| Clusters have attracted intensive attention because of the fundamental research and potential technology application.One of the most important things in cluster science is to determine the ground state structure of the cluster.The general theoretical methods are included three kinds:empirical,semi-empirical method and ab initio calculation or first-principles.By using single-parent genetic algorithm and a genetic algorithm,we investigate the stable isomer of(H2O)n clusters and(MgO)n clusters. Besides,some 'magic number' that they are chose from calculation of genetic algorithm are further optimized on the MP2/6-31G level.A single-parent genetic algorithm(SPGA)is developed from the traditional genetic algorithm to optimize molecular cluster structures.Two mutation operators are used to search for the global minimum on the potential surface.The first mutation operator is that some molecular freedoms of clusters are moved randomly with small steps.The second one is that cluster is rotated any angle about any axis,and is cut through the center of mass on XY plane.Following,the top part of cluster is rotated by random angle about Z axis,and is connected it to the bottom part of the other.We study the connection between real-valued Cartesian variables and molecular freedoms,which is feasible for optimizing any molecular cluster.Combined with the TIP3P potential energy,the structures of water clusters(H2O)n (n≤14)are studied using SPGA.The calculation shows that the planar structures are the lowest energy isomers for(H2O)n(n≤5)clusters.(H2O)6 is open-book structure. (H2O)8 is a cube with D2dsymmetry.Double-pentagonal-cyclic and double-hexagonal- cyclic are lowest energy structures for(H2O)10and(H2O)12. According to the secondary differences of energy,n=4,8,10,12 is specially stable.The stable structures of neutral(MgO)n clusters(n=2-20)are investigated using genetic algorithm based on Buckingham empirical potential.(MgO)n clusters are made up of quadrangular-ring and hexagonal-ring except(MgO)8,(MgO)13,(MgO)17,(MgO)20which are composed of octagonal-ring.According to the secondary differences of energy,n=3,6,9,12,15,18 is 'magic number'of(MgO))n clusters.We optimize the stable structures of(MgO)3,(MgO)6 and(MgO)9 cluster on the MP2/6-31G level in order to ensure further(MgO)n(n≤20)clusters 'magic number'. The calculation result shows that the lowest energy structures for(MgO)6,(MgO)9, (MgO)12are made up of hexagonal-ring.The metastable structures for(MgO)6 and (MgO)9 are cube.The metastable structure for(MgO)12is truncated octahedron.
|
PDF Full Text Request