Single Point Energy in Gaussian software I used Gaussian software to determine the single point energy (SPE) of 3 molecules namely ethane, ethene and phenol.

The SPE for the molecules are -79.83, -78.59 and for phenol it is quite large at -307. However, I am not able to explain the trend.

I am thinking does SPE have anything to do with the molecular weight of the molecule for example, larger mass = more negative SPE?? I cant think of a better explanation. Can someone please help me out?

thank you
The energies you're seeing are those resulting from solving for the eigenfunctions of the Coulomb Hamiltonian under the Born-Oppenheimer approximation, where the Hamiltonian can be broken down into the following 5 components:











The most important thing to take away from these expressions is that they all contain summations over nuclei, electrons, or both; combined with the presence of the set of nuclear charges $\{Z\}$, there is a non-linear dependence of the molecular energy on more than just the nuclear masses.

This is one reason why it usually isn't instructive to compare absolute energies of unrelated molecules.