New answers tagged molecular-orbital-theory
8
When thinking about orbital energy levels, it is not a good idea to think of them as fixed energy 'levels' that can be filled with electrons. This kind of picture is often found in introductory textbooks, but it is wrong. For example, it is known that even for the Hartree-Fock theory, the energy of a molecule is not a sum of the orbital energies. In this ...
1
The conclusion is left to the reader. I believe that there is no useful value in involving d-orbitals in the description of "hypervalent" molecules while remaining on the level of the octet rule.
A typical example for when the octet rule seems to fail is sulfur hexafluoride.
The naive approach
Let's do a quantum chemistry calculation. Using a PBE-D3/def2-...
3
You probably already understand that the oxygen 2s orbital is lower in energy than the hydrogen 1s to begin with. For the unconvinced reader, consider the fact that the ionisation energies of oxygen and hydrogen (1314 and 1312 kJ/mol respectively) are quite similar, which implies that O2p and H1s have (somewhat) similar energies.
The question is then about ...
4
An internal or interior eigenvalue problem seeks a solution somewhere in the middle of the spectrum of eigenvalues as opposed to trying to find the smallest or largest eigenvalue. Its generally trickier to solve for a particular eigenvalue than it is to solve for the smallest/largest one. The HOMO and LUMO are necessarily somewhere in the middle of the ...
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