5

What am I actually changing when I click 631G(d) or 631G(d,p), etc. when I choose a "basis" in restricted Hartree Fock calculations? What is the difference between using an alternative basis? Many thanks.

Geoff Hutchison
  • 27,740
  • 3
  • 79
  • 145
RedPen
  • 963
  • 10
  • 16

1 Answers1

8

The basis set is used to describe the molecular orbital wave functions.

I suggest reading through a number of links:

Community
  • 1
Geoff Hutchison
  • 27,740
  • 3
  • 79
  • 145
  • Really nice links thank you! I'm still a little confused as to what the "diffuse function" is though. – RedPen Nov 10 '14 at 19:53
  • In some cases, the electron distribution is far from the nucleus. A great example is $\ce{H-}$, where the extra electron is really diffuse. So a "diffuse function" might be a very large "s" or "p" function to describe the wave function far from the nucleus. – Geoff Hutchison Nov 10 '14 at 19:57
  • Diffuse functions are critical when dealing with anions (among other situations) since the extra electrons are usually.. well, diffuse. – Geoff Hutchison Nov 10 '14 at 19:58
  • ah thank you I see now, its just how spread out the electron density is then? It seems though that if I change the Basis set to whatever I want I will get a different answer for the energies, how do we know which to trust, let alone which programme? – RedPen Nov 10 '14 at 20:02
  • 3
    @RedPen If you're using Hartree Fock, you can use the variational principal, which clarifies that the lower the energy, the closer to the "true" correct total energy. In general, you use the largest basis set you have time to use, e.g. http://chemistry.stackexchange.com/questions/7920/suggest-methods-and-basis-sets-for-a-variety-of-systems?rq=1 – Geoff Hutchison Nov 10 '14 at 21:48
  • Thank you very much for your help, I'll read over these links over the next few days and see if anything comes up I don't quite understand :) – RedPen Nov 10 '14 at 23:34