Thermochemistry from ab initio calculations

Unlike MM or SEMOT software, ab initio software does not report enthalpies of formation. Instead, the user is responsible for choosing, performing, and combining appropriate computations. Assuming that the target quantity is the enthalpy of formation of molecule M, the following are the basic steps  

Write a balanced chemical reaction involving M, in which the enthalpies of formation of all other species are known reliably. 

Optimize the geometries of all molecules in the reaction using an adequate model. B3LYP/6-31G(d) is a common choice for production work. 

Compute the vibrational frequencies of all molecules using the same model as was used to obtain the geometries. Vibrational frequencies are frequently scaled by an empirical correction factor before use [108]. 

Compute the electronic energies of all molecules at their optimized geometries. Choose a model that balances your requirements for accuracy and computational economy. B3LYP/6-31G(d) is a common choice for production work. CCSD(T)/aug-cc-pVTZ or CCSD(T)/cc-pVTZ is a good choice for more precise work. 

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