Cerowski V, Rao B K, Khanna S N, Jena P, Ishii Soh, Ohno Kaoru, Kawazoe Yoshiyuki
Physics Department, Virginia Commonwealth University, Richmond, Virginia 23284-2000, USA.
J Chem Phys. 2005 Aug 15;123(7):074329. doi: 10.1063/1.2001655.
Using a modified symbiotic genetic algorithm approach and many-body interatomic potential derived from first principles, we have calculated equilibrium geometries and binding energies of the ground-state and low-lying isomers of Be clusters containing up to 41 atoms. Molecular-dynamics study was also carried out to study the frequency of occurrence of the various geometrical isomers as these clusters are annealed during the simulation process. For a selected group of these clusters, higher-energy isomers were more often found than their ground-state structures due to large catchment areas. The accuracy of the above ground-state geometries and their corresponding binding energies were verified by carrying out separate ab initio calculations based on molecular-orbital approach and density-functional theory with generalized gradient approximation for exchange and correlation. The atomic orbitals were represented by a Gaussian 6-311G** basis, and the geometry optimization was carried out using the GAUSSIAN 98 code without any symmetry constraint. While the ground-state geometries and their corresponding binding energies obtained from ab initio calculations do not differ much from those obtained using the molecular-dynamics approach, the relative stability of the clusters and the energy gap between the highest occupied and the lowest unoccupied molecular orbitals show significant differences. The energy gaps, calculated using the density-functional theory, show distinct shell closure effects, namely, sharp drops in their values for Be clusters containing 2, 8, 20, 34, and 40 electrons. While these features may suggest that small Be clusters behave free-electron-like and, hence, are metallic, the evolution of the structure, binding energies, coordination numbers, and nearest-neighbor distances do not show any sign of convergence towards the bulk value. We also conclude that molecular-dynamics simulation based on many-body interatomic potentials may not always give the correct picture of the evolution of the structure and energetics of clusters although they may serve as a useful tool for obtaining starting geometries by efficiently searching a large part of the phase space.
我们采用改进的共生遗传算法方法以及基于第一性原理得出的多体原子间势，计算了含多达41个原子的铍团簇基态和低激发态异构体的平衡几何结构及结合能。还进行了分子动力学研究，以考察在模拟过程中这些团簇退火时各种几何异构体的出现频率。对于这些团簇中的一组选定团簇，由于较大的捕获面积，发现高能异构体比其基态结构更常见。通过基于分子轨道方法和采用广义梯度近似进行交换和关联的密度泛函理论进行单独的从头算计算，验证了上述基态几何结构及其相应结合能的准确性。原子轨道用高斯6 - 311G**基组表示，几何结构优化使用GAUSSIAN 98代码，无任何对称性约束。虽然从头算计算得到的基态几何结构及其相应结合能与使用分子动力学方法得到的结果相差不大，但团簇的相对稳定性以及最高占据分子轨道和最低未占据分子轨道之间的能隙显示出显著差异。使用密度泛函理论计算的能隙显示出明显的壳层闭合效应，即对于含2、8、20、34和40个电子的铍团簇，其值会急剧下降。虽然这些特征可能表明小的铍团簇表现得类似自由电子，因此是金属性的，但结构、结合能、配位数和最近邻距离的演化并未显示出向体相值收敛的任何迹象。我们还得出结论，基于多体原子间势的分子动力学模拟可能并不总是能正确描绘团簇结构和能量学的演化，尽管它们可作为通过有效搜索相空间的很大一部分来获得初始几何结构的有用工具。
