Hi all,

I'm a student currently struggling to understand Roothaans SCF method for open shell electronic systems. I'd be really thankful if you could help me a bit with that!

What exactly is the basic idea? From what I get, symmetry restricted orbitals (?) are used to describe the system, and instead of explicitly describing the (naturally unsymmetric) open shell orbitals, the average of all possible open shell orbital states is used. Is that right?

But, what does he need the symmetry for? Is it to prevent spin contamination? Or is it essential for the Hartee-Fock like scheme he's using?

Thanks for any help!
Sera