Reexamination of low energy structures of Au₄̄ and Au ₄

Low energy isomers of Au₄ and Au₄ were reexamined using the hybrid density functional B3LYP method and the couple-cluster method with the aug-cc-pVDZ-PP and aug-cc-pVTZ-PP basis sets. For Au₄, the B3LYP method favors the zigzag isomer and the second order MollerPlesset perturbation (MP2) total energy calculation favors the D_2h rhombus isomer, whereas the couple-cluster singles and doubles with perturbative triples [CCSD(T)] level of theory favors the Y-shaped C_2v isomer. The pyramid isomer is much higher in energy and could be easily excluded. The Gibbs free energy correction based on harmonic approximation suggests that the zigzag isomer is lower in free energy than the D_2h rhombus isomer at 298.15 K. These results confirm that the Y-shaped C_2v isomer is the global minimum at both 0 K and room temperature and is thus the major isomer to account for the experimental photoelectron spectrum. The zigzag isomer is suggested, as a minor isomer, to account for the weak second peak at 3.40 eV in the experimental photoelectron spectrum. For neutral Au₄, the zigzag isomer is more stable than D_2h rhombus isomer at the B3LYP level and the D_2h rhombus isomer is the global minimum on basis of all post HartreeFock levels of theory.