Boundary-controlled barostats for slab geometries in molecular dynamics simulations

Abstract

Molecular dynamics simulation barostat schemes are derived for achieving a given normal pressure for a thin liquid or solid layer confined between two parallel walls. This work builds on the boundary-controlled barostat scheme of Lupkowski and van Swol [J. Chem. Phys. 93, 737 (1990)]. Two classes of barostat are explored, one in which the external load is applied to a virtual regular lattice to which the wall atoms are bound using a tethering potential. The other type of barostat applies the external force directly to the wall atoms, which are not tethered. The extent to which the wall separation distribution is Gaussian is shown to be an effective measure of the quality of the barostat. The first class of barostat can suffer from anomalous dynamical signatures, even resonances, which are sensitive to the effective mass of the virtual lattice, whose value lacks any rigorous definition. The second type of barostat performs much better under equilibrium and wall-sliding nonequilibrium conditions and in not being so prone to resonance instabilities in the wall separation and does not require so many largely arbitrary parameters. The results of exploratory simulations which characterize the dynamical response of the model systems for both dry and wet or lubricated systems using the different barostats are presented. The barostats which have an inherent damping mechanism, such as the ones analogous to a damped harmonic oscillator, reduce the occurrence of large fluctuations and resonances in the separation between the two walls, and they also achieve a new target pressure more quickly. Near a nonequilibrium phase boundary the attributes of the barostat can have a marked influence on the observed behavior.

Non-equilibrium phase behavior and friction of confined molecular films under shear: A non-equilibrium molecular dynamics study

“…These conditions may be found in MEMS devices or elastohydrodynamic contacts. 27,31,32 Some of the previously observed NEMD steady states have been detected experimentally. For example, Plug-Slip, 34,35 and Central Localization, 36 steady states have been detected using a novel methodology based on phosphorescence imaging.…”

“…Other NEMD studies have instead made stress-resilient walls by introducing a stronger binding energy between the wall atoms than between the confined molecules (and the cross terms). 18,32 In our calculations the walls are built without any artificial springs attached to the atoms but with a stronger binding energy between the wall atoms than between the confined molecules.…”

“…28,29 Friction arising from a solid lubricant has been modeled using NEMD. 30 NEMD simulations have revealed a rich non-equilibrium steady state phase diagram of the confined system, 31,32 which under pressure and high shear rates can transform into a non-flowing plug with slip at the walls. This we have called the 'Plug Slip' (PS) state.…”

“…The relationship between the friction coefficient and the physical state of the intervening film has been reported by us in Refs. 27,31,32 .…”

“…33 NEMD investigations of the impact of applied pressure and shear rate on the confined film states have been reported previously by us. 27,31,32 Boundary-driven simulations mimicking a typical traction drive experimental arrangement were carried out with thermostatted sliding walls as a means of initiating and sustaining shear flow. The calculations showed that an immobilised plug can adopt various structural arrangements and orientations with respect to the walls.…”