Local order parameters for use in driving homogeneous ice nucleation with all-atom models of water

We present a local order parameter based on the standard Steinhardt-Ten Wolde approach that is capable both of tracking and of driving homogeneous ice nucleation in simulations of all-atom models of water. We demonstrate that it is capable of forcing the growth of ice nuclei in supercooled liquid water simulated using the TIP4P/2005 model using overbiassed umbrella sampling Monte Carlo simulations. However, even with such an order parameter, the dynamics of ice growth in deeply supercooled liquid water in all-atom models of water are shown to be very slow, and so the computation of free energy landscapes and nucleation rates remains extremely challenging.Comment: This version incorporates the minor changes made to the paper following peer revie

La Vida Cotidiana de la Niñez en el Campamento "Felipe Camiroaga"

Tesis (Trabajo Social)En la presente investigación, se expondrán los antecedentes de las denominadas “Tomas” en la Quinta región y el país, enfatizando la gran cantidad de campamentos, y como se evidencia en Chile desde la historia Socio-Política, desde la “Cuestión social” y cómo se construye la problematización del fenómeno investigado, a partir de las políticas de vivienda y sus distintas falencias en el sistema económico hegemónico. Esta investigación fue realizada en el territorio denominado “Campamento Felipe Camiroaga” por la observación constante del equipo de investigación, debido a la práctica profesional realizada en el mismo lugar. En el marco teórico se trabaja desde la visión Crítico-Marxista basada en Kosik y sus distintos documentos entorno a la dialéctica. Luego, se revisan las distintas visiones de “Vida Cotidiana” principalmente por Henri Lefebvre, Michel De Certau, entre otros autores basados en el paradigma mencionado anteriormente esto llegando a la urbanización Capitalista desde el análisis de Christian Topalov y el Derecho a la ciudad. Posteriormente se detalla el enfoque metodológico cualitativo, y la etnografía como método siendo lo que más se ajusta a la investigación. El Campamento Felipe Camiroaga, en relación a otros campamentos de la región, aún no cuenta con la antigüedad suficiente para estar dentro del “Registro Nacional de Campamentos” del MINVU, a pesar de ser uno de los más grandes a nivel Nacional. Mediante un acercamiento con las y los pobladores se expone de manera innovadora al grupo de investigación la riqueza e importancia de reconstruir a través de sus miembros la historia y sobre todo visibilizar cómo se plantea la convivencia a diario en el campamento con el objetivo de recabar en las diferentes variables que se presentan como obstaculizadores y facilitadores en su diario vivir, este interés se presenta para el equipo de investigación ya que, la práctica profesional que se está realizando ha permitido observar los distintos discursos planteados por parte de las y los pobladores quienes en la situación que se encuentran, en situación de precariedad, por las diferentes experiencias personales y por una falencia esencial del Estado en las Políticas de Vivienda y urbanismo, han tomado la decisión de tomarse un terreno y vivir de manera permanente, asumiendo todos los riesgos a los que se exponen, sobre todo la falta de servicios básicos y la irregularidad a la que se enfrentan, en los discursos planteados, se visualiza un enojo con la autoridad actual tanto Local como Nacional

Computing the free energy of molecular solids by the Einstein molecule approach: Ices XIII and XIV, hard-dumbbells and a patchy model of proteins

The recently proposed Einstein molecule approach is extended to compute the free energy of molecular solids. This method is a variant of the Einstein crystal method of Frenkel and Ladd[J. Chem. Phys. 81,3188 (1984)]. In order to show its applicability, we have computed the free energy of a hard-dumbbells solid, of two recently discovered solid phases of water, namely, ice XIII and ice XIV, where the interactions between water molecules are described by the rigid non-polarizable TIP4P/2005 model potential, and of several solid phases that are thermodynamically stable for an anisotropic patchy model with octahedral symmetry which mimics proteins.Our calculations show that both the Einstein crystal method and the Einstein molecule approach yield the same results within statistical uncertainty.In addition, we have studied in detail some subtle issues concerning the calculation of the free energy of molecular solids. First, for solids with non-cubic symmetry, we have studied the effect of the shape of the simulation box on the free energy. Our results show that the equilibrium shape of the simulation box must be used to compute the free energy in order to avoid the appearance of artificial stress in the system that will result in an increase of the free energy. In complex solids, such as the solid phases of water, another difficulty is related to the choice of the reference structure. As in some cases there is not an obvious orientation of the molecules, it is not clear how to generate the reference structure. Our results will show that,as long as the structure is not too far from the equilibrium structure,the calculated free energy is invariant to the reference structure used in the free energy calculations. Finally, the strong size dependence of the free energy of solids is also studied.Comment: 43 pages, 5 figure

Solid helium at high pressure: A path-integral Monte Carlo simulation

Solid helium (3He and 4He) in the hcp and fcc phases has been studied by path-integral Monte Carlo. Simulations were carried out in the isothermal-isobaric (NPT) ensemble at pressures up to 52 GPa. This allows one to study the temperature and pressure dependences of isotopic effects on the crystal volume and vibrational energy in a wide parameter range. The obtained equation of state at room temperature agrees with available experimental data. The kinetic energy, E_k, of solid helium is found to be larger than the vibrational potential energy, E_p. The ratio E_k/E_p amounts to about 1.4 at low pressures, and decreases as the applied pressure is raised, converging to 1, as in a harmonic solid. Results of these simulations have been compared with those yielded by previous path integral simulations in the NVT ensemble. The validity range of earlier approximations is discussed.Comment: 7 pages, 5 figure

The Feynman effective classical potential in the Schr\"odinger formulation

New physical insight into the correspondence between path integral concepts and the Schr\"odinger formulation is gained by the analysis of the effective classical potential, that is defined within the Feynman path integral formulation of statistical mechanics. This potential is related to the quasi-static response of the equilibrium system to an external force. These findings allow for a comprehensive formulation of dynamical approximations based on this potential.Comment: 10 pages, 4 figure

Phase diagram of model anisotropic particles with octahedral symmetry

We computed the phase diagram for a system of model anisotropic particles with six attractive patches in an octahedral arrangement. We chose to study this model for a relatively narrow value of the patch width where the lowest-energy configuration of the system is a simple cubic crystal. At this value of the patch width, there is no stable vapour-liquid phase separation, and there are three other crystalline phases in addition to the simple cubic crystal that is most stable at low pressure. Firstly, at moderate pressures, it is more favourable to form a body-centred cubic crystal, which can be viewed as two interpenetrating, and almost non-interacting, simple cubic lattices.Secondly, at high pressures and low temperatures, an orientationally ordered face-centred cubic structure becomes favourable. Finally, at high temperatures a face-centred cubic plastic crystal is the most stable solid phase.Comment: 12 pages,10 figure