Solvent-scaling as an alternative to coarse-graining in adaptive-resolution simulations: The adaptive solvent-scaling (AdSoS) scheme

Abstract

A new approach termed Adaptive Solvent-Scaling (AdSoS) is introduced for performing simulations of a solute embedded in a fine-grained (FG) solvent region itself surrounded by a coarse-grained (CG) solvent region, with a continuous FG ↔ CG switching of the solvent resolution across a buffer layer. Instead of relying on a distinct CG solvent model, the AdSoS scheme is based on CG models defined by a dimensional scaling of the FG solvent by a factor s, accompanied by an s-dependent modulation of the atomic masses and interaction parameters. The latter changes are designed to achieve an isomorphism between the dynamics of the FG and CG models, and to preserve the dispersive and dielectric solvation properties of the solvent with respect to a solute at FG resolution. This scaling approach offers a number of advantages compared to traditional coarse-graining: (i) the CG parameters are immediately related to those of the FG model (no need to parameterize a distinct CG model); (ii) nearly ideal mixing is expected for CG variants with similar s-values (ideal mixing holding in the limit of identical s-values); (iii) the solvent relaxation timescales should be preserved (no dynamical acceleration typical for coarse-graining); (iv) the graining level NG (number of FG molecules represented by one CG molecule) can be chosen arbitrarily (in particular, NG = s3 is not necessarily an integer); and (v) in an adaptive-resolution scheme, this level can be varied continuously as a function of the position (without requiring a bundling mechanism), and this variation occurs at a constant number of particles per molecule (no occurrence of fractional degrees of freedom in the buffer layer). By construction, the AdSoS scheme minimizes the thermodynamic mismatch between the different regions of the adaptive-resolution system, leading to a nearly homogeneous scaled solvent density s3ρ. Residual density artifacts in and at the surface of the boundary layer can easily be corrected by means of a grid-based biasing potential constructed in a preliminary pure-solvent simulation. This article introduces the AdSoS scheme and provides an initial application to pure atomic liquids (no solute) with Lennard-Jones plus Coulomb interactions in a slab geometry.

Simulation of aqueous solutes using the adaptive solvent-scaling (AdSoS) scheme

“…The solvent-scaling approach for the definition of CG solvent models with arbitrary graining levels and the implementation of this approach into an adaptive-resolution scheme have been described in details in Ref. 82. This information is only briefly summarized below, and the reader is referred to the original publication for more details.…”

“…As detailed in Ref. 82, the scalings of the different physical quantities Q (force-field parameters, configurational variables, thermodynamic observables) follow equations of the form…”

“…In the AdSoS scheme 82 , the scaling factor s i assigned to an atom i is adapted on-the-fly based on the position vector R R R I of the reference atom within molecule I which atom i belongs to, i.e.…”

“…62,63,83 Note that AdResS can also be used in a grand-canonical fashion, where the CG model is replaced by non-interacting tracer particles. 84,85 In a recent article, 82 we introduced a simple and elegant approach to derive CG models of arbitrary graining levels directly based on a given FG solvent model. The mapping involves a scaling of the spatial dimensions by a factor s, with the FG model corresponding to an s-value of one, along with a corresponding s-dependent adjustment of the force-field parameters of the solvent molecule.…”

“…This is not the case for AdSoS, where the scaled solvent in the SG and CG regions provide (by construction of the scaling) appropriate solvation contributions to such a solute (see Fig. 1 The original article 82 provided the theory along with an initial application of AdSoS considering a pure monoatomic solvent (no solute) in a slab geometry. The goal of this article is to extend the AdSoS scheme to more practically relevant situations, involving: (i) a molecular dipolar solvent (e.g.…”