A comparison of various approaches in internally contracted multireference configuration interaction: the carbon dimer as a test case

Various variational spaces deriving from different contraction schemes of the first-order interacting space have been examined and their performances have been tested on the carbon dimer. The contraction schemes stemming from the strongly contracted and from the partially contracted approach of the n-electron valence state perturbation theory have been analysed along with the mixed approach of Werner and Knowles as well as with the totally uncontracted approach where the full dimensionality of the first-order interacting space is taken into account. The results are very similar for all the spaces considered, despite their widely varying dimensionality. A comparison with the second-order and the third-order results of NEVPT is also reported.