Subject: Summary: Gaussian98 Warning Message

From: "Fernando De La Vega" <[email-masked]>
Date: Tue, 23 Apr 2002 08:56:51 -0700

Dear All,
  Last week I asked a question with regard to the warning received in 
Gaussian98 that reads:

"Warning!!: The  largest alpha MO coefficient is #########", where the #'s 
represent a given (usually large)  number.

I want to thank Douglas Fox (from Gaussian Tech Support), Antonio Marquez 
and Christoph van Wüllen for their response to my inquiry. Below I 
reproduced their answers.

----------------------------------------------------
Douglas J. Fox:

Fernando, The warning is most relevant for post-HF calculations like Moller 
Plesset perturbation theory or coupled cluster, where the accuracy of the 
result is related to the accuracy of the MO integrals. When you have near 
linear dependencies in the basis set you will get large MO coefficients 
related to maintaining orthogonality. The side effect of large coefficients 
is loss of precision on any existing machine. Depending on the order that 
you add C1*I1+C2*I2-C3*I3+C4*I4 where C1 and C3 are large and of the same 
sign, if the integrals are all about the same size and C1*I1-C3*I3 is near 
zero the contribution from C2 or C4 can be lost if they are added before C3 
is subtracted. For HF and DFT calculations the code which checks this is run 
but seldom is it an issue, it is actually just after the SCF completes. For 
any post-HF method look to see that the correlation corrections are a 
moderate fraction of the total energy. It may not be an error but it is 
worth comparing with a slightly smaller basis or a different correlation 
method, CCSD is less sensitive than MP4, as a check.

---------------------------------------------------------------
Antonio Marquez ([email-masked]):

Dear Fernando,
I guess that you mean that ####### is a LARGE number. This means that you 
have a nearly linearly dependent AO basis set. The closer you are to have a 
really linearly dependent basis set the biggest will be the number. For HF 
calculations this is not usually a problem as the large number(s) is(are) in 
one (or more) of the virtual MO that are just the orthogonal complement to 
your occupied MO. Problems may arise if these orbitals are used for a 
subsequent correlated ab initio calculation. During the 2e-integrals 
transformation to the MO basis the AO integrals will be mutiplied by the all 
the MO coefficients. If one or more of your coefficients if very high the 
numerical precision on your transformed MO integrals will be compromised and 
you have to be carefull with the result. The ways to circumvent this problem 
are either reduce the linear dependency in your AO basis set or use an 
algorithm to compute the 2e repulsion integrals that mantains a high degree 
of numerical accuracy.
I hope this clarifies a little bit your doubts.

----------------------------------------------------------------

Christoph van Wüllen ([email-masked]):

This means that you have a near-linear-dependency in your basis sets, e.g. 
if
you have diffuse functions on two atoms which are quite close.
Since very similar wavefunctions can result from very different 
coefficients,
numerical problems might arise in force constant or correlation 
calculations.

If you are sure that there is no input error, proceed (with care).

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