The third (PNOF3), fourth (PNOF4) and fifth (PNOF5) versions of the Piris natural orbital functional were used to characterize the beryllium dimer. The results obtained were compared to those gained afforded by CASSCF and CASPT2 as well as experimental data. The equilibrium distances (Re), dissociation energies (De), effective bond orders (EBOs), and rovibrational levels were calculated. PNOF3, PNOF4, and CASPT2 predicted a bonded Be2 molecule, while PNOF5 and CASSCF did not, which demonstrates the importance of the dynamical electron correlation. We observed that PNOF3 yields the most accurate equilibrium distances, while PNOF4 most accurately calculates the rovibrational levels. However, both of these functionals overestimate dissociation energies. Both PNOF3 and PNOF4 predict EBOs that agree with that obtained using CASPT2.
Natural orbital functional theory PNOF Dynamical correlation Beryllium dimer
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Financial support comes from Eusko Jaurlaritza (GIC 07/85 IT-330-07 and S-PC11UN003) and the Spanish Office for Scientific Research (CTQ2011-27374). The SGI/IZO–SGIker UPV/EHU is gratefully acknowledged for generous allocation of computational resources. JMM would like to thank the Spanish Ministry of Science and Innovation for funding through a Ramon y Cajal fellowship position (RYC 2008-03216).
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