Electrostatic guidelines and molecular tailoring for density functional investigation of structures and energetics of (Li)n clusters

Probing the structural evolution and electronic properties of divalent metal Be2Mgn clusters from small to medium-size

Bimetallic clusters have aroused increased attention because of the ability to tune their own properties by changing size, shape, and doping. In present work, a structural search of the global minimum for divalent bimetal Be₂Mg_n (n = 1-20) clusters are performed by utilizing CALYPSO structural searching method with subsequent DFT optimization. We investigate the evolution of geometries, electronic properties, and nature of bonding from small to medium-sized clusters. It is found that the structural transition from hollow 3D structures to filled cage-like frameworks emerges at n = 10 for Be₂Mg_n clusters, which is obviously earlier than that of Mg_n clusters. The Be atoms prefer the surface sites in small cluster size, then one Be atom tend to embed itself inside the magnesium motif. At the number of Mg larger than eighteen, two Be atoms have been completely encapsulated by caged magnesium frameworks. In all Be₂Mg_n clusters, the partial charge transfer from Mg to Be takes place. An increase in the occupations of the Be-2p and Mg-3p orbitals reveals the increasing metallic behavior of Be₂Mg_n clusters. The analysis of stability shows that the cluster stability can be enhanced by Be atoms doping and the Be₂Mg₈ cluster possesses robust stability across the cluster size range of n = 1-20. There is s-p hybridization between the Be and Mg atoms leading to stronger Be-Mg bonds in Be₂Mg₈ cluster. This finding is supported by the multi-center bonds and Mayer bond order analysis.

Electrostatics-Assisted Building-Up Procedure for Capturing Energy Minima of Metal Clusters: Test Case of Agn Clusters

Global geometry optimization of metal clusters is an important problem in nanophysics. The starting geometries of the clusters generated with empirical or other model potentials are generally optimized further by density functional theory (DFT)-based energy minimization. For this purpose, several algorithms such as simulated annealing, genetic algorithms, basin hopping, etc. are used. Our building-up procedure generates putative lower-energy structures of metal (M) clusters, M_n+1, M_n+2, etc., by anchoring one or more metal atoms in the vicinity of the minima of the molecular electrostatic potential (MESP) of M_n. Here, we report an application of this method to Ag_n clusters, for 5 ≤ n ≤ 20, followed up by DFT-based geometry optimization, generating several lower-energy structures than those reported in the literature. New low-energy isomers are obtained by applying the same procedure to the test case of mixed-metal clusters, Ni_nAg_m, for n + m = 4 and 5. In conclusion, our MESP-based building-up procedure offers a new general methodology for generating lower-energy geometries of metal clusters.

Materials discovery via CALYPSO methodology

The structure prediction at the atomic level is emerging as a state-of-the-art approach to accelerate the functionality-driven discovery of materials. By combining the global swarm optimization algorithm with first-principles thermodynamic calculations, it exploits the power of current supercomputer architectures to robustly predict the ground state and metastable structures of materials with only the given knowledge of chemical composition. In this Review, we provide an overview of the basic theory and main features of our as-developed CALYPSO structure prediction method, as well as its versatile applications to design of a broad range of materials including those of three-dimensional bulks, two-dimensional reconstructed surfaces and layers, and isolated clusters/nanoparticles or molecules with a variety of functional properties. The current challenges faced by structure prediction for materials discovery and future developments of CALYPSO to overcome them are also discussed.

Molecular cluster building algorithm: electrostatic guidelines and molecular tailoring approach

Nano-sized clusters of various materials are recent experimental targets, since they exhibit size-dependent physico-chemical properties. A vast amount of literature is available on the study of molecular clusters but general methods for systematic evolution of their growth are rather scarce. The present work reports a molecular cluster building algorithm based on the electrostatic guidelines, followed by ab initio investigations, enabled by the application of molecular tailoring approach. Applications of the algorithm for generating geometries and interaction energies of large molecular clusters of zinc sulfide, benzene, and water are presented.

An appraisal of Poincaré-Hopf relation and application to topography of molecular electrostatic potentials

The Poincaré-Hopf relation is studied for molecular electrostatic potentials (MESPs) of a few test systems such as cyclopropane, cyclobutane, pyridine, and benzene. Appropriate spheres centered at various points, including the center of mass of the system under study, are constructed and the MESP gradient is evaluated on the corresponding spherical grid. The change in directional nature of MESP gradient on the surface of these spheres gives indication of the critical points of the function. This is used for developing a method for locating the critical points of MESP. The strategy also enables a general definition of the Euler characteristic (EC) of the molecule, independent of any region or space. Further, the effect of basis set and level of theory on the EC is discussed.