Principle Computer Simulations Research Articles

Microscopic Pair Potentials and the Physical Properties of the Condensed Phases of Parahydrogen

Equilibrium physical properties of the solid and liquid phases of parahydrogen, computed by first principle computer simulations, are compared for different choices of pair-wise, spherically symmetric intermolecular potentials. The most recent ab initio potential [Patkowski et al., J. Chem. Phys., 2008, 129, 094304], which has a stiffer repulsive core than the commonly used Silvera-Goldman, yields results for structural quantities in better agreement with the most recent experimental measurements, while possibly overestimating the kinetic energy per molecule by as much as 10%. Altogether, the comparison between theory and the available experimental evidence suggests that the potential of Patkowski et al. may be a better choice for simulations of condensed phases of parahydrogen at moderate pressure.

Mechanical properties of AlMgB14-related boron carbide structures. A first principle study

We examine the effects produced by replacing B–B interlayer bonds by C–C bonds in AlMgB14-related boron network on its mechanical properties. The elastic constants, Vickers hardness and shear strength are evaluated by means of first principle computer simulations on the basis of density functional theory. The results of simulations suggest a possibility of existence of several orthorhombic boron carbide phases with strongly enhanced mechanical properties with the Young's modulus and Vickers hardness being within the range of 550–600 GPa and 43–50 GPa, respectively.

Thin $$^4$$He Films on Alkali Substrates: Where Do $$^3$$He Atoms Bind?

The possible occurrence of bound states of $$^3$$ He atoms in the vicinity of a weakly attractive substrate coated with a thin superfluid $$^4$$ He film is investigated by first principle computer simulations. No evidence is seen of such bound states, even in the case of the weakest substrate, i.e., Cs; a single $$^3$$ He atom always binds to the free $$^4$$ He surface, regardless of the thickness of the $$^4$$ He film. A comparison of $$^4$$ He density profiles computed in this work with those yielded by the Density Functional approach that led to the prediction of $$^3$$ He bound states near the substrate shows that the latter may not have afforded a sufficiently accurate structural description of the adsorbed $$^4$$ He film.

Quasi-2D $$\hbox {H}_2$$: On the Verge of Turning Superfluid?

First principle computer simulations of a thin parahydrogen film adsorbed on a silica substrate at low temperature (below 6 K) yield no evidence that the top layer is liquid and/or in the proximity of a superfluid transition, as claimed in recent experimental work (Makiuchi et al. in Phys Rev Lett 123:245301, 2019). Computed values of first and second layer completion densities are in quantitative agreement with experiment, but as observed also on other substrates, the top layer is an insulating crystal, quantum-mechanical exchanges of molecules are non-existent, and the overall physical behavior of the system can be understood largely along classical lines.

Superfluid Response of Parahydrogen Clusters in Superfluid $$^4$$He

First principle computer simulations yield evidence of a finite superfluid response at low temperature of a parahydrogen cluster of 15 molecules in bulk superfluid He-4. The superfluid fraction is worth ~44% at T=0.25 K, growing to about 66% at T=0.15 K, i.e., it is substantially reduced compared to that of the same cluster in vacuo, due to higher molecular localization. The implications of these findings on the interpretation of experiments with linear molecules embedded in parahydrogen clusters immersed in superfluid helium are discussed.

Properties of bulk liquid Pd and Pt and their free liquid surface studied with first principles techniques

We have performed first principles computer simulations in order to study the structural and dynamic properties of bulk liquid Pd and Pt near their melting points. We find good agreement with the available experimental static structure and transport properties, and furthermore we provide more detailed information that is not available from experiments. Additional simulations have also been undertaken so as to study the free liquid surface of both liquid metals. The calculated longitudinal ionic density profile exhibits an oscillatory behavior whose properties have been analyzed. For both metals, the associated intrinsic surface structure factor presents a marked maximum related to surface layering.

Phases of dipolar bosons in a bilayer geometry

We study by first principle computer simulations the low temperature phase diagram of bosonic dipolar gases in a bilayer geometry, as a function of the two control parameters, i.e., the in-plane density and the interlayer distance. We observe four distinct phases, namely paired and decoupled superfluids, as well as a crystal of dimers and one consisting of two aligned crystalline layers. A direct quantum phase transition from a dimer crystal to two independent superfluids is observed in a relatively wide range of parameters. No supersolid phase is predicted for this system.

