Efficient moves for global geometry optimization methods and their application to binary systems

We locate the putative global minimum structures of Na x Cs 55−x and Li x Cs 55−x nanoalloys through combined empirical potential and density functional theory calculations, and compare them to the structures of 55-atom Li-Na and Na-K nanoalloys obtained in a recent paper [A. Aguado and J. M. López, J. Chem. Phys. 133, 094302 (2010)]. Alkali nanoalloys are representative of isovalent metallic mixtures with a strong tendency towards core-shell segregation, and span a wide range of size mismatches. By comparing the four systems, we analyse how the size mismatch and composition affect the structures and relative stabilities of these mixtures, and identify useful generic trends. The Na-K system is found to possess a nearly optimal size mismatch for the formation of poly-icosahedral (pIh) structures with little strain. In systems with a larger size mismatch (Na-Cs and Li-Cs), frustration of the pIh packing induces for some compositions a reconstruction of the core, which adopts instead a decahedral packing. When the size mismatch is smaller than optimal (Li-Na), frustration leads to a partial amorphization of the structures. The excess energies are negative for all systems except for a few compositions, demonstrating that the four mixtures are reactive. Moreover, we find that Li-Cs and Li-Na mixtures are more reactive (i.e., they have more negative excess energies) than Na-K and Na-Cs mixtures, so the stability trends when comparing the different materials are exactly opposite to the trends observed in the bulk limit: the strongly non-reactive Li-alkali bulk mixtures become the most reactive ones at the nanoscale. For each material, we identify the magic composition x m which minimizes the excess energy. x m is found to increase with the size mismatch due to steric crowding effects, and for Li x Cs 55−x the most stable cluster has almost equiatomic composition. We advance a simple geometric packing rule that suffices to systematize all the observed trends in systems with large size mismatch (Na-K, Na-Cs, and Li-Cs). As the size mismatch is reduced, however, electron shell effects become more and more important and contribute significantly to the stability of the Li-Na system.

Section: Methodssupporting

confidence: 72%

Identifying structural and energetic trends in isovalent core-shell nanoalloys as a function of composition and size mismatch

Aguado

López²

2011

“…39 Furthermore, low energy barriers are generally connected to low frequency eigenmodes of local minima. 40 These properties can be readily extended to the configurational enthalpy. Therefore, the probability of finding low enthalpy configurations can be expected to increase when the direction of the initial velocity vector of a MD run points toward a direction with low curvature.…”

Section: B Softening and Optimizing Cell Parametersmentioning

confidence: 99%

Crystal structure prediction using the minima hopping method

Amsler

Goedecker²

2010

Self Cite

292

238

A structure prediction method is presented based on the Minima Hopping method. Optimized moves on the configurational enthalpy surface are performed to escape local minima using variable cell shape molecular dynamics by aligning the initial atomic and cell velocities to low curvature directions of the current minimum. The method is applied to both silicon crystals and binary Lennard-Jones mixtures and the results are compared to previous investigations. It is shown that a high success rate is achieved and a reliable prediction of unknown ground state structures is possible.

“…The same caveat applies to approximate swap gains. 10 A detailed analysis of this issue and more benchmark calculations are in progress.…”

Section: Klmentioning

confidence: 99%

“…However, the performance of this strategy is expected to deteriorate as the correlation between E * ij and E ij gets worse (e.g., lattice mismatched systems). 10 We now define the flip gain (analogous to the "cell gain" of Fiduccia and Mattheyses 11 ), quantified by E i , the energy change due to a single particle i switching type (i.e., "flipping"). There are only N possibilities for a flip at any given instance: fewer than the number of unlike pairs, unless min (N A , N B ) ≤ 1.…”

Section: While C Is Falsementioning

confidence: 99%

Communication: A new paradigm for structure prediction in multicomponent systems

Schebarchov¹,

Wales²

2013

We analyse the combinatorial aspect of global optimisation for multicomponent systems, which involves searching for the optimal chemical ordering by permuting particles corresponding to different species. The overall composition is presumed fixed, and the geometry is relaxed after each permutation in order to relieve local strain. From ideas used to solve graph partitioning problems we devise a deterministic search scheme that outperforms (by orders of magnitude) conventional and self-guided basin-hopping global optimisation. The search is guided by the energy gain from either swapping particles i and j (ΔEij) or changing the identity of particles i (ΔEi). These quantities are derived from the underlying (arbitrary) energy function, hence not constituting external bias, and for site-separable force fields each ΔEi can be approximated simply and efficiently. In our self-guided variant of basin-hopping, particles are weighted by an approximate ΔEi when randomly selected for an exchange, yielding a significant improvement for segregated multicomponent systems with modest particle size mismatch.