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In the present work we investigate the adequacy of broken-symmetry unrestricted density functional theory for constructing the potential energy curve of nickel dimer and nickel hydride, as a model for larger bare and hydrogenated nickel cluster calculations. We use three hybrid functionals: the popular B3LYP, Becke’s newest optimized functional Becke98, and the simple FSLYP functional (50% Hartree – Fock and 50% Slater exchange and LYP gradient-corrected correlation functional) with two basis sets: all-electron (AE) Wachters+ f basis set and Stuttgart RSC effective core potential (ECP) and basis set. We find that, overall, the best agreement with experiment, comparable to that of the high-level CASPT2, is obtained with B3LYP/AE, closely followed by Becke98/AE and Becke98/ECP. FSLYP/AE and B3LYP/ECP give slightly worse agreement with experiment, and FSLYP/ECP is the only method among the ones we studied that gives an unacceptably large error, underestimating the dissociation energy of Ni2 by 28%, and being in the largest disagreement with the experiment and the other theoretical predictions. We also find that for Ni2, the spin projection for the broken-symmetry unrestricted singlet states changes the ordering of the states, but the splittings are less than 10 meV. All our calculations predict a δδ-hole ground state for Ni2 and δ-hole ground state for NiH. Upon spin projection of the singlet state of Ni2 , almost all of our calculations: Becke98 and FSLYP both AE and ECP and B3LYP/AE predict 1 ( d A x2-y2d B x2-y2 d B ) or 1 ( d A xy d Bxy) ground state, which is a mixture of 1 Σg+ and 1Γg. B3LYP/ECP predicts a 3(d Ax 2 - y 2 dBxy )(mixture of 3Σ g - and 3Γu ) ground state virtually degenerate with the 1 (d Ax2 - y 2 dBx 2 - y 2 )/ 1 (dAxy dBxy ) state. The doublet δ-hole ground state of NiH predicted by all our calculations is in agreement with the experimentally predicted 2 ^ ground state. For Ni2 , all our results are consistent with the experimentally predicted 0g+ (a mixture of l Σg+and 3Σ g-) or 0u-(a mixture of l Σu-and Σu +).

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© 2004 American Institute of Physics.