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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.

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

I suggest reading through a number of links:

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  • $\begingroup$ Really nice links thank you! I'm still a little confused as to what the "diffuse function" is though. $\endgroup$ – RedPen Nov 10 '14 at 19:53
  • $\begingroup$ 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. $\endgroup$ – Geoff Hutchison Nov 10 '14 at 19:57
  • $\begingroup$ Diffuse functions are critical when dealing with anions (among other situations) since the extra electrons are usually.. well, diffuse. $\endgroup$ – Geoff Hutchison Nov 10 '14 at 19:58
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    $\begingroup$ @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. chemistry.stackexchange.com/questions/7920/… $\endgroup$ – Geoff Hutchison Nov 10 '14 at 21:48
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    $\begingroup$ Feel free to post other questions on Chemistry.SE if you'd like some advice on picking methods and/or basis sets. $\endgroup$ – Geoff Hutchison Nov 10 '14 at 21:48

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