Documentation

• First principles molecular dynamics - actual research projects

   

  - high-pressure physics

  
  

  High-pressure physics: - Insulator to metal transition in hydrogen under high pressures - Polyacetylene and a-C:H from Acetylene at High Pressure - Hydrostatic and uniaxial compression of diamond - Magnetic collapse and metallization of molecular oxygen - Methane, Ammonia and Water at Planetary Conditions - Decomposition and Polymerization of Solid CO - Liquid and solid iron at earth's core conditions Insulator to metal transition in hydrogen under high pressures By using a combination of constant pressure ab-initio molecular dynamics simulations and density-functional linear response theory we analyze the high pressure phases of molecular solid hydrogen. We use a gradient corrected LDA, and, in the case of ab-initio molecular dynamics, a freshly implemented efficient technique for Brillouin zone sampling. An extremely good k-point sampling turns out to be crucial for obtaining the correct ground state. This approach allows us to optimize simultaneously the orientational degrees of freedom, the lattice constants, and the space group. This can be done either by a local optimization technique, or by running molecular dynamics (MD) trajectories. The MD allows for the system to undergo structural transformations spontaneously. In the lower pressure, namely for the broken symmetry phase (BSP or phase II), we find that the best candidates are the "quadrupolar" orthorhombic structures of Pca2_1 and P21/c symmetry. Equation of state, lattice parameters and vibronic frequencies, are in very good agreement with experimental observations. We are presently extending our simulations to higher pressures, with the aim of understanding the nature of phase III (H-A) and of the long-sought hypothetical metallic phase. [J. Kohanoff, S. de Gironcoli, S. Scandolo, G.L. Chiarotti, E. Tosatti] [top] Polyacetylene and a-C:H from Acetylene at High Pressure We have simulated the polymerization of crystalline acetylene into polyacetylene and its subsequent transformation into into a-C:H under pressure shock. We find that crystalline acetylene polymerizes at 15 GPa, with formation of both cis and trans chains. Subsequent compression shows that polyacetylene develops interchain links, causing a gradual saturation of C-C bonds, and ending up at 50 GPa with a-C:H containing about 80 % of sp3 carbons. The sp2 to sp3 conversion is irreversible and is not undone reverting back to zero pressure. The final a-C:H is a wide gap insulator and, unlike the conventionally generated a-C:H, is highly anisotropic keeping some memory of the original polyacetylene chains axis. [ M. Bernasconi, M. Parrinello, P. Focher, Guido L. Chiarotti, E. Tosatti] [top] Hydrostatic and uniaxial compression of diamond We have used constant-pressure ab-initio molecular dynamics to simulate the high-pressure induced transformation of graphite into diamond. The two simulation cells adopted contain 48 and 64 carbon atoms, described by a state-of-the-art LDA pseudopotential scheme. Within a few ps simulation time, and under increasing pressure, a spontaneous conversion from the initial hexagonal graphite to a final diamond structure is obtained. We find that the conversion path proceeds trough a preliminary sliding of the graphite planes into an orthorhombic stacking wherefrom an abrupt collapse and buckling of the planes leads to both cubic and hexagonal (Lonsdaleite) forms of diamond in comparable proportions. This result confirms earlier speculations by Wheeler and Lewis [Mat. Res. Bull. 10, 687 (1975)]. The mutual orientation of the initial and final phases - a crucial indicator of the actual conversion path - is identical to that observed in shock-wave experiments. A new metallic phase of carbon, metastable at terapascal pressure, is also discovered at higher pressures (see S. Scandolo et al.). The simulation shows that, upon fast compression, diamond survives in a metastable state up to about 3 TPa, where it collapses into a six-fold coordinated structure, named SC4, metallic, never considered so far. Although SC4 is not the lowest energy phase at these pressures, it is argued that large activation barriers may