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 heat s of formation via atomization and formation reaction procedures is ev aluated. While both approaches give similar results, the former is sho wn to perform somewhat better, especially for organic molecules. The r ecommended general procedure for obtaining heats of formation at 0 K f rom G2-type total energies is to use G2-type atomization energies in c onjunction with experimental gas-phase heats of formation for the cons tituent 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 o n experimental data. While the harmonic oscillator model is a good app roximation for most vibrational modes, internal rotations with relativ ely low frequencies are more accurately treated within the free rotor approximation.