Heats of formation from G2, G2(MP2), and G2(MP2,SVP) total energies

The translation of G2, G2(MP2), and G2(MP2,SVP) total energies to heats of formation via atomization and formation reaction procedures is evaluated. While both approaches give similar results, the former is shown to perform somewhat better, especially for organic molecules. The recommended general procedure for obtaining heats of formation at 0 K from G2-type total energies is to use G2-type atomization energies in conjunction with experimental gas-phase heats of formation for the constituent atoms, The corrections required to give heats of formation at finite temperatures can be obtained using scaled theoretically derived vibrational frequencies for the species under consideration together with temperature correction terms for the constituent elements based on experimental data. While the harmonic oscillator model is a good approximation for most vibrational modes, internal rotations with relatively low frequencies are more accurately treated within the free rotor approximation.