Slow relaxation dynamics molecular systems

The nature of the excitation has a profound influence on the subsequent relaxation of molecular Uquid systems, as the molecular dynamics simulations show. This influence can be exerted at field-on equiUbrium and in decay transients (the deexdtation effect). GrigoUni has shown that the effect of high-intensity excitation is to slow the time decay of the envelope of such oscillatory functions as the angular velocity autocorrelation function. The effect of high-intensity pulses is the same as that of ultrafast (subpicosecond laser) pulses. The computer simulation by Abbot and Oxtoby shows that... 

Molecular dynamics is a true first principles dynamic molecular model. It simply solves the equations of motion. Given an intermolecular potential, MD provides the exact spatial and temporal evolution of the system. The stiffness caused by fast vibrations compared with slow molecular relaxations demands relatively small time steps and challenges current simulations. As an example, the time scale associated with vibrations is a fraction of a picosecond, whereas those associated with diffusion or reaction may easily be in the seconds to hours range depending on the activation energy. Consequently, MD on a single processor is usually limited to short time and length scales (e.g., pico- to nanoseconds and 1-2 nm). 

Dynamic Monte Carlo simulations were first used by Verdier and Stockmayer (5) for lattice polymers. An alternative dynamical Monte Carlo method has been developed by Ceperley, Kalos and Lebowitz (6) and applied to the study of single, three dimensional polymers. In addition to the dynamic Monte Carlo studies, molecular dynamics methods have been used. Ryckaert and Bellemans (7) and Weber (8) have studied liquid n-butane. Solvent effects have been probed by Bishop, Kalos and Frisch (9), Rapaport (10), and Rebertus, Berne and Chandler (11). Multichain systems have been simulated by Curro (12), De Vos and Bellemans (13), Wall et al (14), Okamoto (15), Kranbu ehl and Schardt (16), and Mandel (17). Curro s study was the only one without a lattice but no dynamic properties were calculated because the standard Metropolis method was employed. De Vos and Belleman, Okamoto, and Kranbuehl and Schardt studies included dynamics by using the technique of Verdier and Stockmayer. Wall et al and Mandel introduced a novel mechanism for speeding relaxation to equilibrium but no dynamical properties were studied. These investigations indicated that the chain contracted and the chain dynamic processes slowed down in the presence of other polymers. 

Very accurate PCS measurements for different T and P conditions were carried out on different molecular glass-forming systems by Patkowski and co-workers, including epoxy oligomers [129-131], and the van der waals liquids PDE [132,133], BMPC [134], and BMMPC [135], In most of the investigated systems, master curves are obtained for kww(T,P) of the a-relaxation plotted versus xa(T,P), with the value decreasing (broader dispersion) as the dynamics slow (longer xa(T,P).). This is another evidence that the a-dispersion is directly related to the relaxation time. 

From a theoretical standpoint, block copolymers are ideal systems for studying many fundamental issues in the thermodynamics and dynamics of self-assembly of soft materials. This is so because of the relatively large length scale of macromolecules, the slow dynamics associated with the relevant structural relaxations, and the ease of control of the molecular characteristics, such as the molecular weight, compositions, and architecture. 

Molecular dynamics (MD), while computationally very expensive, is often used to relax structures, relieve local strain, and refine models of macro-molecular structures or complexes of these with ligands docked into the receptor site. Basically, MD consists of solving Newton s equations of motion for each atom in a system as a function of time and energy funaions describing interatomic forces. > While calculating the atomic trajeaories is slow, once calculated these can be stored and often played back in real time, thus creating a picture of the system as it evolves over time. 

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