中性小团簇的结构与成键
Structure and Bonding in Small Neutral Alkali-Halide Clusters
摘要：
采用从头算微扰离子（aiPI）模型和限制性结构弛豫准则，研究了小中性碱卤原子簇（AX）n（n ≤ 10，A=Li、Na、K、Rb，X=F、Cl、Br、I）的结构和成键性质。发现岩盐和六方环状异构体之间存在竞争的趋势，并根据相对离子尺寸进行了讨论。本文的主要结论是：六方结构与岩盐结构的分界点r_C/r_A=0.5（r_C和r_A分别为阳离子和阴离子半径）。平衡几何总能量的经典静电部分足以解释这些趋势。在所研究的尺寸范围内的幻数是n= 4、6和9，并且这些幻数是通用的，因为它们发生在所有碱金属卤化物中并且不依赖于特定的基态几何形状。相反，这些数字允许形成紧凑的集群。本文考虑了（LiF）n（n=3-7）和（AX）_3团簇的全几何弛豫，并在少数情况下研究了库仑关联的影响。这两个效应保持了迄今为止所取得的一般结论。
英文摘要：
The structural and bonding properties of small neutral alkali-halide clusters (AX)n, with n less than or equal to 10, A=Li, Na, K, Rb and X=F, Cl, Br, I, are studied using the ab initio Perturbed Ion (aiPI) model and a restricted structural relaxation criterion. A trend of competition between rock-salt and hexagonal ring-like isomers is found and discussed in terms of the relative ionic sizes. The main conclusion is that an approximate value of r_C/r_A=0.5 (where r_C and r_A are the cationic and anionic radii) separates the hexagonal from the rock-salt structures. The classical electrostatic part of the total energy at the equilibrium geometry is enough to explain these trends. The magic numbers in the size range studied are n= 4, 6 and 9, and these are universal since they occur for all alkali-halides and do not depend on the specific ground state geometry. Instead those numbers allow for the formation of compact clusters. Full geometrical relaxations are considered for (LiF)n (n=3-7) and (AX)_3 clusters, and the effect of Coulomb correlation is studied in a few selected cases. These two effects preserve the general conclusions achieved thus far.
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