Subject: Linear Dependency and Gaussian 98: A cautionary tale

From: "Feller, David F" <[email-masked]>
Date: Fri, 21 Jan 2000 08:56:01 -0800

One change in Gaussian 98 that had escaped our notice until
yesterday was that it will "automatically handle" linear dependency
in basis sets.  What this means is that a rectangular transformation
is performed to eliminate eigenvectors of the overlap matrix with
eigenvalues below some threshold.  The default threshold in G98
seems to be 1.0E-5.  To the best of my knowledge, in previous versions 
of Gaussian all basis functions were retained, no matter how small
their eigenvalues.  In particularly severe cases this caused convergence
failure in the SCF. 

Eliminating eigenvectors has both positive and negative consequences.  
The principal advantage is that the code will operate more robustly.  The
disadvantage is that users find *significant* energy differences between
calculations run with earlier versions of Gaussian (or other codes) and
Gaussian 98.  Like us, they may not have been aware of this change
in Gaussian 98.

In the G98 output file, you will see a line such as:
One-electron integrals computed using PRISM.
NBasis=    95 RedAO= T  NBF=    95
NBsUse=    92 1.00D-04 NBFU=    92

The variable NBUse is the actual number of basis functions used.
In this case 3 were elminated.  John Montgomery sent us the 
IOP code necessary to override this new feature.  It is iop(3/32=2)
which should appear on the same line as the level of theory,
basis set, etc.

Dave 

P.S. Thanks to Michel Dupuis, Kirk Peterson and John Montgomery
for unraveling the mystery.

David Feller                                | [email-masked] 
Environmental Molecular Sciences Lab., K8-91| Box 999
Pacific Northwest National Laboratory       | Richland, WA 99352 
                                            | Fax: 509-375-6631