)/6-311g(d,p) + MP2/6-311+G(3df,2p) 
                    - MP2/6-311g(d,p)
          // MP2/6-31g(d)

G2(MP2,SVP) = ZPE*.8929 + HLC(G2,SVP) + QCISD(T)/6-31g(d) + MP2/6-311+G(3df,2p)
                        - MP2/6-31g(d)
          // MP2/6-31g(d)

my method of choice:

deltaE = delta(ZPE*.8929 + QCISD(T)/6-31g(d,p) + MP2/6-311++G(3df,3pd)
                         - MP2/6-31g(d,p)  )
          // MP2/6-31g(d,p)

Jeremy
******************************************************************************
Original Question:

I'm attempting to calculate high accuracy energies for a series of species,
and would like to use a compound method.

G2 energies, or failing that G2(MP2) would be the obvious method of choice. 
Unfortunately, G2 is just out of reach; QCISD(T)/6-311G(d,p) is too
expensive, as is MP4. However, QCISD(T)/6-31G(d,p) is not, nor are MP2
calculations.

Questions:

Are there somewhat less expensive compound methods which are recommended?

Would substituting QCISD(T)/6-31G(d,p) for QCISD(T)/6-311G(d,p) make a 
complete nonsense of G2(MP2)?

Is delta-E-HLC specific to the G-theory basis sets and therefore not portable?

I'm considering the following simple alternative: at MP2/6-31g(d,p) geom
E = (HF)ZPE*0.9 + MP2/6-311G++(3df,3pd) + QCISD(T)/6-31G(d,p) - MP2/6-31g(d,p)

Thanks,

Jeremy
*****************************************************************************
Answers:

>From [email-masked] Tue Aug 27 23:35:27 1996

In regards to your question about a less expensive method to 
calculate molecular energies, you might care to check out
our recent paper:  J. Phys. Chem.  v100, p10083.

Here we calculated the energy differences (electron affinites) between
a number of quinones and their respective radical anions.  We compare
several methods and find that the best is B3LYP hybrid HF/DF method
with a 6-311G(dp) or 6-311G(3dp) basis set.  We found that the G2
method, which you mention specifically, gives very bad results
for parabenzoquinone.

I would very much like to hear about you work as it
progresses.

Sincerely

Anthony K. Grafton
Department of Chemistry and Biochemistry
University of Oklahoma
Norman, OK

>From [email-masked] Thu Aug 29 00:51:15 1996

Whoops.  In the mail I sent you earlier, I said we had tried G2 on 
quinone energies.  That was a bit wrong.  We sis try it, but did not
have the computing resources to complete the jobs (which includes 
an 8 processor IBM/SP2 with 512MB/processor).

The method that we did use and get poor results from was the CBS-4 
calculation.  Sorry about the confusion.

Anthony Grafton
Department of Chemistry 
University of Oklahoma

--
From: "Frederick R. Bennett" <[email-masked]>
Subject: CCL:G:G2 with reduced basis sets

Hi,
   someone was asking about attempting G2 calculations with reduced basis
sets. Well I found a paper addressing this issue exactly

 Gaussian-2 (G2) theory: Reduced basis set requirements, Curtis, Redfern,
Smith,  and Radom: JCP, 104 (13) 5184

Hope this helps

Ciao

===============================================================================
                    Frederick R. Bennett
                    
 Papernet Address:  Institut Fur anorganische, analytische 
                    und physikalische Chemie                  
                    Freiestrasse 3
                    CH-3000 Bern 9 
                    Switzerland
                    
 Mouthnet Address:  [41] (031) 631 4231

 Faxnet Address     [41] (031) 631 3994
 
 Internet Address:  [email-masked]

===============================================================================
From: Anthony P Scott <[email-masked]>
(also thanks to James W Gauld <[email-masked]>)

Jeremy,

I understand that James has already sent you two references for the 
G2(MP2,SVP) method.

Here are the two leading references for G2(MP2,SVP).

