Ab initio study of many-body decomposition of the interaction energy in small beryllium clusters

We present the decomposition of interaction energies in beryllium clusters, Be m = 3 - 6 , into a series of nonidentical m-body nonadditivities up to m = 6 . We analyse the role of the electron correlation and compare nonadditivities at HF, CASSCF, MP2, CASPT2 and CCSD levels of theory with the reference aug-cc-pVTZ CCSD(T) results. The dominant nonadditivity terms are the three- and four-body. Five and six-body terms are relatively large in Be 5 - 6 clusters, but the total interaction energy of Be 6 is affected only marginally, due to their mutual cancellation.