How to choose a basis set for ab-initio evaluations ?

Question

How do I pick a basis set for an ab-initio Hartree-Fock evaluation ? In other words, what are the important characteristics of a basis set so that a proper choice can be made ?

Answer 1

This is a pretty big topic, but the basic important qualities of an atom-centered basis set are:

• The number of separate, unlinked functions it uses at the valence level (the so-called *zeta* count), which allow the modeling of that many different electronic environments, practically speaking.
• Additional higher angular momentum so-called *polarisation* functions to allow for more angular complexity.
• Whether it has diffuse functions to model more spacially extensive orbitals - so called *augmented* functions.

Generally, though, for Hartree-Fock:

• Double-zeta for very quick, very rough structural parameters.
• Double-zeta + polarisation for slightly less rough structural information, large systems (100+ atoms), or simple bonding environments like alkanes.
• Triple-zeta for better accuracy and useful energies.
• Augmented functions if you're working with anions.

Going to a better basis set than this for Hartree-Fock probably won't gain you much. In any case, unless you're moving on to a more complex method, you should probably be using density-functional theory (DFT) instead.

For more information on basis sets including how different basis sets are labelled, I recommend chapter 5.2 in Jensen's "Introduction to Computational Chemistry".

Answer 2

Here are some references that might be useful:

• http://pubs.acs.org/doi/abs/10.1021/cr00074a002

• http://www.sciencedirect.com/science/article/pii/0167797785900036

• http://books.google.com/books?id=Gt4pnp-UFhUC&lpg=PA853&vq=helgaker&pg=PA725#v=snippet&q=helgaker&f=false (if you see nothing, click next then prev). See http://www.worldscientific.com/doi/abs/10.1142/9789812832115_0001 to find the analog version in your library.

• http://vergil.chemistry.gatech.edu/courses/chem6485/pdf/basis-sets.pdf