Commun. Comput. Phys., 13 (2013), pp. 129-149.


Effect of the Reaction Field on Molecular Forces and Torques Revealed by an Image-Charge Solvation Model

Wei Song 1, Yuchun Lin 2, Andrij Baumketner 3, Shaozhong Deng 4, Wei Cai 4, Donald J. Jacobs 5*

1 Departments of Bioinformatics and Genomics, University of North Carolina at Charlotte, Charlotte, NC 28262, USA.
2 Departments of Physics and Optical Science, University of North Carolina at Charlotte, Charlotte, NC 28262, USA; and Departments of Mathematics and Statistics, University of North Carolina at Charlotte, Charlotte, NC 28262, USA.
3 Departments of Physics and Optical Science, University of North Carolina at Charlotte, Charlotte, NC 28262, USA. On leave from Institute for Condensed Matter Physics, 1 Svientsitsky Str., Lviv 79011, Ukraine.
4 Departments of Mathematics and Statistics, University of North Carolina at Charlotte, Charlotte, NC 28262, USA.
5 Departments of Physics and Optical Science, University of North Carolina at Charlotte, Charlotte, NC 28262, USA.

Received 29 July 2011; Accepted (in revised version) 18 July 2011
Available online 12 June 2012
doi:10.4208/cicp.290711.180711s

Abstract

We recently developed the Image-Charge Solvation Model (ICSM), which is an explicit/implicit hybrid model to accurately account for long-range electrostatic forces in molecular dynamics simulations [Lin et al., J. Chem. Phys., 131, 154103, 2009]. The ICSM has a productive spherical volume within the simulation cell for which key physical properties of bulk water are reproduced, such as density, radial distribution function, diffusion constants and dielectric properties. Although the reaction field (RF) is essential, it typically accounts for less than 2% of the total electrostatic force on a water molecule. This observation motivates investigating further the role of the RF within the ICSM. In this report we focus on distributions of forces and torques on water molecules as a function of distance from the origin and make extensive tests over a range of model parameters where Coulomb forces are decomposed into direct interactions from waters modeled explicitly and the RF. Molecular torques due to the RF typically account for 20% of the total torque, revealing why the RF plays an important role in the dielectric properties of simulated water. Moreover, it becomes clear that the buffer layer in the ICSM is essential to mitigate artifacts caused by the discontinuous change in dielectric constants at the explicit/implicit interface.


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PACS: 61.20.Gy, 61.20.Ja, 41.20.Cv, 83.10.Rs
Key words: Image charges, reaction field, force distribution, torque distribution, dipole moment.

*Corresponding author.
Email: wsong4@uncc.edu (W. Song), yuchun.lin@berkeley.edu (Y. Lin), abaumket@uncc.edu (A. Baumketner), shaodeng@uncc.edu (S. Deng), wcai@uncc.edu (W. Cai), djacobs1@uncc.edu (D. J. Jacobs)
 

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