Constant volume molecular dynamics

Vessal2 4 also used a constant volume molecular dynamics method to simulate vitreous silica. The short-range interaction between different ions was modeled by a Buckingham potential. The work employed a three-body potential of the form ... 

An algorithm for performing a constant-pressure molecular dynamics simulation that resolves some unphysical observations in the extended system (Andersen s) method and Berendsen s methods was developed by Feller et al. [29]. This approach replaces the deterministic equations of motion with the piston degree of freedom added to the Langevin equations of motion. This eliminates the unphysical fluctuation of the volume associated with the piston mass. In addition, Klein and coworkers [30] present an advanced constant-pressure method to overcome an unphysical dependence of the choice of lattice in generated trajectories. 

Ti-wadeite, but structural similarity in the case of Zr-wadeite. These differences/ similarities influence the energetics of nucleation, with the result that it is more difficult to form a critical nucleus of Ti-wadeite. Vessal and Dickinson (1994) have undertaken both constant volume and constant pressure molecular dynamics simulations to test this hypothesis. 

For many problems, however, it is more convenient to keep the temperature, pressure, or chemical potential constant, instead of the total energy, volume, and number of particles. Generalizations of the molecular dynamics technique to virtually any ensemble have been developed, and they will be discussed in the following chapters. Of course, constant temperature MD does not conserve the total system energy, allowing it to fluctuate as is required at constant temperature. Similarly, volume is allowed to fluctuate in constant pressure molecular dynamics. The trick is to make these quantities fluctuate in a manner consistent with the probability distribution of the desired ensemble. 

Just as one may wish to specify the temperature in a molecular dynamics simulation, so may be desired to maintain the system at a constant pressure. This enables the behavior of the system to be explored as a function of the pressure, enabling one to study phenomer such as the onset of pressure-induced phase transitions. Many experimental measuremen are made under conditions of constant temperature and pressure, and so simulations in tl isothermal-isobaric ensemble are most directly relevant to experimental data. Certai structural rearrangements may be achieved more easily in an isobaric simulation than i a simulation at constant volume. Constant pressure conditions may also be importai when the number of particles in the system changes (as in some of the test particle methoc for calculating free energies and chemical potentials see Section 8.9). 

In a normal molecular dynamics simulation with repeating boundary conditions (i.e., periodic boundary condition), the volume is held fixed, whereas at constant pressure the volume of the system must fluemate. In some simulation cases, such as simulations dealing with membranes, it is more advantageous to use the constant-pressure MD than the regular MD. Various schemes for prescribing the pressure of a molecular dynamics simulation have also been proposed and applied [23,24,28,29]. In all of these approaches it is inevitable that the system box must change its volume. 

Several material properties exhibit a distinct change over the range of Tg. These properties can be classified into three major categories—thermodynamic quantities (i.e., enthalpy, heat capacity, volume, and thermal expansion coefficient), molecular dynamics quantities (i.e., rotational and translational mobility), and physicochemical properties (i.e., viscosity, viscoelastic proprieties, dielectric constant). Figure 34 schematically illustrates changes in selected material properties (free volume, thermal expansion coefficient, enthalpy, heat capacity, viscosity, and dielectric constant) as functions of temperature over the range of Tg. A number of analytical methods can be used to monitor these and other property changes and... 

The overfired batch conversion process, as well as the combustion process, of wood fuels is shown to be extremely dynamic. The dynamic ranges for the air factor of the conversion system is 10 1 and for the stoichiometric coefficients is CHs.iOiCHoOo during a batch for a constant volume flux of primary air. The dynamics of the stoichiometry indicates the dynamics of the molecular composition of the conversion gas during the course of a run. From the stoichiometry it is possible to conclude that... 

Once the boundary conditions have been implemented, the calculation of solution molecular dynamics proceeds in essentially the same manner as do vacuum calculations. While the total energy and volume in a microcanonical ensemble calculation remain constant, the temperature and pressure need not remain fixed. A variant of the periodic boundary condition calculation method keeps the system pressure constant by adjusting the box length of the primary box at each step by the amount necessary to keep the pressure calculated from the system second virial at a fixed value (46). Such a procedure may be necessary in simulations of processes which involve large volume changes or fluctuations. Techniques are also available, by coupling the system to a Brownian heat bath, for performing simulations directly in the canonical, or constant T,N, and V, ensemble (2,46). 

Atomistic molecular dynamics simulations of one molecule of 1ETN soaked in water were performed under constant volume and temperature conditions (NVT). Details of the simulation can be found elsewhere [13]. 

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