Yield improvement factor, definition

To use the verbal definition, interaction means that the effects of the factors on the response are not independent that is, the effect of one factor on the response is not the same at each level of an other factor. In the example given, increasing the catalyst concentration gave improved yields at 20 mint and 40 min reactions, the opposite at 60 min reaction. Or, conversely, with 0 1% catalyst, yield increased with time up to 60 min with 0 2% and 0 3% catalyst, yield went through a maximum at 40 min, and then decreased. 

However, the Hartree-Fock method does not treat (by definition) what is called electron correlation, which physically represents instantaneous interactions between individual electrons rather than the average as done in HF. Electron correlation is a key factor to find important properties of chemical interest such as binding energies. One way to solve this problem is to include additional determinants to the wavefunction. Unfortunately, improving the single-determinant wavefimction, by adding additional determinants, yields methods that are extremely expensive in computational resources. 

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