Concerning the Question: Does Quantum Mechanics Predict Molecular Structure? An Examination of the Energetics

There is a controversy concerning the role of the Born-Oppenheimer approximation (BOA) in molecular orbital calculations. Some philosophers argue that because the BOA clamps the position of the nuclei in space the BOA fixes the outcome of the calculation, thus calling into question the calculation’s legitimacy. As a contribution to the debate, we perform a thermochemical analysis to determine the absolute energies of the quantum Hamiltonian operator, Ĥψ , in kJmol ^–1 , with respect to the constitutional isomers ethanol and dimethyl ether and their constituent particles. Our results show that the difference between the quantum Hamiltonian enthalpies, the Δ _QH H , is tiny, − 407,023 kJmol ^– 1 vs . − 407,116 kJmol ^–1 , a 0.02% difference, which is far smaller than the ~ 1% chemical error deemed acceptable/tolerable in many chemical procedures and calculations.