Thermal effects on CH3_3NH3_3PbI3_3 perovskite from ab-initio molecular dynamics simulations

We present a molecular dynamics simulation study of CH$_3$NH$_3$PbI$_3$ based on forces calculated from density functional theory. The simulation were performed on model systems having 8 and 27 unit cells, and for a total simulation time of 40 ps in each case. Analysis of the finite size effects, in particular the mobility of the organic component, suggests that the smaller system is over correlated through the long range electrostatic interaction. In the larger system this finite size artifact is relaxed producing a more reliable description of the anisotropic rotational behavior of the methyl ammonium molecules. The thermal effects on the optical properties of the system were also analyzed. The HOMO-LUMO energy gap fluctuates around its central value with a standard deviation of approximately 0.1 eV. The projected density of states consistently place the Fermi level on the $p$ orbitals of the I atoms, and the lowest virtual state on $p$ orbitals of the Pb atoms throughout the whole simulation trajectory.Comment: 16 pages, 11 figure

Molecular kinetics of solid and liquid CHCl3_3

We present a detailed analysis of the molecular kinetics of CHCl$_3$ in a range of temperatures covering the solid and liquid phases. Using nuclear quadrupolar resonance we determine the relaxation times for the molecular rotations in solid at pre-melting conditions. Molecular dynamics simulations are used to characterize the rotational dynamics in the solid and liquid phases and to study the local structure of the liquid in terms of the molecular relative orientations. We find that in the pre-melting regime the molecules rotate about the C-H bond, but the rotations are isotropic in the liquid, even at supercooled conditions.Comment: Chemical Physics Letter (in press). 14 pages, 7 figure

Self-assembly of pseudo-dipolar nanoparticles at low densities and strong coupling

Nanocolloids having directional interactions are highly relevant for designing new self-assembled materials easy to control. In this article we report stochastic dynamics simulations of finite-size pseudo-dipolar colloids immersed in an implicit dielectric solvent using a realistic continuous description of the quasi-hard Coulombic interaction. We investigate structural and dynamical properties near the low-temperature and highly-diluted limits. This system self-assembles in a rich variety of string-like configurations, depicting three clearly distinguishable regimes with decreasing temperature: fluid, composed by isolated colloids; string-fluid, a gas of short string-like clusters; and string-gel, a percolated network. By structural characterization using radial distribution functions and cluster properties, we calculate the state diagram, verifying the presence of string-fluid regime. Regarding the string-gel regime, we show that the antiparallel alignment of the network chains arises as a novel self-assembly mechanism when the characteristic interaction energy exceeds the thermal energy in two orders of magnitude, ud/kBT ≈ 100. This is associated to relevant structural modifications in the network connectivity and porosity. Furthermore, our results give insights about the dynamically-arrested nature of the string-gel regime, where we show that the slow relaxation takes place in minuscule energy steps that reflect local rearrangements of the network.Fil: Brito, Mariano Exequiel. Helmholtz Gemeinschaft. Forschungszentrum Jülich; Alemania. Consejo Nacional de Investigaciones Científicas y Técnicas. Centro Científico Tecnológico Conicet - Córdoba. Instituto de Física Enrique Gaviola. Universidad Nacional de Córdoba. Instituto de Física Enrique Gaviola; ArgentinaFil: Carignano, Marcelo A.. Qatar Environment And Energy Research Institute; QatarFil: Marconi, Veronica Iris. Consejo Nacional de Investigaciones Científicas y Técnicas. Centro Científico Tecnológico Conicet - Córdoba. Instituto de Física Enrique Gaviola. Universidad Nacional de Córdoba. Instituto de Física Enrique Gaviola; Argentin

The water supercooled regime as described by four common water models

The temperature scale of simple water models in general does not coincide with the natural one. Therefore, in order to make a meaningful evaluation of different water models a temperature rescaling is necessary. In this paper we introduce a rescaling using the melting temperature and the temperature corresponding to the maximum of the heat capacity to evaluate four common water models (TIP4P-Ew, TIP4P-2005, TIP5P-Ew and Six-Sites) in the supercooled regime. Although all the models show the same general qualitative behavior, the TIP5P-Ew appears as the best representation of the supercooled regime when the rescaled temperature is used. We also analyze, using thermodynamic arguments, the critical nucleus size for ice growth. Finally, we speculate on the possible reasons why atomistic models do not usually crystalize while the coarse grained mW model do crystallize.Comment: 8 pages, 8 figure

