Basis Set Selection for Geometry Optimization

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jayeshjawandhia
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Joined: 16 Jun 2020, 09:27
First name(s): Jayesh
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Affiliation: IIT Delhi
Country: India

Basis Set Selection for Geometry Optimization

Post by jayeshjawandhia » 17 Jun 2020, 10:42

Hey,

We are doing HF geometry optimizations to find out structures for charged molecules i.e. we optimize molecule like C3H6o (acetone) in Dalton to get the structure of C3H6O+ but we are not sure how should we go about selecting the basis set for the optimizations for a range of molecules like if we do it for Amines and molecules like Ozone can we keep the basis set the same or there are some rules to decide the basis set. Right now we are using 6-31G**. Please help!

Thanks
Jayesh
IIT Delhi

taylor
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Joined: 15 Oct 2013, 05:37
First name(s): Peter
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Affiliation: Tianjin University
Country: China

Re: Basis Set Selection for Geometry Optimization

Post by taylor » 18 Jun 2020, 11:00

Hartree-Fock geometries are OK, but not great. You would probably get more reliable results from a DFT with a suitable functional. (I leave that choice to DFT experts: probably the most popular is B3LYP.)

All the basis sets of the popular 6-311G** general type are poor quality. They involve compromises like equal s and p exponents that are major constraints for such small basis sets. Much better basis sets are available, such as Frank Jensen's "polarization-consistent" sets like pc-1 and pc-2. These are in the Dalton basis set library.

I notice that you are coming to the forum with some rather general questions. If you have not done many quantum-chemical calculations before, it would be worth looking at some of the texts available, such as Jensen's book, or Cramer's. There are many others.

Best regards
Pete

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