Constant Temperature Molecular Dynamics Simulation And Its Application

Based on the basic model of molecular dynamics, we introduce the theoretical model of constant temperature molecular dynamics.Taking a common Lennard-Jones fluid for example, we simulate the evolvement of a constant temperature system. After simulating, we analyse the numerical data, and then we conculde that generalized mass Q introduced in extented Hamiltonian is the inertia of system which determines sensitivity of system to external influence.If Q is bigger, macroscopical physical quantity of system liking temperature and energy fluctuate gently, and system is stable. If Q become smaller, macroscopical physical quantity of system fluctuate more tempestuously, and system is more sensitive to external influence.After studying the theory of constant temperature molecular dynamics,we apply it to practise. Through simulating the progress that molecular gas diffuse in siloxane polymer, we calculate diffusion coefficient of molecular gas in siloxane polymer. Comparing the calculated value with experimental value, we find that for same polymer, if theoretical radius of gas molecule is larger, diffusion coefficient is smaller, and gas molecules diffuse more slowly. For same gas, if glass-transition temperature of polymer increases, diffusion coefficient will decrease, and gas molecules diffuse in siloxane more slowly. Further more, we become conscious of that MSD would change abruptly if simulation is long enough. The explain is that there are many microcavities in siloxane polymer, when molecule jump from one microcavity to another microcavity, MSD would change abruptly.