Gałyńska Marta, Ásgeirsson Vilhjálmur, Jónsson Hannes, Bjornsson Ragnar
Science Institute and Faculty of Physical Sciences, VR-III, University of Iceland, 107 Reykjavík, Iceland.
Institute of Physics, Faculty of Physics, Astronomy and Informatics, Nicolaus Copernicus University, Grudziadzka 5, 87-100 Toruń, Poland.
J Phys Chem Lett. 2021 Feb 4;12(4):1250-1255. doi: 10.1021/acs.jpclett.0c03651. Epub 2021 Jan 26.
Recent Rydberg spectroscopy measurements of a diamine molecule, ,'-dimethylpiperazine (DMP), indicate the existence of a localized electronic state as well as a delocalized electronic state. This implies that the cation, DMP, can similarly have its positive charge either localized on one of the N atoms or delocalized over both. This interpretation of the experiments has, however, been questioned based on coupled cluster calculations. In this article, results of high-level multireference configuration interaction calculations are presented where a localized state of DMP is indeed found to be present with an energy barrier separating it from the delocalized state. The energy difference between the two states is in excellent agreement with the experimental estimate. The results presented here, therefore, support the original interpretation of the experiments and illustrate a rare shortcoming of CCSD(T), the "gold standard" of quantum chemistry. These results have implications for the development of density functionals, as most functionals fail to produce the localized state.
