Path-integral simulation of ice Ih: The effect of pressure

The effect of pressure on structural and thermodynamic properties of ice Ih has been studied by means of path-integral molecular dynamics simulations at temperatures between 50 and 300 K. Interatomic interactions were modeled by using the effective q-TIP4P/F potential for flexible water. Positive (compression) and negative (tension) pressures have been considered, which allowed us to approach the limits for the mechanical stability of this solid water phase. We have studied the pressure dependence of the crystal volume, bulk modulus, interatomic distances, atomic delocalization, and kinetic energy. The spinodal point at both negative and positive pressures is derived from the vanishing of the bulk modulus. For P < 0, the spinodal pressure changes from -1.38 to -0.73 GPa in the range from 50 to 300 K. At positive pressure the spinodal is associated to ice amorphization, and at low temperatures is found between 1.1 and 1.3 GPa. Quantum nuclear effects cause a reduction of the metastability region of ice Ih.Comment: 12 pages, 9 figure

The phase diagram of ice: a quasi-harmonic study based on a flexible water model

The phase diagram of ice is studied by a quasi-harmonic approximation. The free energy of all experimentally known ice phases has been calculated with the flexible q-TIP4P/F model of water. The only exception is the high pressure ice X, in which the presence of symmetric O-H-O bonds prevents its modeling with this empirical interatomic potential. The simplicity of our approach allows us to study ice phases at state points of the T-P plane that have been omitted in previous simulations using free energy methods based on thermodynamic integration. The effect in the phase diagram of averaging the proton disorder that appears in several ice phases has been studied. It is found particularly relevant for ice III, at least for cell sizes typically used in phase coexistence simulations. New insight into the capability of the employed water model to describe the coexistence of ice phases is presented. We find that the H-ordered ices IX and XIV, as well as the H-disordered ice XII, are particularly stable for this water model. This fact disagrees with experimental data. The unexpected large stability of ice IX is a property related to the TIP4P-character of the water model. Only after omission of these three stable ice phases, the calculated phase diagram becomes in reasonable qualitative agreement to the experimental one in the T-P region corresponding to ice Ih, II, III, V, and VI. The calculation of the phase diagram in the quantum and classical limits shows that the most important quantum effect is the stabilization of ice II due to its lower zero-point energy when compared to that one of ices Ih, III, and V.Comment: 13 pages, 8 figures, 5 table

Semiclassical ordering in the large-N pyrochlore antiferromagnet

We study the semiclassical limit of the $Sp(N)$ generalization of the pyrochlore lattice Heisenberg antiferromagnet by expanding about the $N \to \infty$ saddlepoint in powers of a generalized inverse spin. To leading order, we write down an effective Hamiltonian as a series in loops on the lattice. Using this as a formula for calculating the energy of any classical ground state, we perform Monte-Carlo simulations and find a unique collinear ground state. This state is not a ground state of linear spin-wave theory, and can therefore not be a physical (N=1) semiclassical ground state.Comment: 4 pages, 4 eps figures; published versio