Proton or hydrogen transfer in model hydrogen bonded systems

The population analysis of the hydrogen bond atoms was analyzed within the different basis sets for model molecular systems for the ground and low-lying excited electronic states. The systems were considered as multireference ones and at the CAS SCF level the Mulliken, Lowdin and Hirshfeld methods were used in our investigations. It has been shown that in some cases the proton is transferred. However, there are systems where in the excited electronic states rather hydrogen atom is responsible for the tautomeric interconversion. The density functional method has also been applied for cases known to require the multireference description. It should be noted that the calculations including correlation energy within the DFT formalism, in some cases, do not properly account for the multireference character of the wavefunction.