Understanding 6-31G in Molecules: N2

[hiltac]

Hello !

When we say that we use a 6-31G for a molecule (ex: N₂), it is this number of gaussian (6+3+1) for both of the atoms or for each atom ?
Thank you

 

[DrDu]

For each atom, obviously, as the gaussians are used to approximate the atomic orbitals of each atom.

 

[hiltac]

And how can we explain to use a 6-31G basis set to study a molecule like H² ? 6 gaussian for the 1s and what about the splitting in 3+1 ?
Thank you again

 

[DrDu]

You should read the original publication
http://scitation.aip.org/content/aip/journal/jcp/54/2/10.1063/1.1674902
There you can see that for hydrogen, a basis of two orbitals is used, one being a contraction of 3 GTO and the other one just 1GTO (GTO = Gaussian type orbital).