# 3-21G vs. 6-31G(d) geom opt

Mon, 1998 June 22
Hello,
The 3-21G (strictly, 3-21G*) basis set gives geometries which are actually a
bit better than those from the 6-31G* set (Hehre, "Practical Strategies for
Electronic Structure Calculations", p 23; Hehre, Radom, Schleyer and Pople,
"Ab Initio Molecular Orbital Theory", pp 175 and 185; this is for
HF-level
calculations; for correlated-level work one needs at least 6-31G*, of course).
HF/3-21G* geometries are even used for Petersson's high-accuracy CBS-4 method.
So why (judging by the literature) is the 6-31G* basis usually preferred for
HF-level geometries? Is it because of an unexamined belief that bigger is
better? Or is it because *relative energies* are better at the HF/6-31G* level
than at the HF/3-21G level? If this is the reason, then in those cases where
*both* HF and MP2 (or other correlated) calculations are reported, would it not
be better to do the HF calculations using 3-21G optimizations (then use MP2/
6-31G* or bigger for the post-HF jobs)?
E. Lewars
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