Scientists in silico?
The end (for human scientists) is nigh? The posit of this discourse is that the majority, if not all, scientific research will eventually be undertaken by one, or a number of, weak artificial intelligences.
physics.pop-ph↗
arXiv subjects
Carl McBride
Publications and source records attributed to Carl McBride.
A study of the influence of isotopic substitution on the melting point and temperature of maximum density of water by means of path integral simulations of rigid models
The melting point of ice Ih, as well as the temperature of maximum density (TMD) in the liquid phase, has been computed using the path integral Monte Carlo method. Two new models are introduced; TIP4PQ_D2O and TIP4PQ_T2O which are specifically designed to study D2O and T2O respectively. We have also used these models to study the "competing quantum effects" proposal of Habershon, Markland and Manolopoulos; the TIP4PQ/2005, TIP4PQ/2005 (D2O) and TIP4PQ/2005 (T2O) models are able to study the isotopic substitution of hydrogen for deuterium or tritium whilst constraining the geometry, while the TIP4PQ_D2O and TIP4PQ_T2O models, where the O-H bond lengths are progressively shortened, permit the study of the influence of geometry (and thus dipole moment) on the isotopic effects. For TIP4PQ_D2O - TIP4PQ/2005 we found a melting point shift of 4.9 K (experimentally the value is 3.68K) and a TMD shift of 6K (experimentally 7.2K). For TIP4PQ_T2O - TIP4PQ/2005 we found a melting point shift of 5.2 K (experimentally the value is 4.49K) and a TMD shift of 7K (experimentally 9.4K).
cond-mat.stat-mech↗
The phase diagram of water from quantum simulations
The phase diagram of water has been calculated for the TIP4PQ/2005 model, an empirical rigid non-polarisable model. The path integral Monte Carlo technique was used, permitting the incorporation of nuclear quantum effects. The coexistence lines were traced out using the Gibbs-Duhem integration method, once having calculated the free energies of the liquid and solid phases in the quantum limit, which were obtained via thermodynamic integration from the classical value by scaling the mass of the water molecule. The resulting phase diagram is qualitatively correct, being displaced to lower temperatures by 15-20K. It is found that the influence of nuclear quantum effects are correlated to the tetrahedral order parameter.
cond-mat.stat-mech↗
Path integral Monte Carlo simulations for rigid rotors and their application to water
In this work the path integral formulation for rigid rotors, proposed by Müser and Berne [Phys. Rev. Lett. {\bf 77}, 2638 (1996)], is described in detail. It is shown how this formulation can be used to perform Monte Carlo simulations of water. Our numerical results show that whereas some properties of water can be accurately reproduced using classical simulations with an empirical potential which, implicitly, includes quantum effects, other properties can only be described quantitatively when quantum effects are explicitly incorporated. In particular, quantum effects are extremely relevant when it comes to describing the equation of state of the ice phases at low temperatures, the structure of the ices at low temperatures, and the heat capacity of both liquid water and the ice phases. They also play a minor role in the relative stability of the ice phases.
cond-mat.stat-mech↗
Non-Markovian melting: a novel procedure to generate initial liquid like phases for small molecules for use in computer simulation studies
Computer simulations of liquid phases require an initial configuration from which to begin. The preparation of such an initial configuration or `snapshot' often involves the melting of a solid phase. This melting is usually undertaken by heating the system at low pressure, followed by a lengthy re-compression and cooling once the melt has formed. This note looks at a novel technique to produce a liquid phase from a perfect crystal using a standard Monte Carlo simulation code.
cond-mat.stat-mech↗
Quantum contributions in the ice phases: the path to a new empirical model for water -- TIP4PQ/2005
With a view to a better understanding of the influence of atomic quantum delocalisation effects on the phase behaviour of water, path integral simulations have been undertaken for almost all of the known ice phases using the TIP4P/2005 model, in conjunction with the rigid rotor propagator proposed by Muser and Berne [Phys. Rev. Lett. 77, 2638 (1996)]. The quantum contributions then being known, a new empirical model of water is developed (TIP4PQ/2005) which reproduces, to a good degree, a number of the physical properties of the ice phases, for example densities, structure and relative stabilities.
