Subject: On basis sets for calculating thermochemistry properties

From: Wai-To Chan <chan-.at.-curl.gkcl.yorku.ca>
Date: Sun, 8 Jun 2003 18:41:11 -0400 (EDT)

This is an amendment of my previous message. Perhaps 
the recommendation of the 6-31+G(d,p) as the minimal basis 
set to use is too strong and not always pragmatic. My 
experience is that the '+' can improve accuracy significantly
for characterization of the potential surface of 
floppy radical structures. But diffuse functions can also 
create convergence difficulties. For calculation of 
the equilibrium structures of more 'usual' type of molecules 
I think it would be fine to begin with 6-31G(d,p).

Wai-To Chan 

<<<<<<<<<<<<<<<<<<<<<<<<<<<<<<<<<<<<<<<<<,,
I used the xalpha/3-21g method for geometry optimization
of LARGE molecules in the past and got some decent results. But 
sto-3g and 3-21g won't be a good choice for reliable thermochemistry 
calculation with Hybrid DFT method like B3LYP. Use a basis set at least 
as large as 6-31+G(d)/6-31+G(d,p). The inclusion of '+' can make a difference. 
You may refer to 

   J. Phys. Chem. A, (2003) vol 107 pg 1384-1388 by Lynch, Zhao and Truhlar

for an assessment of the accuracy of the MPW1PW91/6-31+G(d,p) method. 

Wai-To Chan