We propose a time-dependent boundary condition coupling an atomistic simulation system to linear surroundings such that reflection of elastic waves across the boundary is minimized. Interdomain interactions expressed in terms of memory kernel functions within linear-response theory are treated in a natural dynamical manner, albeit numerically. The approach is shown to give significantly reduced phonon reflections at the domain boundaries relative to existing coupling methods. In addition, we demonstrate that the framework is also effective in the context of static relaxation of displacement fields associated with embedded inhomogeneities.

No abstract

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...

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