Enhanced superfluid response of parahydrogen in nanoscale confinement

Confinement has generally the effect of suppressing order in condensed matter. Indeed, phase transitions such as freezing, or the superfluid transition in liquid helium, occur at lower temperatures in confinement than they do in the bulk. We provide here an illustration of a physical setting in which the opposite takes place. Specifically, the enhancement of the superfluid response of parahydrogen confined to nanoscale size cavities is demonstrated by means of first principle computer simulations. Prospects to stabilize and observe the long sought but yet elusive bulk superfluid phase of parahydrogen in purposefully designed porous media are discussed.

Excitation Spectrum of a Supersolid

Conclusive experimental evidence of a supersolid phase in any known condensed matter system is presently lacking. On the other hand, a supersolid phase has been recently predicted for a system of spinless bosons in continuous space, interacting via a broad class of soft-core, repulsive potentials. Such an interaction can be engineered in assemblies of ultracold atoms, providing a well-defined pathway to the unambiguous observation of this fascinating phase of matter. In this Letter, we study by first principles computer simulations the elementary excitation spectrum of the supersolid, and show that it features two distinct modes, namely, a solidlike phonon and a softer collective excitation, related to broken translation and gauge symmetry, respectively.

Mapping the shape and phase of palladium nanocatalysts

Understanding the size, shape and morphological transformations that may occur under different temperature conditions is vital to ensuring the long term stability of metal nanoparticle catalysts. This information can be conveniently captured in a nanoscale phase diagram, that maps the thermally stable structure as a function of size. This paper presents results of theoretical modeling, based on first principle computer simulations, exploring the equilibrium and non-equilibrium morphologies of palladium nanoparticles over the entire nanoscale. A total of 18 different ideal and imperfect structures and shapes are considered and compared, to reveal a delicate balance between the sphericity, the degree of twinning, and the surface anisotropy. The phase diagram is then discussed in relation to the experiments reported in the literature.

Bonding and structure in BxNy nanotubes (x,y = 1,2)

Although it has been widely established that the electronic properties of carbon nanotubes are strongly linked to their chirality, boron nitride nanotubes, which are structurally similar, are uniformly insulating irrespective of their chiral index. The production of boron nitride nanotubes with metallic or semiconducting behaviour will therefore require more specific intervention, by chemically altering the stoichiometry or distribution of the boron and nitrogen atoms with respect to one another. Some of these chemical and structural combinations will be stable, while others will not. In the present study we use first principles computer simulations to investigate the bonding and structure of BxNy nanotubes, for the case of x,y = 1,2. The results indicate that B2N nanotubes represent viable candidates for synthesis efforts, although the resultant structures are decorated with defective sites produced by the breaking of unstable N–N bonds.

Critical Size of the Phase Transition from Cubic to Tetragonal in Pure Zirconia Nanoparticles

Pure (unstabilized) zirconia nanoparticles in the fluorite-type cubic phase are observed by transmission electron microscopy. Microscopic observations at a pressure of 5 × 10 -5 Pa suggest that the phase transition from the cubic to tetragonal phase arises for particles near 2 nm in diameter. First principles computer simulations of large zirconia clusters show that the critical size of the transition could occur for a diameter of 2.05 ± 0.15 nm, which is in good agreement with the experimental value.

Cascading morphoregulatory energy and the biological form: Theory and reality

It is proposed that biological forms are not determined by a single molecule or a single process but by multiple forms of energies, acting on a network of molecules. The molecular organization of biological form is determined by not only its molecular composition but also the different forms of internal and external energies acting on those molecules. A concept of cascading morphoregulatory energy is proposed which is defined as the quantal, differential energy — total internal minus total external energy — that acts on the biological form at a given moment in time. It is a quantal composite form of energy consisting of chemical, electromagnetic and phononic or quantum fluid mechanical energy. It is further proposed that biological forms are extremely dynamic and are constantly dominated by internal and external flow-induced fluid forces and energy patterns. Some of the well known variables that influence these fluid patterns include internal and external temperature, salinity, proton and other ionic gradients, surface tension, hydrostatic pressures and organic compositions. A biological process and its products form a unit. A biological process serves as a source and its products are subject to local dispersive fluid forces constrained by chaotic streamlines. The process and products together form a unitary biological form. The biomolecular fluid patterning seems to follow curvilinear geometry. On the basis of these general principles computer simulations of most biological structures seem to be feasible. This would provide a good beginning for the physical modelling of biological forms.