Coupled-Cluster study of 'no-pair' bonding in the tetrahedral Cu4 cluster

► High level ab initio study of no pair bond in copper. ► CCSD(T) benchmark confirming the bonding nature of the state. ► Analysis of the basis set effect. ► Analysis of the core correlation effect. ► Analysis of the BSSE effect. ► Comparison with alkali metal no pair bond clusters.