Adaptive resolution molecular dynamics simulation through coupling to an internal particle reservoir

For simulation studies of (macro) molecular liquids it would be of significant interest to be able to adjust or increase the level of resolution within one region of space, while allowing for the free exchange of molecules between open regions of different resolution or representation. We generalize the adaptive resolution idea and suggest an interpretation in terms of an effective generalized grand canonical approach. The method is applied to liquid water at ambient conditions.

Figure 1

Pictorial representation of the adaptive simulation box and local molecular representation. On the right is the low resolution (coarse-grained) region, indicated by $B$, and on the left is the high resolution (atomistic) region $A$. In the middle is the transition (hybrid) region $H$ with the switching function $w(x)$ (curve in grey).

Figure 2

(a) Density profile for the water simulation for the uncorrected AdResS simulation of water and several iterations of the thermodynamic force with $A$ being the all-atom SPC/E region and $B$ the coarse-grained region. The final density profile shows deviation of less than 0.4%. (The arrow indicates the development of the density with the iteration.) (b) Compensation of the local pressure change between regions by the thermodynamic force. (c) Molecule number fluctuations in slabs (width $\Delta x=1\mathrm{nm}$) along the $x$ axis for pressure corrected and uncorrected coarse-grained models.

Figure 3

Particle number distribution in a slab of width $l=4\mathrm{nm}$ and for an atomistic zone in adaptive resolution with the same width. Both distributions match the expected Gaussian behavior with the same $⟨N⟩$ and variance ${\sigma }^2=\rho k_{B}T{\kappa }_T⟨N⟩$.