Gold standard coupled-cluster study of acetylene pentamers and hexamers via molecular tailoring approach

Due to high scaling order of MP2 and CCSD(T) methods, it is either difficult or at times even impossible to treat even moderately sized molecular systems with elaborate basis sets such as aug-cc-pVXZ (X = D, T, Q). In the present work, several structures of acetylene pentamers and hexamers are explored at MP2 and CCSD(T) levels of theory as prototypical examples of clusters bound by CH center dot center dot center dot pi interactions. To enable this investigation, fragment-based method Molecular Tailoring Approach (MTA) is employed. It is shown that these acetylene assemblies can be treated with substantial reduction in computational resources and time, yet retaining a sub-millihartree accuracy in the energy. Further, using standard extrapolation methodologies, stabilization energies at the complete basis set limit of the acetylene clusters under consideration are determined at MP2 and CCSD(T) levels of theory. To test out the feasibility of treating a large cluster at MP2 level, a demonstrative calculation on a dodecamer of acetylene is reported.