Smith, B. J. and Radom, L. J. Phys. Chem. 99 (1995) 6468.
        - where it is first introduced
Curtiss, L. A., Redfern, P. C., Smith, B. J. and Radom, L. J. Chem. Phys.
104 (1996) 5148.
        - a detailed description of G2(MP2,SVP), including the new HLC to 
use.

In your email you didn't say why the qcisd(t)/6-311g** calculataions are 
not possible.

Is it disk space or cpu time that limits you?

If you are trying to do these calculations with Gaussian you are really 
beating your head against a brick wall. Two other programs are 
substantially more efficient that G94 for these type of calculations.

Molpro96 ( limited to closed shell however) or ACES2 will result in 
(usually) much better cpu times that G94. They both make explicit use of 
symmetry, however ACES2 in C1 symmetry will often take longer than gaussian!!

If it is disk space that is the problem then try to get more!

Hope this is of some use.

Kind Regards,

Tony

--
>From [email-masked] Wed Aug 28 00:57:14 1996

If you have access to Gaussian 94, you might consider the CBS model
chemistries - see J. Chem. Phys. 104 (1996) 2598-2619 for details.

John Montgomery
Lorentzian, Inc.
--
>From [email-masked] Wed Aug 28 13:45:26 1996

The Complete Basis Set methods (CBS) of Petersson and coworkers are an
attractive alternative, especially CBS-4 and CBS-Q. They are also
compound models. They are automated in G94 and we discuss them
in chapter 7 of Exploring Chemistry.

--
From: Elmer Valderrama <[email-masked]>

 Just to call your attention on the work

 "Combining MCSCF and DFT for dyn. electron correlation"
 N.O.J. Malcom and J.J.W.McDouall, J. Phys. Chem 100(24) 10131-10134 (1996)

 where an interesting proposal is advanced concerning compound methods.

 This possibility (MCSCF (or CASSCF) + DFT) was probably first noted
 by Clementi in 1974, but a way to do it was not advanced (see ref in
 work cited aboved)

 Elmer

--
From: [email-masked] (Bernd Engels)

hello
Jeremy R Greenwood asked for cheaper methods than
QCISD or MP4. I would try to use DFT methods.
There are also some extensions to the normal
G2 which uses the DFT.

bernd engels
[email-masked]
--
Reply-To: [email-masked]

I'm not particularly experienced in this issue, but there are several
related works in our group (K. Morokuma), which might provide some useful
info.

First of all, there are always those scaling approaches, which works
rather well in many cases, including transition metal chemistry. One of
them is the so-called PCI-80, proposed by M. Svensson and P. Sigbahn.

Secondly, some of our group memebrs are working on the so-called IMOMO
method, which is basically treating the active part with high accuracy,
and the rest with relatively low accuracy. Indeed, in most chemical
reactions, correlation effects are rather localized, the inactive part are
either correlation-insensitive, or effects simply cancels out if one
considers the relative reaction profile. In those cases, one can do really
high level calculations such as G2 on the active site. For example, people
in our group did some benchmark calculations on a series D-A reactions(
with large substituents) and found really good agreement with several gas
phase exp. results. They've also done some systematic studies on the S/T
transition energies for a series of large molecules,and again found
promising results. I guess the merit of those approach is, one does't
really need any new code to do the calculation, (provided that u have a
decent geometry, people in our group are also doing geom. optimization
using IMOMO/IMOMM). All u have to do is, do a few single points calc. at
different levels, and put them together. Sophisticated mathmatics doesn't
always mean better results. (although personally I like rigorous math
better:-)

Any way, that's what I can think of and what I'm relatively familar with.
If u r interested, u can contact me or [email-masked] for
some references.

Hope this will help in some way..

______________________________________________________________

Qiang Cui
Dept. of Chem. Emory Univ.         508 Webster Dr. Apt.#2
Atlanta, GA 30322.                 Decatur, GA 30033.
(404)-727-2381                     (404)-636-6149

http://tswww.cc.emory.edu/~qcui
______________________________________________________________