Structures and properties of neutral gallium clusters: A theoretical investigation

A systematic and unbiased structure search based on a genetic algorithm in combination with density functional theory (DFT) procedures has been carried out to locate low-energy isomers of Ga-n up to n = 25. For the smaller clusters up to n = 8 results are checked by coupled cluster singles and doubles with perturbative triples corrections (CCSD(T)) employing a quadruple zeta type basis set. The CCSD(T) calculations confirm a (3)Pi(u) ground state for the dimer. Ga-3 has a doublet ground state 0.2 eV below two quartet states, whereas two isoenergetic triplet states are predicted for Ga-4 with D-4h and a rhombus structure (D-2h). Three low-lying isomers with doublet electronic states are found for Ga-5: a W-structure (C-2v), a planar envelope (C-s) at 0.015 eV, and a non-planar envelope (C-1) 0.086 eV above the ground state. A triplet state for a trigonal prism (D-3h) and a singlet for an open prism (C-2v) are computed with virtually identical energy for Ga-6. The global minimum for Ga-7 is a capped trigonal prism (C-s) and that for Ga-8 a distorted cube in D-2h. DFT provides a fair agreement with CCSD(T), deviations in dissociation energies are up to 0.2 eV for n <= 8. The structures for Ga-n are mostly irregular for n >= 9, those for Ga-12 to Ga-17 can be derived from the truncated decahedron with D-5h symmetry though highly distorted by Jahn-Teller effects, for example. For Ga-18 to Ga-23 we find stacks of five- and six-membered rings as global minima, e.g., 5-1-5-1-6 for Ga-18. Ga-24 and Ga-25 consist of layers with packing sequence ABCBA similar to those found for clusters of aluminum. The most important feature of computed cohesive energies is a rapid increase with n: for Ga-25 it reaches 2.46 eV, the experimental bulk value is 2.84 eV. Particularly stable clusters for Ga-n are seen for n = 7, 14, and 20. (C) 2011 American Institute of Physics. [doi: 10.1063/1.3615501]