Subject: Summary: Unused basis function in g98?

From: Jens Antony <[email-masked]>
Date: Fri, 21 Jan 2000 14:47:41 +0100 (MET)

Dear CCL!

A few days ago I posted a question concerning the difference of the HF
results on the Cd2+ ion between Gaussian98 and Gaussian94. It turned out
that it was caused by the fact that Gaussian98 automatically checks the
basis set for linear dependency and reduces the expansion space if
necessary. Below as summary two replies and the original question.

Thanks for the help and interest of all replicants.

Jens

---------- Original question ----------

Comparing the HF energies of Gaussian98 with Gaussian94, DALTON, and
MOLCAS we observe a difference of about 0.01 a.u. with Gaussian94,
DALTON, and MOLCAS having the same lower energy. We see this for the
Cd ion and for cadmium containing molecules.

We use the Gen keyword to invoke Kelloe and Sadlej's contracted basis for
Cd (Theor Chim Acta (1995) 91: 353). Looking into the output with Pop=Full
we find that Gaussian98 apparently does not use basis function no. 4 of
s-type. It occurs for revisions A5 and A7 on both LINUX and SGI machines.

Is this a bug in g98 and if so when does it occur?

---------- Reply from Karl K. Irikura ----------

Dear Jens,

G98 may have decided that the basis has a linear dependency and to
delete one function.  You can try turning this off using IOP(3/32=2).

Good luck!

Karl

---------- Reply from Douglas J. Fox (Gaussian support) ----------

There are indeed checks for linear dependencies on the basis set and you
have an unusual case in that you have a near linear dependency in the core
functions. The more common occurance of such dependencies is in diffuse
functions where little or no change in the energy can be detected. For HF
energies you can generally include these functions by turning down the
threshold for discarding basis functions with IOP(3/59=5), the default is
4, or turn off checking as you did with IOP(3/32=2).

This has drawbacks for geometry optimizations and post-HF calculations
where near linear dependencies can change with geometry or cause numerical
problems with correlation expansions and integrals over the MO basis.

Also while the total energy is affected it is likely that this energy
defect will be evident in all molecular systems as the core orbitals don't
interact strongly on bonding. Thus relative energies are likely to be
unaffected.

Dependencies in core functions are uncommon and while you have checked the
Sadlej publication it is possible that there is an error there. You might
want to look at the uncontracted atomic calculation and associated
eigenvectors to see how well than energy compares with the contracted
value.