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:
- Wikipedia
- The Gaussian manual - Basis Sets - a nice description of the different basis sets, if a bit technical at times.
- Naming of quantum chemistry basis sets
- GAMESS Crash Course - applies to Gaussian or other packages
answered Nov 10, 2014 at 19:42
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$\begingroup$ Really nice links thank you! I'm still a little confused as to what the "diffuse function" is though. $\endgroup$– RedPenNov 10, 2014 at 19:53
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$\begingroup$ 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. $\endgroup$ Nov 10, 2014 at 19:57
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$\begingroup$ Diffuse functions are critical when dealing with anions (among other situations) since the extra electrons are usually.. well, diffuse. $\endgroup$ Nov 10, 2014 at 19:58
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$\begingroup$ 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? $\endgroup$– RedPenNov 10, 2014 at 20:02
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3$\begingroup$ @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/… $\endgroup$ Nov 10, 2014 at 21:48