Canonical Dynamics Simulations of Single-Chain Polyethylene

Cagin-T, Goddard-WA, Ary, ML
Computational Polymer Science, 1, 241-248 (1991).


Conventional microcanonical molecular dynamics (Newtonian dynamics) generates trajectorie confined to a constant energy surface in a 6N-dimensional phase space ( N = number of atoms ). In contrast, canonical dynamics (Nose) utilizes a modified lagrangian which can be shown to yield a canonical distribution of points in phase space. The total energy fluctuates in canoncial dynamics, leading to a rapid equilibration of polymer conformations. We illustrate this feature of canonical dynamics by applying it to study the equilibrium distribution of conformational states of medium length (122 to 302 atoms), single chain ethylene oligomers. For T=300 K, we find that canonical molecular dynamics reaches equilibrium thermal distributions in about 10 ps whereas microcanonical dynamics has not produced an equilibrium distribution in about 1ns simulation. The equilibrium distributions calculated by time-averaging canonical molecular dynamics over 10-30 ps agree with each other and with the results obtained from static molecular mechanics calcualtions (using the same classical interaction potential for polyethylene). < hr>

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