Exploration of the potential energy surface of the ethanol hexamer.

The potential energy surfaces (PESs) of the neutral ethanol clusters is among the complex PESs of the neutral clusters. This is due to the fact that the ethanol monomer has three different isomers. In this work, we propose a systematic procedure to thoroughly explore the PES of the neutral ethanol hexamer that can be extended to other ethanol clusters. Thus, we started with a thorough exploration using the ABCluster code which uses the Lennard-Jones potential model. The resulting structures are further optimized at the APFD/6-31++g(d,p) level of theory {APFD refers to the initials of the first four authors in Austin et al. [J. Chem. Theory Comput. 8, 4989-5007 (2012)]}. Finally, 68 APFD structures have been fully re-optimized using the second order Møller-Plesset perturbation (MP2) method associated to the aug-cc-pVDZ basis set As a result, an isomer constituted of two trans ethanol monomers, two gauche+ ethanol monomers, and two gauche- ethanol monomers, is predicted to be the most stable structure using ABCluster. Full optimizations at the APFD/6-31++g(d,p) and MP2/aug-cc-pVDZ levels of theory confirm that this isomer is among the iso-energetic most stable structures of the ethanol hexamer. We found that most of the iso-energetic most stable structures are constituted of at least two different ethanol monomers. This highlights the importance of taking into account all the possible monomers in the exploration of the neutral ethanol clusters. In addition, we found that all the structures having their relative energies within 1.7 kcal mol-1 are cyclic structures. The results show that the most stable branched cyclic structures lies 1.7 kcal mol-1 above the most stable at the APFD/6-31++g(d,p) level of theory.