Combined or scripted methods

G3(MP2) calculations are especially well adapted to hydrocarbons, having mean absolute deviations from experiment of 1.0 kcal mol 1 for 

The geometry is at the MP2(full)/6-31G(d) level, and the zero-point energy is from the scaled (0.8929) HF/6-31G(d) harmonic oscillator frequencies. Other calculations are carried out on valence electrons only (the frozen core approximation). A typical script is 

HF geometry ZPE and vib freq MP2(full) geo QCI energy GTMP2Large energy 

Upon carrying out the thermal conversion from 0 to 298 K, we obtain the desired enthalpy at 298 K 

The spin-orbit coupling energy has been ignored because of cancellation. 

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