First-principles calculations have been performed for V, Nb, Ta, Mo, and W. The recently discovered bcc→ rhombohedral transition for vanadium ͓Phys. Rev. Lett. 98, 085502 ͑2007͔͒ was confirmed as the mechanical instability of c 44 was found at P = 80 GPa. Furthermore, the c 11 , c 12 , and c 44 constants for the group-V elements showed erratic behaviors whereas the constants for the group-VI elements were monotonically increasing with pressure. The metals were analyzed with Fermi surface calculations, showing shrinking nesting vectors with pressure for V, Nb, and Ta but were not seen for Mo and W. From electronic topological transition contributions, a critical energy closely situated to the Fermi level for vanadium could be the reason why the elastic constants of V and Nb were difficult to reproduce at ambient pressure.

Although studied experimentally for centuries, the melting of solids is still a fascinating phenomenon whose underlying mechanisms are not yet well understood.[1] 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 [2] or Gibbs-Duhem [3] integration; this approach suffers from the difficulty of calibration. Alternatively, in coexistence methods the interface between the two phases must be described explicitly; [4] this interface is often hard to stabilize.[5]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 Ne N and Ar N 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 cluster corresponds to the "magic numbers": N = 1 + 2 P n k¼1 (5k 2 +1) = 13, 55, 147, 309, 561, and 923.[6] These "magic numbers" have been provided by mass spectra in free-jet expansions of rare-gas clusters. [7,8] The unusual stability of these clusters is explained by the structure in which one to six completed shells of atoms surround a central atom (see the inset of Figure 1). [6] Previous work on determining melting curves for rare-gas clusters include Monte Carlo (MC) simulations by Labastie and Whetten [9] (N = 13, 55, 147). They found a well-defined single peak in the heat capacity, which becomes higher more intense and narrower as the cluster size increases. The apparent convergence towards the bulk limit was, however, questioned by Noya and Doye, [10] who found an additional premelting peak for the 309-atom cluster. As we show below, even though complicated premelting phenomena remain for all clusters with more than 309 atoms, the melting peak itself always turns out to be well defined. Furthermore, and to our knowledge, the 309-atom system is the largest rare-gas cluster studied so far for melting. To determine the bulk melting point T m by extrapolation to the macroscopic limit, inclusion of two more shells is required, as we demonstrate in this work.

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.

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Using nonequilibrium molecular dynamics ͑MD͒ simulations we study shock-induced melting in Ni with an embedded atom method ͑EAM͒. Dynamic melting is probed by the pair correlation function, and we find a melting lattice temperature of T melt = 6400± 300 K for a melting pressure of P melt = 275± 10 GPa. When a combined MD+ TTM ͑two-temperature model͒ approach is used to include electronic heat conduction and electron-phonon coupling, P melt and T melt change. For a given pressure, the temperature behind the shock decreases due to electronic heat diffusion into the cold, unshocked material. This cooling of the material behind the shock slightly increases the melting pressure compared to simulations without electronic heat conduction and electron-phonon coupling. The decrease in the temperature behind the shock front is enhanced if the electron-phonon coupling is artificially made larger. We also explore the feasibility of using x-ray diffraction to detect melting.