hinder the transition of diamond to other phases at lower or comparable pressure. In order to understand the behavior of diamond in the so-called Diamond Anvil Cell (DAC), widely used in high-pressure research, we are currently studying the more realistic case of a combined hydrostatic and uniaxial compression of diamond. In particular, we have calculated the nonlinear stress-vs-strain curves for tetragonal (001) distorsion of the diamond lattice. These curves are of fundamental importance in the modeling of diamond-anvil cell devices. [Z. Ji-Jun, S. Scandolo, G.L. Chiarotti, J. Kohanoff, E. Tosatti] [top] Magnetic collapse and metallization of molecular oxygen The behavior of solid oxygen in the pressure range between 5-116 GPa is studied by ab-initio simulations, showing a spontaneous phase transformation from the antiferromagnetic insulating delta-O2 phase to a non-magnetic, metallic molecular phase. The calculated static structure factor of this phase is in excellent agreement with X-ray diffraction data in the metallic zeta-O2 phase above 96 GPa [Y. Akahama et al, Phys. Rev. Lett. 74, 4690 (1995)]. We thus propose that zeta-O2 should be base centered monoclinic with space group C2/m and 4 molecules per cell, suggesting a re-indexing of experimental diffraction peaks. Physical constraints on the intermediate-pressure epsilon-O2 phase are also obtained. [S. Serra, G.L. Chiarotti, S. Scandolo, E. Tosatti] [top] Methane, Ammonia and Water at Planetary Conditions Knowledge of the equation of state, phase diagram and other physical properties such as the electric conductivity of Methane, Water and Ammonia at high pressures and temperatures is of great interest in planetary physics. It is in fact known that the interiors of planets like Uranus and Neptune are mainly composed of such molecules, in solar proportions. Pressures and temperatures inside these planets are thought to range from some GPa and about 1000 K, to several hundred GPa and several thousand kelvin. Using our constant-pressure molecular dynamics tool we have first investigated the behavior of methane in these conditions, and found that, contrary to the current interpretation of shock-wave experiments, methane dissociates below 100 GPa into a mixture of hydrocarbons, and it fully separates into hydrogen and carbon only above 300 GPa. This may explain the anomalous abundance of hydrocarbons in the atmosphere of Neptune. Simulations on water and ammonia along the planetary isentrope show that they instead behave as fully dissociated ionic, electronically insulating fluid phases, which turn metallic for temperatures exceeding 7000 K for water and 5500 K for ammonia. At lower temperatures, the phase diagrams of water and ammonia exhibit a superprotonic solid phase between the solid and the ionic liquid. [C. Cavazzoni, F. Ancilotto, G.L. Chiarotti, S. Scandolo, E. Tosatti, M. Bernasconi, M. Parrinello] [top] Decomposition and Polymerization of Solid CO By performing constant-pressure deformable-cell ab-initio molecular dynamics simulations we have studied the pressure-induced chemical instability of CO above 5 GPa [Katz et al, J. Phys. Chem. 88, 3176 (1984)]. The simulation shows that, contrary to previous speculations, polymerization proceeds without CO bond dissociation. The resulting polymer consists of a disordered network of small polycarbonyl (CO)_n chains connecting fivefold C4O cycles. The computed vibrational spectra and electronic gap agree very well with (and shed light on) very recent experimental data obtained at Livermore. [S. Bernard, G.L. Chiarotti, S. Scandolo, E. Tosatti] [top] Liquid and solid iron at earth's core conditions The behavior of Iron at earth's core conditions is far from being fully understood [see e.g. O.L. Anderson, Science 278, 821 (1997)]. Open issues include the melting temperature at the inner core boundary, whose estimates range from 5000 to 9000 K, and the hypothetical presence of a new solid phase above 200 GPa and 4000 K, whose presence would reconcile most of the experimental data. We are currently undertaking extensive ab-initio simulations of both liquid and solid iron, in order to clarify such issues. [A. Laio, S. Bernard, G.L. Chiarotti, S. Scandolo, E. Tosatti] [top]