Structural transitions and dipole moment of water clusters (H2 O) n=4-100

The properties of water clusters (H2 O) n over a broad range of sizes (n=4-100) were studied by microcanonical parallel tempering Monte Carlo and replica exchange molecular dynamics simulations at temperatures between 20 and 300 K, with special emphasis in the understanding of relation between the structural transitions and dipole behavior. The effect of the water interaction potential was analyzed using six nonpolarizable models, but more extensive calculations were performed using the TIP4P-ice water model. We find that, in general, the dipole moment of the cluster increases significantly as the cluster melts, suggesting that it could be used to discriminate between the solidlike and liquidlike phases. The effect of a moderate electric field on the cluster heat capacity and total dipole moment was found to be negligible.Fil: Gelman Constantin, Julián. Consejo Nacional de Investigaciones Científicas y Técnicas. Oficina de Coordinación Administrativa Ciudad Universitaria. Instituto de Química, Física de los Materiales, Medioambiente y Energía. Universidad de Buenos Aires. Facultad de Ciencias Exactas y Naturales. Instituto de Química, Física de los Materiales, Medioambiente y Energía; ArgentinaFil: Carignano, Marcelo A.. Northwestern University; Estados UnidosFil: Szleifer, Igal. Northwestern University; Estados UnidosFil: Marceca, Ernesto José. Consejo Nacional de Investigaciones Científicas y Técnicas. Oficina de Coordinación Administrativa Ciudad Universitaria. Instituto de Química, Física de los Materiales, Medioambiente y Energía. Universidad de Buenos Aires. Facultad de Ciencias Exactas y Naturales. Instituto de Química, Física de los Materiales, Medioambiente y Energía; ArgentinaFil: Corti, Horacio Roberto. Consejo Nacional de Investigaciones Científicas y Técnicas. Oficina de Coordinación Administrativa Ciudad Universitaria. Instituto de Química, Física de los Materiales, Medioambiente y Energía. Universidad de Buenos Aires. Facultad de Ciencias Exactas y Naturales. Instituto de Química, Física de los Materiales, Medioambiente y Energía; Argentina. Comisión Nacional de Energía Atómica. Centro Atómico Constituyentes; Argentin

Hydrogen bonding and orientation effects on the accommodation of methylamine at the air-water interface

Methylamine is an abundant amine compound detected in the atmosphere which can affect the nature of atmospheric aerosol surfaces, changing their chemical and optical properties. Molecular dynamics simulation results show that methylamine accommodation on water is close to unity with the hydrophilic head group solvated in the interfacial environment and the methyl group pointing into the air phase. A detailed analysis of the hydrogen bond network indicates stronger hydrogen bonds between water and the primary amine group at the interface, suggesting that atmospheric trace gases will likely react with the methyl group instead of the solvated amine site. These findings suggest new chemical pathways for methylamine acting on atmospheric aerosols in which the methyl group is the site of orientation specific chemistry involving its conversion into a carbonyl site providing hydrophilic groups for uptake of additional water. This conversion may explain the tendency of aged organic aerosols to form cloud condensation nuclei. At the same time, formation of NH2 radical and formaldehyde is suggested to be a new source for NH2 radicals at aerosol surfaces, other than by reaction of absorbed NH3. The results have general implications for the chemistry of other amphiphilic organics, amines in particular, at the surface of atmospherically relevant aerosol

Hydrogen bonding and orientation effects on the accommodation of methylamine at the air-water interface

Methylamine is an abundant amine compound detected in the atmosphere which can affect the nature of atmospheric aerosol surfaces, changing their chemical and optical properties. Molecular dynamics simulation results show that methylamine accommodation on water is close to unity with the hydrophilic head group solvated in the interfacial environment and the methyl group pointing into the air phase. A detailed analysis of the hydrogen bond network indicates stronger hydrogen bonds between water and the primary amine group at the interface, suggesting that atmospheric trace gases will likely react with the methyl group instead of the solvated amine site. These findings suggest new chemical pathways for methylamine acting on atmospheric aerosols in which the methyl group is the site of orientation specific chemistry involving its conversion into a carbonyl site providing hydrophilic groups for uptake of additional water. This conversion may explain the tendency of aged organic aerosols to form cloud condensation nuclei. At the same time, formation of NH2 radical and formaldehyde is suggested to be a new source for NH2 radicals at aerosol surfaces, other than by reaction of absorbed NH3. The results have general implications for the chemistry of other amphiphilic organics, amines in particular, at the surface of atmospherically relevant aerosol

Utilization of medicines in a program of primary health care in rural- marginal populations

Se determinaron indicadores básicos del uso de medicamentos en Atención Primaria de Salud (APS) en un programa solidario y se implementaron estrategias de Atención Farmacéutica para mejorar la calidad de la dispensación y la educación sanitaria, asegurando el suministro de medicamentos esenciales a una población rural-marginal de Argentina. Se analizaron las prescripciones médicas siguiendo parámetros de la Organización Mundial de la Salud (OMS) sobre el uso de medicamentos, mediante la determinación de tres indicadores básicos: las prácticas de prescripción, la asistencia al paciente y los factores específicos del servicio de Farmacia. El lugar de realización fue la zona rural de Cachi Yaco (Córdoba). La utilización de los indicadores básicos OMS fue una herramienta simple que permitió evaluar con rapidez y seguridad aspectos importantes de la APS. El trabajo coordinado y comprometido de los distintos profesionales del equipo de salud, alumnos y voluntarios, permitió satisfacer en un alto porcentaje el acceso a medicamentos esta comunidad.The aim of this work was to determine the basic indicators of medicines in Primary Health Care (PHC) in a social program, and to implement the strategies of Pharmaceutical Care in order to improve the quality of the dispensation and sanitary education, thus assuring the supply of essential medicines in rural-marginal populations. The method chosen was to apply the parameters of the World Health Organization (WHO) to determine three basic indicators: the practice of prescription, the assistance to the patient and the specific factors of the pharmaceutical service. The place of the study was Cachi Yaco in the province of Córdoba, Argentina. The utilization of the basic indicators of the WHO were a simple tool that allowed us to evaluate quickly important safety aspects of the pharmaceutical practice in the field of the PHC. Using coordinated work of the professionals, students and volunteers, the access to medicines in this community was realized in an efficient way.Colegio de Farmacéuticos de la Provincia de Buenos Aire