R. O. Jones

Structure and spin in small iron clusters

Abstract Density functional calculations have been performed for iron clusters with up to seven atoms. The electron-ion interaction is treated by a pseudopotential, and the large effect of core electrons on the spin configuration by modifying the exchange-correlation function. The scheme allows us to perform molecular dynamics (MD, Car-Parrinello) simulations and to explore the potential energy surface of the cluster without symmetry or other constraints. The most stable structures have magnetic moments ≈3 μ B per atom and are generally compact. We discuss the structural trends, particularly the similarities with the close-packed structures found in clusters where the atoms interact with a pairwise Lennard-Jones potential.

Density functional study of amorphous, liquid and crystalline Ge2Sb2Te5: homopolar bonds and/or AB alternation?

The amorphous, liquid and crystalline phases of the phase change material Ge(2)Sb(2)Te(5) (GST) have been studied by means of density functional/molecular dynamics simulations. The large sample (460 atoms and 52 vacancies in the unit cell) and long simulations (hundreds of picoseconds) provide much new information. Here we extend our original analysis (2007 Phys. Rev. B 76 235201) in important ways: partial coordination numbers and radial distribution functions, bond angle distributions, new local order parameters, vibration frequencies, and the charges on atoms and vacancies. The valence band densities of states in amorphous and crystalline GST are compared with ones from x-ray photoemission spectroscopy. The results for the liquid phase are new and those for the crystalline phase much expanded. GST shows pronounced AB alternation (A: Ge, Sb; B: Te), especially in its amorphous phase, and ABAB squares play a central role in the amorphous to crystalline transition. We comment on earlier speculations concerning the nature of the amorphous to crystalline transition.

Rings and chains in sulfur cluster anions S− to S9−: Theory (simulated annealing) and experiment (photoelectron detachment)

Density functional calculations with simulated annealing have been performed for singly charged anions of sulfur clusters with up to nine atoms. The calculations predict the existence of two classes of structure: open (chain‐like) and closed (ring‐like), for which the vertical detachment energies (VDE) and the adiabatic electron affinities for transitions to states of the neutral clusters show pronounced differences. These calculations are complemented by photoelectron detachment measurements on sulfur cluster ions with up to 11 atoms using a pulsed arc cluster ion source (PACIS). The measurements provide unambiguous evidence for the existence of two types of isomers in both S6− and S7−. Although ring structures are generally energetically more stable than chain structures, the environment used to generate the larger clusters (n≳7) favors the formation of the latter. The measured VDE and vibration frequencies agree well with the calculated values.

Structure and spectroscopy of phosphorus cluster anions: Theory (simulated annealing) and experiment (photoelectron detachment)

Photoelectron detachment measurements have been performed on singly charged phosphorus cluster anions with up to nine atoms, generated by a pulsed arc cluster ion source (PACIS). Transitions between the anion ground states and states of the neutral clusters are observed for all clusters, and vibrational fine structure in both dimer and trimer. A comparison with the results of density functional calculations with simulated annealing—an extension to negative ions of earlier work on neutral and positively charged clusters—provides a consistent overall picture for all cluster sizes and the first experimental structural information on several.

Density functional and Monte Carlo studies of sulfur. I. Structure and bonding in Sn rings and chains (n=2–18)

Density functional calculations have been performed for ring isomers of sulfur with up to 18 atoms, and for chains with up to ten atoms. There are many isomers of both types, and the calculations predict the existence of new forms. Larger rings and chains are very flexible, with numerous local energy minima. Apart from a small, but consistent overestimate in the bond lengths, the results reproduce experimental structures where known. Calculations are also performed on the energy surfaces of S8 rings, on the interaction between a pair of such rings, and the reaction between one S8 ring and the triplet diradical S8 chain. The results for potential energies, vibrational frequencies, and reaction mechanisms in sulfur rings and chains provide essential ingredients for Monte Carlo simulations of the liquid–liquid phase transition. The results of these simulations will be presented in Part II.

Density functional study of molecular crystals: Polyethylene and a crystalline analog of bisphenol-A polycarbonate

Density functional calculations have been performed on two types of molecular crystal: (a) crystalline (orthorhombic) polyethylene comprising covalently bonded parallel chains with weak interchain interactions, and (b) a crystalline analog of bisphenol-A polycarbonate with a unit cell containing two molecules with 59 atoms each. The local density approximation for the exchange-correlation energy overestimates the strength of the intermolecular bonds in both, and the Becke–Perdew functional (gradient corrected) gives no intermolecular binding in the former and a very weak bond in the latter. The functional of Perdew, Burke, and Ernzerhof leads to binding in both molecules.

Density functional study of crystalline polyethylene

Abstract Density functional calculations have been performed for single chains and the crystalline (orthorhombic) form of polyethylene (PE). The geometrical structures are optimized without constraints, and the exchange-correlation energy is calculated using local density (LD) and non-local (gradient-corrected, GC) approximations. Both give good descriptions of the structure of a single PE chain, but LD calculations overestimate the binding energies between chains, and GC calculations lead to no interchain binding at all.

Density functional study of carbonic acid clusters

Density functional calculations on carbonic acid H2CO3 are extended to clusters of up to five such units. The most stable forms are the linear, hydrogen-bonded analogs of the dimer with anti–anti orientation. We calculate structures and vibration frequencies, as well as the energy required to bend and stretch the linear isomers. Linear chains of up to ∼20 units should be favored over ring structures, and they have a tensile strength reminiscent of chains of water molecules. We also discuss planar, nonlinear structures as well as three-dimensional isomers.

A reactive force field simulation of liquid-liquid phase transitions in phosphorus

A force field model of phosphorus has been developed based on density functional (DF) computations and experimental results, covering low energy forms of local tetrahedral symmetry and more compact (simple cubic) structures that arise with increasing pressure. Rules tailored to DF data for the addition, deletion, and exchange of covalent bonds allow the system to adapt the bonding configuration to the thermodynamic state. Monte Carlo simulations in the N-P-T ensemble show that the molecular (P(4)) liquid phase, stable at low pressure P and relatively low temperature T, transforms to a polymeric (gel) state on increasing either P or T. These phase changes are observed in recent experiments at similar thermodynamic conditions, as shown by the close agreement of computed and measured structure factors in the molecular and polymer phases. The polymeric phase obtained by increasing pressure has a dominant simple cubic character, while the polymer obtained by raising T at moderate pressure is tetrahedral. Comparison with DF results suggests that the latter is a semiconductor, while the cubic form is metallic. The simulations show that the T-induced polymerization is due to the entropy of the configuration of covalent bonds, as in the polymerization transition in sulfur. The transition observed with increasing P is the continuation at high T of the black P to arsenic (A17) structure observed in the solid state, and also corresponds to a semiconductor to metal transition.

Structures and energy barriers in small hydrocarbon molecules — a density functional study

Abstract Density functional calculations with simulated annealing have been performed for a range of small hydrocarbon molecules (CH, CH 2 , C 2 H 2 , C 2 H 4 , C 3 H 6 , C 3 H 8 ). In addition to the geometries and relative energies of low-lying states, we focus on the barriers for internal rotation about both single and double bonds. There is satisfactory overall agreement with experiment using both local spin density and gradient-corrected non-local energy functionals, so that energy surfaces provided by DF calculations on structural units should then provide a useful database for parameterizing the forces in extended systems, such as organic polymers.