Looking for info about cluster orbitals

[phz]

Looking for info about "cluster orbitals"

Homework Statement

Why is the orbital 1d present in eg an Al₂-cluster but not in a H-atom?

Homework Equations

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The Attempt at a Solution

This is a question for an assignment. The answer is probably not meant to be very elaborate, "Because it is specific for a cluster which differs from a single atom" probably suffices (the question isn't formulated exactly like I wrote before, it is about interpreting a diagram. My own wording of the question might even have made it faulty for all I know), but there is no info about cluster orbitals in my course literature ("Quantum mechanics" by Robinett), and all the references I can find on Google are to advanced it seems, I can't find a basic explanation of what it is. I'm just curious :-)

Before (for the atoms we've studied) the rule has always been that if n=1 as in this case, the only allowed values for l is { 0, 1, ..., n-1 }, but in a 1d-construction it seems that n=1 and l=2. Confusing for me.

 

[BigJDubs]

Cluster orbitals are specific to molecules and ions made up of multiple atoms. The 1d orbital in an Al2-cluster is a molecular orbital that is created by the overlap of the 1s orbitals from both atoms in the cluster. This orbital is not present in a single H-atom because it is specific to the geometry of the Al2-cluster.