L. Koci

Accurate Melting Temperatures for Neon and Argon from Ab Initio Monte Carlo Simulations

Although studied experimentally for centuries, the melting of solids is still a fascinating phenomenon whose underlying mechanisms are not yet well understood. Predicting melting points is a nontrivial task: The standard computational method relies on analyzing the free energies of the solid and liquid phases obtained independently by thermodynamic or Gibbs–Duhem integration; this approach suffers from the difficulty of calibration. Alternatively, in coexistence methods the interface between the two phases must be described explicitly; this interface is often hard to stabilize. An alternative idea, which we pursue herein, is to obtain information about the melting transition by studying finite clusters and extrapolating the results to infinitely large systems. Here we present for the first time calculated melting temperatures reaching experimental accuracy obtained from Monte Carlo simulations of NeN and ArN clusters consisting of a “magic number” N of atoms (N = 13, 55, 147, 309, 561, 923) and of bulk samples. This was achieved by the use of accurate interaction potentials obtained from precise ab initio data having the same computational efficiency as the widely used empirical Lennard-Jones (LJ) potential, and without any experimental input whatsoever. Argon and neon adopt the face-centered cubic (fcc) periodic packing in the solid state, but their clusters with N< 1000 are most stable as complete Mackay icosahedra. The number of atoms in the first six shells of the

Dynamical stability of the hardest known oxide and the cubic solar material: TiO2

The authors have studied dynamical stability of different polymorphs of TiO2 using ab initio phonon calculations based on density functional theory in conjunction with force-constant method. Rutile TiO2 was found stable at ambient pressure, but unstable at high pressure. The calculated Raman frequency and phonon density of states (PDOS) of rutile TiO2 are in a good agreement with experiment. Concerning two cubic phases (solar materials), fluorite stabilized under pressure, whereas pyrite showed instability throughout the whole pressure range. Furthermore, the PDOS of cotunnite (the hardest known oxide) phase confirmed that it exists at high pressure and can be quenched down to a low pressure limit.

Mechanical stability of TiO2 polymorphs under pressure : ab initio calculations

First-principles calculations using plane-wave basis sets and ultrasoft pseudopotentials have been performed to study the mechanical stabilities of the rutile, pyrite, fluorite and cotunnite phases of titanium dioxide (TiO2). For these polymorphs, we have calculated the equilibrium volumes, equations of state, bulk moduli and selected elastic constants. Compared to the three phases rutile, pyrite and fluorite, the recently discovered cotunnite phase shows the highest c44 for all pressures considered. Cotunnite also shows the highest bulk modulus amongst the four studied phases at an ambient pressure of B0 = 272 GPa.

Study of the high-pressure helium phase diagram using molecular dynamics

The rich occurrence of helium and hydrogen in space makes their properties highly interesting. By means of molecular dynamics ( MD), we have examined two interatomic potentials for He-4. Both poten ...

Ab initio and classical molecular dynamics calculations of the high-pressure melting of Ne

Classical molecular dynamics (CMD) calculations are fast but are heavily dependent on the potential feasibility. On the other hand, first-principles (ab initio) molecular dynamics (AIMD) does not use any empirical knowledge, but can be extremely time consuming. As both techniques have been applied to study melting at extreme conditions, a comparison of the methods is motivated. Furthermore, when melting is studied with MD, the use of coexistent solid and liquid structures (two-phase) in the initial simulation configuration, instead of a only a solid structure (one-phase), can have a significant impact. In this work, comparisons have been made between CMD and AIMD methods applied to one- and two-phase systems for the melting of Ne at high pressure.