Paul Tangney

An ab initio parametrized interatomic force field for silica

We present a classical interatomic force field for liquid SiO2 which has been parametrized using the forces, stresses and energies extracted from ab initio calculations. We show how inclusion of more electronic effects in a phenomenological way and parametrization at the relevant conditions of pressure and temperature allow the creation of more accurate force fields. We compare the results of simulations with this force field both to experiment and to the results of ab initio molecular dynamics simulations and show how our procedure leads to comparisons which are greatly improved with respect to the most widely used force fields for silica.

How well do Car–Parrinello simulations reproduce the Born–Oppenheimer surface? Theory and examples

We derive an analytic expression for the average difference between the forces on the ions in a Car–Parrinello simulation and the forces obtained at the same ionic positions when the electrons are at their ground state. We show that for common values of the fictitious electron mass, a systematic bias may affect the Car–Parrinello forces in systems where the electron–ion coupling is large. We show that in the limit where the electronic orbitals are rigidly dragged by the ions the difference between the two dynamics amounts to a rescaling of the ionic masses, thereby leaving the thermodynamics intact. We study the examples of crystalline magnesium oxide and crystalline and molten silicon. We find that for crystalline silicon the errors are very small. For crystalline MgO the errors are very large but the dynamics can be quite well corrected within the rigid-ion model. We conclude that it is important to control the effect of the electron mass parameter on the quantities extracted from Car–Parrinello simulat...

Polarizable interatomic force field for TiO2 parametrized using density functional theory

We report a classical interatomic force field for TiO2, which has been parametrized using density functional theory forces, energies, and stresses in the rutile crystal structure. The reliability of this classical potential is tested by evaluating the structural properties, equation of state, phonon properties, thermal expansion, and some thermodynamic quantities such as entropy, free energy, and specific heat under constant volume. The good agreement of our results with ab initio calculations and with experimental data, indicates that our force field describes the atomic interactions of TiO2 in the rutile structure very well. The force field can also describe the structures of the brookite and anatase crystals with good accuracy.

A many-body interatomic potential for ionic systems: Application to MgO

An analytic representation of the short-range repulsion energy in ionic systems is described that allows for the fact that ions may change their size and shape depending on their environment. This function is extremely efficient to evaluate relative to previous methods of modeling the same physical effects. Using a well-defined parametrization procedure we have obtained parameter sets for this energy function that reproduce closely the density functional theory potential energy surface of bulk MgO. We show how excellent agreement can be obtained with experimental measurements of phonon frequencies and temperature and pressure dependences of the density by using this effective potential in conjunction with ab initio parametrization.

A first principles based polarizable O(N) interatomic force field for bulk silica

We present a reformulation of the Tangney-Scandolo interatomic force field for silica [J. Chem. Phys. 117, 8898 (2002)], which removes the requirement to perform an Ewald summation. We use a Yukawa factor to screen electrostatic interactions and a cutoff distance to limit the interatomic potential range to around 10 Å. A reparametrization of the potential is carried out, fitting to data from density functional theory calculations. These calculations were performed within the local density approximation since we find that this choice of functional leads to a better match to the experimental structural and elastic properties of quartz and amorphous silica than the generalized gradient approximation approach used to parametrize the original Tangney-Scandolo force field. The resulting O(N) scheme makes it possible to model hundreds of thousands of atoms with modest computational resources, without compromising the force field accuracy. The new potential is validated by calculating structural, elastic, vibrational, and thermodynamic properties of α-quartz and amorphous silica.

Giant Wave-Drag Enhancement of Friction in Sliding Carbon Nanotubes

Molecular dynamics simulations of coaxial carbon nanotubes in relative sliding motion reveal a striking enhancement of friction when phonons whose group velocity is close to the sliding velocity of the nanotubes are strongly excited. The effect is analogous to the dramatic increase in air drag experienced by aircraft flying close to the speed of sound but differs in that it can occur in multiple velocity ranges with varying magnitude, depending on the atomic level structures of the nanotubes. The phenomenon is a general one that may occur in other nanoscale mechanical systems.

Calculations of the A 1 Phonon Frequency in Photoexcited Tellurium

Calculations of the

Factors influencing the distribution of charge in polar nanocrystals

{A}_{1}

Melting slope of MgO from molecular dynamics and density functional theory.

phonon frequency in photoexcited tellurium are presented. The phonon frequency as a function of photoexcited carrier density and phonon amplitude is determined. Recent pump-probe experiments are interpreted in light of these calculations. It is proposed that, in conjunction with measurements of the phonon period in ultrafast pump-probe reflectivity experiments, the calculated frequency shifts can be used to infer the evolution of the density of photoexcited carriers on a subpicosecond time scale.

Atomistic force field for alumina fit to density functional theory

We perform first-principles calculations of wurtzite GaAs nanorods to explore the factors determining charge distributions in polar nanostructures. We show that both the direction and magnitude of the dipole moment d of a nanorod, and its electric field, depend sensitively on how its surfaces are terminated and do not depend strongly on the spontaneous polarization of the underlying lattice. We identify two physical mechanisms by which d is controlled by the surface termination, and we show that the excess charge on the nanorod ends is not strongly localized. We discuss the implications of these results for tuning nanocrystal properties, and for their growth and assembly.