# Computational determination of the diagram of molecular orbitals of a polyatomic chemical species

A diagram of molecular orbitals for $$\ce{H2O}$$ would be as follows:

The above diagram has been constructed qualitatively following the steps that I have been instructed in various classes on this bonding theory.

Nevertheless, I have seen that the Guassian software allows to visualize the molecular orbitals of a species. For example, for the case of water one obtains:

The computational results agree with the qualitative diagram data, since all electrons are paired and there are also five atomic orbitals.

What I do not know is how to obtain the energy of the orbitals of the oxygen atom and the two hydrogen atoms. On the other hand, I would not know how to construct the diagram either, that is, I would not know that molecular orbital 5, the LUMO, comes only from the 2p orbital of the oxygen.

For the oxygen-atom orbitals you would need to calculate a lone oxygen atom. For the $$\ce{H2}$$-orbitals you would need to calculate a lone $$\ce{H2}$$ molecule. You can also calculate with symmetry and Gaussian will assign the molecular orbitals to the irreducible representations. This helps to identify molecular orbitals. This information is given somewhere in the log file if the symmetry was properly determined by Gaussian during the calculation.
• +1 But please use MathJax! Chemical formulae can be rendered with $\ce{...}$. Jul 18, 2021 at 8:23