Ground state structures, electronic and optical properties of medium-sized Na+_n (n = 9, 15, 21, 26, 31, 36, 41, 50 and 59) clusters from ab initio genetic algorithm

摘要

Employing genetic algorithm incorporated with density functional theory calculations we determined the lowest-energy structures of cationic Na ~+ _n clusters (n = 9, 15, 21, 26, 31, 36, 41, 50 and 59). We revealed a transition of growth pattern from "polyicosahedral" sequence to the Mackay icosahedral motif at around n = 40. Based on the ground-state structures the size dependent electronic properties of Na ~+ _n clusters including the binding energies, HOMO-LUMO gaps, electron density of states and photoabsorption spectra were discussed. As cluster size increases, the HOMO-LUMO gap of Na~+ _n cluster gradually reduces and converges to metallic behavior of bulk crystal rapidly. The photoabsorption spectra of Na~+ _n clusters from our calculations agree with experimental data rather well, confirming the reliability of our theoretical approaches.