Subject: generating or augmenting basis sets

From: "Shobe, David" <DShobe[at]sud-chemieinc.com>
Date: Fri, 10 Oct 2003 13:24:17 -0400

When simulating "heavy heavy atoms" such as Pt or Te, the following problem arises: the lack of large basis sets for these elements.  What is typically available is a pseudopotential with a double-zeta basis set (e.g. LANL2DZ).  There are papers in which a single polarization function (d or f as appropriate) is derived for these elements.  For many calculations, this is adequate.

But what if you need a larger basis set?  I know people sometimes "decontract" basis sets to add more "zetas", and that (3d) sets of polarization functions are often derived from the single (d) function by multiplying and dividing the exponent by 4.  Are these universally accepted methods, or are there critics?   (The GAMESS manual seems a bit critical of the multiply and divide by 4 strategy).  Are there other acceptable methods besides these?  What if one wants a higher angular momentum function (e.g. to add an f function when only s, p, and d functions are available)?

Or is all of this a waste because of inherent limitations of the pseudopotential?

At any rate it seems like only a few groups produce basis sets, and I  get the impression that it's a rather arcane area of computational chemistry.  

--David Shobe
Süd-Chemie, Inc.