Pushing the mass limit for intact launch and photoionization of large neutral biopolymers

Since their first discovery by Louis Dunoyer and Otto Stern, molecular beams have conquered research and technology. However, it has remained an outstanding challenge to isolate and photoionize beams of massive neutral polypeptides. Here we show that femtosecond desorption from a matrix-free sample in high vacuum can produce biomolecular beams at least 25 times more efficiently than nanosecond techniques. While it has also been difficult to photoionize large biomolecules, we find that tailored structures with an abundant exposure of tryptophan residues at their surface can be ionized by vacuum ultraviolet light. The combination of these desorption and ionization techniques allows us to observe molecular beams of neutral polypeptides with a mass exceeding 20,000 amu. They are composed of 50 amino acids – 25 tryptophan and 25 lysine residues – and 26 fluorinated alkyl chains. The tools presented here offer a basis for the preparation, control and detection of polypeptide beams

Revised Basin-Hopping Monte Carlo Algorithm for Structure Optimization of Clusters and Nanoparticles

Suggestions for improving the Basin-Hopping Monte Carlo (BHMC) algorithm for unbiased global optimization of clusters and nanoparticles are presented. The traditional basin-hopping exploration scheme with Monte Carlo sampling is improved by bringing together novel strategies and techniques employed in different global optimization methods, however, with the care of keeping the underlying algorithm of BHMC unchanged. The improvements include a total of eleven local and nonlocal trial operators tailored for clusters and nanoparticles that allow an efficient exploration of the potential energy surface, two different strategies (static and dynamic) of operator selection, and a filter operator to handle unphysical solutions. In order to assess the efficiency of our strategies, we applied our implementation to several classes of systems, including Lennard-Jones and Sutton-Chen clusters with up to 147 and 148 atoms, respectively, a set of Lennard-Jones nanoparticles with sizes ranging from 200 to 1500 atoms, binary Lennard-Jones clusters with up to 100 atoms, (AgPd)₅₅ alloy clusters described by the Sutton-Chen potential, and aluminum clusters with up to 30 atoms described within the density functional theory framework. Using unbiased global search our implementation was able to reproduce successfully the great majority of all published results for the systems considered and in many cases with more efficiency than the standard BHMC. We were also able to locate previously unknown global minimum structures for some of the systems considered. This revised BHMC method is a valuable tool for aiding theoretical investigations leading to a better understanding of atomic structures of clusters and nanoparticles

The Role of Charge States in the Atomic Structure of Cu_<i>n</i> and Pt_<i>n</i> (<i>n</i> = 2–14 atoms) Clusters: A DFT Investigation

In general, because of the high computational demand, most theoretical studies addressing cationic and anionic clusters assume structural relaxation from the ground state neutral geometries. Such approach has its limits as some clusters could undergo a drastic structural deformation upon gaining or losing one electron. By engaging symmetry-unrestricted density functional calculations with an extensive search among various structures for each size and state of charge, we addressed the investigation of the technologically relevant Cu_<i>n</i> and Pt_<i>n</i> clusters for <i>n</i> = 2–14 atoms in the cationic, neutral, and anionic states to analyze the behavior of the structural, electronic, and energetic properties as a function of size and charge state. Moreover, we considered potentially high-energy isomers allowing foresight comparison with experimental results. Considering fixed cluster sizes, we found that distinct charge states lead to different structural geometries, revealing a clear tendency of decreasing average coordination as the electron density is increased. This behavior prompts significant changes in all considered properties, namely, energy gaps between occupied and unoccupied states, magnetic moment, detachment energy, ionization potential, center of gravity and “bandwidth” of occupied d-states, stability function, binding energy, electric dipole moment and sd hybridization. Furthermore, we identified a strong correlation between magic Pt clusters with peaks in sd hybridization index, allowing us to conclude that sd hybridization is one of the mechanisms for stabilization for Pt_<i>n</i> clusters. Our results form a well-established basis upon which a deeper understanding of the stability and reactivity of metal clusters can be built, as well as the possibility to tune and exploit cluster properties as a function of size and charge