Interplay of local hydrogen-bonding and long-ranged dipolar forces in simulations of confined water

Abstract

Spherical truncations of Coulomb interactions in standard models for water permit efficient molecular simulations and can give remarkably accurate results for the structure of the uniform liquid. However, truncations are known to produce significant errors in nonuniform systems, particularly for electrostatic properties. Local molecular field (LMF) theory corrects such truncations by use of an effective or restructured electrostatic potential that accounts for effects of the remaining long-ranged interactions through a density-weighted mean field average and satisfies a modified Poisson's equation defined with a Gaussian-smoothed charge density. We apply LMF theory to 3 simple molecular systems that exhibit different aspects of the failure of a naïve application of spherical truncations-water confined between hydrophobic walls, water confined between atomically corrugated hydrophilic walls, and water confined between hydrophobic walls with an applied electric field. Spherical truncations of 1/r fail spectacularly for the final system, in particular, and LMF theory corrects the failings for all three. Further, LMF theory provides a more intuitive way to understand the balance between local hydrogen bonding and longer-ranged electrostatics in molecular simulations involving water.effective short-ranged model | spherical truncation | confinement | hydrophobic walls | electric field A n accurate and efficient treatment of Coulomb interactions in molecular simulations represents an important and ongoing challenge. Standard biomolecular simulation packages like CHARMM (1) and AMBER (2), in general, assign effective point charges to interaction sites even in neutral molecules to approximate the charge separation of the electron cloud along polar bonds. Effective point charges are also found in most standard water models such as the extended simple point charge (SPC/E) model (3) shown in Fig. 1A, and water molecules are increasingly being included explicitly in biomolecular simulations.To minimize edge effects in necessarily finite simulation cells, these simulations use periodic boundary conditions (4). Researchers have long sought to devise spherical truncations of the Coulomb 1/r potential so that the truncated potential accurately describes the strong Coulomb forces between closely spaced charges that lead to hydrogen bonding in water but then vanish quickly beyond some properly chosen cutoff radius. Then, only the minimum (closest) image of an interacting charge needs to be accounted for and fast and efficient simulations that scale linearly with system size are possible. In bulk-like systems these spherical truncation approaches can be surprisingly accurate, but standard estimates of thermodynamic properties in the bulk liquid are less satisfactory (5-12).However, it has long been established (13) that when these spherical truncations are employed in nonuniform geometries, such as near a lipid bilayer, there can be pronounced errors in structure, thermodynamics, and particularly electrostatic properties. In those ...

Understanding the Phase Behavior of Coarse-Grained Model Lipid Bilayers through Computational Calorimetry

“…The truncation makes the simulations much more efficient, so we employ this here as we are not examining charge density effects themselves in the simulations and the results for many properties with and without long-ranged interactions were found to be similar for this model. 44 However, this is not strictly correct and may lead to errors; 45,46 to capture the true longrange character of charges and polarization, it is essential to use special techniques such as Ewald summation or particle mesh methods.…”

Long-ranged contributions to solvation free energies from theory and short-ranged models

“…However, small differences in the nonuniform water density ρðrÞ can be seen. These arise because the long-ranged electrostatics generates subtle long wavelength perturbations of the local hydrogen bond network around the solute to produce dielectric screening and related effects (30)(31)(32).…”

Nanoflow hydrodynamics

“…From this we see that the hydrophilic wall-fluid interactions introduce an additional orientation that corresponds to a deviation in the order of ∼5% away from a perfectly uniform orientation. The relatively small increased orientation for a zero field system is likely linked to the Wolf method applied [32,33] and is removed when the field is switched on. It is important to point out that we have applied the Wolf method in order to enable a direct comparison between the NEMD simulations and the continuum prediction based on the transport coefficients in Ref.…”