Nondynamical correlation effects in multidimensional potential energy curves of prototypical hydrocarbons

In this work, we demonstrate that simultaneously stretching all bonds in a molecule by small increments provides a systematic approach to gradually increasing its multireference character. We show this for four hydrocarbons—methane, ethane, ethylene, and acetylene—each representing progressively more complex electronic structures, i.e., with no C–C bond, and with a single, double, and triple C–C bond, respectively. We show that the 𝒯1, D1, and %TAE[(T)] multireference diagnostics increase as all the bonds are stretched simultaneously. Additionally, we show that post-CCSD(T) correlation components of the total atomization energy (TAE) up to CCSDTQ systematically increase as the bonds are stretched. The extent of the increase in the multireference character correlates with the complexity of the electronic structure in the order methane  ethane  ethylene  acetylene. We proceed to examine the ability of density functional theory (DFT) methods to cope with the increasing degree of nondynamical correlation along the multidimensional potential energy curves. We find that the conventional DFT methods B97-D, TPSS, and TPSSh and double-hybrid DFT methods are more robust toward multireference effects. Finally, we propose that the CCSDTQ/CBS multidimensional potential energy curves generated in this study could serve as benchmarks for assessing the performance of DFT methods for strong correlation effects.