Subject: On basis sets for calculating thermochemistry properties

From: Wai-To Chan <chan*at*curl.gkcl.yorku.ca>
Date: Sun, 8 Jun 2003 01:42:18 -0400 (EDT)

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Dear all,

I am calculating thermochemistry properties by Gaussian DFT method. Can 
anyone give me some suggestions on basis sets for the calculation? 
Which is the minimum basis set I can use in this calculation is 
reasonable? With small basis sets (sto-3g, 3-21g), which one can get 
better results for thermochemistry properties by Gaussian DFT method?

Thanks to you all,

Holly

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