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Divacancies are among the most important defects that alter the charge transport properties of single-walled carbon nanotubes (SWNT), and we here study, using ab initio calculations, their properties. Two structures were investigated, one that has two pentagons side by side with an octagon (585) and another composed of three pentagons and three heptagons (555777). We investigate their stability as a function of tube diameter, and calculate their charge transport properties. The 585 defect is less stable in graphene due to two broken bonds in the pentagons. We estimate that the 555777 becomes more stable than the 585 for a diameter of about 40 Å (53 Å) for an armchair (zigzag) SWNTs, indicating that they will prevail in large diameter multiwalled carbon nanotubes and graphene ribbons.

We present a method, for highly efficient free-energy calculations by means of molecular dynamics and Monte Carlo simulations, which is an optimized combination of coupling parameter and adiabatic switching formalisms. This approach involves dynamical reversible scaling of the potential energy function of a system of interest, and allows accurate determination of its free energy over a wide temperature interval from a single simulation. The method is demonstrated in two applications: crystalline Si at zero pressure and a fcc nearest-neighbor antiferromagnetic Ising model. PACS numbers: 02.70.Lq, 02.70.Ns, 65.50. + m In the study of thermodynamic properties of materials [1,2], free-energy calculation is a unique application of computer simulation techniques. For this purpose, the coupling parameter formalism [2,3] provides a powerful and robust framework which underlies state-of-the-art techniques such as thermodynamic integration (TI) [2] and adiabatic switching (AS) [4][5][6][7][8][9][10]. Standard application of this approach involves the evaluation of reversible work along a path connecting a physical system of interest to a reference. Usually, the path is constructed using a composite Hamiltonian H͑l͒ coupling the two systems through a parameter l. Upon varying l the coupled system evolves along a reversible trajectory, changing continuously from the system of interest to the reference. The reversible work done by the generalized force ≠H͞≠l along this path is then equal to the free-energy difference between the systems. While this approach is very powerful, it is not optimal in that only the initial and final points on the trajectory correspond to physically relevant systems. The information gathered at the intermediate states of the path has no physical meaning, serving only to connect the end points of the path. As a consequence, one obtains only one value of the desired free energy per simulation.In this Letter we describe a formulation which fully utilizes all the information available along a reversible path and thereby allows the evaluation of free energies over a wide temperature interval from a single simulation. This approach, which effectively exploits both the coupling parameter formalism and the adiabatic switching technique, is based on the use of a specific path which is defined by the introduction of a scaling factor l in the potential energy function of the physical system of interest. The fundamental difference from the usual coupling approach is that for this particular path all intermediate states provide physically relevant information. In fact, an exact relation between the partition functions of the original and the scaled systems shows that all the states along the scaling path correspond to the original physical system at different temperatures. The combination of this reversiblescaling concept with the dynamical variation of l within the adiabatic switching method results in a highly optimized technique with a significant efficiency gain without loss of accuracy.The idea of determ...

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We study the application of the adiabatic switching molecular dynamics method to determine bulk and vacancy-formation Gibbs free energies as a function of temperature at zero pressure for copper. The bulk free energy has been determined through isochoric isothermal switching procedures in which a system consisting of 500 copper atoms interacting through a semiempirical tight-binding potential is turned into a system of 500 independent identical three-dimensional harmonic oscillators. The equilibrium volumes of these simulations were determined from equilibrium isobaric isothermal molecular dynamics simulations. The frequency of the oscillators is chosen to be of the order of a principal phonon frequency of copper in order to achieve competitive convergence. The resulting bulk free energy and entropy are in excellent agreement with experimental values. The vacancy-formation Gibbs free energy has been computed from isobaric isothermal switching procedures in which the interactions of a single copper atom are switched off. Considering the limited accuracy of the interatomic potential and the numerical noise present in the small energy differences measured, the estimated formation enthalpies and entropies agree remarkably well with experimental data. The method has shown to be computationally efficient. Typically, 6 h of CPU time on a Digital Alpha 3000/900 were required per data point for the bulk as well as the vacancy-formation parameters. ͓S0163-1829͑97͒07901-0͔

In this paper we investigate the behavior of an Einstein crystal as a reference system in adiabatic switching procedures. We study the canonical massive Nose-Hoover chain (MNHC) dynamics [G.J.

The effects of hydrostatic pressure on the energetics of self-diffusion in silicon are investigated via parameter-free total-energy calculations. The three microscopic mechanisms, vacancy, interstitial, and concerted exchange, which have very similar activation energies in Si, exhibit different pressure dependences.The results suggest that a set of experiments carried out at different pressures can unravel their relative contributions by a comparison to the present results. In addition, it is shown that in contrast to the (111) surface, the nearest neighbors of the Si vacancy relax inwards, rather than outwards.Self-diffusion in silicon has been the subject of extensive experimental and theoretical studies. Various microscopic mechanisms have been considered, but no consensus has yet been achieved regarding their relative importance. Experimentally, the measured activation enthalpy varies from 4.1 to 5.1 eV (Ref. 1) depending on the measurement technique and temperature range. Due to the nature of the bonding in silicon, it was initially thought that native defects mediate self-diffusion. In addition, it was shown experimentally that the concentration of native defects is small, and therefore the contributions of complex defects, e.g. , divacancies, are negligible.Theoretical studies " obtained very similar activation enthalpies for the vacancy and interstitial mechanisms, as well as proposed a new one, namely that of concerted exchange. All three mechanisms have activation energies in the experimentally observed range. Therefore, due to both experimental uncertainties and theoretical approximations, it has not been possible to determine the relative contributions of these mechanisms to the self-diffusion process. In this paper we present the results of parameter-free totalenergy calculations of the effects of hydrostatic pressure on the energetics of these mechanisms. Although the three mechanisms have very similar activation enthalpies at zero pressure, the response to external pressure of each mechanism is qualitatively different, making the determination of their relative contributions possible. The use of pressure as a probe to investigate diffusive processes in solids is not new. Several results for metals and ionic compounds were reported over twenty years ago. In semiconductors, however, the 6rst results of pressure effects on self-diffusion in germanium' and silicon, and on diffusion of As in silicon, have appeared only recently.In the high-temperature regime the self-diffusion coef6cient can be written as Dsa Doexp( -hH, d/kT), where hH, d denotes the activation enthalpy. The prefactor Do in (1) is proportional to exp(hS, d/k ), where AS,4 is the activation entropy. For diffusion involving defects, the activation enthalpy is the sum of the formation enthalpy dHf and the migration enthalpy hM .We will now analyze the process whereby a native defeet is created in a crystal and the effect of an external pressure on its formation energy. When a vacancy is created in a dislocation-free crystal, one atom leav...

We present a dynamic implementation of the Clausius–Clapeyron integration (CCI) method for mapping out phase-coexistence boundaries through a single atomistic simulation run. In contrast to previous implementations, where the reversible path of coexistence conditions is generated from a series of independent equilibrium simulations, dynamic Clausius–Clapeyron integration (d-CCI) explores an entire coexistence boundary in a single nonequilibrium simulation. The method gives accurately the melting curve for a system of particles interacting through the Lennard-Jones potential. Furthermore, we apply d-CCI to compute the melting curve of an ab initio pair potential for argon and verify earlier studies on the effects of many-body interactions and quantum effects in the melting of argon. The d-CCI method shows to be effective in both applications, giving converged coexistence curves spanning a wide range of thermodynamic states from relatively short nonequilibrium simulations.