# Gaussian Calculations - What is the Basis?

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.

• 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