3.3 Conclusion

In conclusion, the $\Pi$2 method is seen to perform similarly to the more expensive CCSD(T) method if criterion (8) is satisfied. This is equally valid for frozen and full core calculations. Especially for "difficult" molecules like N₂ the quality of the calculated spectroscopic constants can be improved significantly using the $\Pi$2 method instead of MP4. For DZ- and DZP-quality basis sets, usually good results are obtained whereas adding diffuse functions does not always yield an improvement for the quality of the perturbational methods.