cond-mat.stat-mech↗
Anomalies in water as obtained from computer simulations of the TIP4P/2005 model: density maxima, and density, isothermal compressibility and heat capacity minima
The so-called thermodynamic anomalies of water form an integral part of the peculiar behaviour of this both important and ubiquitous molecule. In this paper our aim is to establish whether the recently proposed TIP4P/2005 model is capable of reproducing a number of these anomalies. Using molecular dynamics simulations we investigate both the maximum in density and the minimum in the isothermal compressibility along a number of isobars. It is shown that the model correctly describes the decrease in the temperature of the density maximum with increasing pressure. At atmospheric pressure the model exhibits an additional minimum in density at a temperature of about 200K, in good agreement with recent experimental work on super-cooled confined water. The model also presents a minimum in the isothermal compressibility close to 310K. We have also investigated the atmospheric pressure isobar for three other water models; the SPC/E and TIP4P models also present a minimum in the isothermal compressibility, although at a considerably lower temperature than the experimental one. For the temperature range considered no such minimum is found for the TIP5P model.
cond-mat.stat-mech↗
The properties of fully flexible Lennard-Jones chains in the solid phase: Wertheim theory and simulation
NpT ensemble Monte Carlo simulations were performed for fully flexible Lennard-Jones chains in the solid phase. The bond length between monomers within the chains is fixed to $L=σ$ and the molecule is free to adopt any configuration. The solid structure of fully flexible chains is obtained by randomly locating the bonds of the chain within a face centered cubic close packed arrangement of atoms. For fully flexible chains it is believed that the stable solid phase is disordered. Such a solid is considered in this work. Computer simulations were performed for chains with $m=3, 4$ and 5 monomer units, and results were obtained for the equation of state and internal energy of the chains. An extension of Wertheim's TPT1 to the solid phase of Lennard-Jones chains (C.Vega, F.J.Blas and A.Galindo, J.Chem.Phys.,{\bf 116},7645,2002) has been proposed recently. The simulation results of this work provide a check on the performance of this theory. It is found that Wertheim's TPT1 successfully predicts the equation of state and internal energies of fully flexible LJ chains in the solid phase. Finally, a rigid LJ chain in a linear configuration has been considered. Computer simulations were also performed for the rigid chain in an ordered solid structure. It is found that fully flexible and linear rigid chains present quite different equations of state and different thermodynamic properties in the solid phase.
cond-mat.stat-mech↗
The range of meta stability of ice-water melting for two simple models of water
A number of crystal structures of water have been `superheated' in Monte Carlo simulations. Two well known models for water were considered; namely the TIP4P model and the SPC/E model. By comparing the fluid-solid coexistence temperature to the temperature at which the solid becomes mechanically unstable and melts it is possible to determine the typical range of temperatures over which is possible to superheat the ice phases in conventional simulation studies. It is found that the ice phases can be superheated to approximately 90K beyond the fluid-solid coexistence temperature. Beyond this limit they spontaneously melt. This limit appears to depend weakly both on the type of ice phase considered and on the chosen model. Obviously only rigorous free energy calculations can determine the equilibrium fluid-solid coexistence of a model. However, a "rule of thumb" is that, by subtracting 90K from the mechanically stability limit of the the ice phase one is provided with a first guess as to the equilibrium fluid-solid coexistence temperature.
cond-mat.stat-mech↗
wikiFactor: a measure of the importance of a wiki site
A new metric is introduced, inspired by the Hirsch {\it h-index}, to measure the impact of a wiki site. A table of wikiFactors is presented for a number of wikis, in particular those oriented towards scientific topics. The wikiFactor is defined as the number of web pages, wF, that have had $\ge 1000$wF visits.
physics.soc-ph↗
Hard biaxial ellipsoids revisited: numerical results
Monte Carlo simulations are performed for hard ellipsoids for a number of values of its semi-axes in the range $c/a \in \{0.1,10\}$. The isotropic phase results are compared to the Vega equation of state [Mol. Phys. {\bf 92} (1997) 651-665]. The position of the isotropic-nematic transition is also evaluated. The biaxial phase is seen to form only after the previous formation of a discotic phase.
cond-mat.soft↗
Computers and Liquid State Statistical Mechanics
The advent of electronic computers has revolutionised the application of statistical mechanics to the liquid state. Computers have permitted, for example, the calculation of the phase diagram of water and ice and the folding of proteins. The behaviour of alkanes adsorbed in zeolites, the formation of liquid crystal phases and the process of nucleation. Computer simulations provide, on one hand, new insights into the physical processes in action, and on the other, quantitative results of greater and greater precision. Insights into physical processes facilitate the reductionist agenda of physics, whilst large scale simulations bring out emergent features that are inherent (although far from obvious) in complex systems consisting of many bodies. It is safe to say that computer simulations are now an indispensable tool for both the theorist and the experimentalist, and in the future their usefulness will only increase. This chapter presents a selective review of some of the incredible advances in condensed matter physics that could only have been achieved with the use of computers.
cond-mat.stat-mech↗