Coarse-grained modeling of soft condensed matter

Coarse-graining is an important and growing area in molecular modeling. Coarse-graining can mean a number of different things to different people. It includes modeling using simple models without a lot of detail on the one extreme and can mean integrated multiscale modeling using several interconnected models on the other end. So, there is a variety of many different approaches in coarse-graining of soft materials and this Themed Issue wants to showcase the different approaches in a cohesive fashion.

Coarse-graining of soft matter is clearly a transdisciplinary problem. Chemistry, physics, biology and engineering applications draw from the same fundamental principles. It often appears that similar techniques are “reinvented” several times as researchers are not aware of new techniques being developed in neighboring disciplines. In soft materials typically a large variety of interconnected length scales are relevant even if we are not interested in the “full picture” but only want to understand a certain phenomenon on one length scale. Even in this case other scales cannot be neglected because they have at least an indirect influence, be it that more local scales impose a structure or that larger scales, e.g. by bending, introduce boundary conditions. Also meaningful simulations for a large range of systems would be impossible if all details had to be taken into account; the required computer time would be just of reach. Therefore it comes as no surprise that soft matter scientists have out of necessity traditionally been at the forefront of developing multiscale descriptions for their systems. This issue wants to show both new developments in model building and algorithm as well as modern and meaningful applications of such models and algorithms. We try to balance between the two. The reader will find technical advances in coarse-graining with the promise to solve future simulation problems. You will also find contributions apply coarse-grained models and algorithms to scientific questions we otherwise could not address. Several important themes emerged over the course of putting together this issue without any original intent from the editor—the plan was to cover soft materials in all their breadth. One theme which emerged was the self-assembly of complex systems where the interplay of the local length scale leads to structures on larger scales and both scales need to be described by the model. Another focus which evolved is descriptions of phase behavior which is also clearly not reachable in atomistic simulations.

No coarse-grained modeling, indeed no modeling at all, would be possible without adapted algorithms. So researchers are always on the lookout for improving and testing new and advanced algorithms. Several contributions here are devoted to this field. The groups of Wales, Voth, Müller, van Gunsteren and Qian discuss topics from rigid body simulations, effective force algorithms, free energies at interfaces to dynamical implications of timesteps and equations of motion. Most contributions focus on specific scientific problems sometimes including algorithmic and model advances. Even the seemingly simple problem of small molecules is worth a fresh look, as the paper of Shell shows us by applying relative entropy techniques to spherical water models. A number of papers focus on polymers including the contributions by Paul, Binder, Chen, Wilson, Hooper, Chung, or Mukherji. Polymer science has long been one of the front runners in coarse-graining and coarse-grained descriptions. The topics in this issue range from local structural coarse-graining, cluster formation over the area of polymer phase behavior to questions of ductility in polymer alloys. It is also not surprising that biomolecules are a strong contributor as the life sciences encompass questions from molecules to molecular assemblies, cells and beyond. Several groups including Pandey, Allen, Peter, van der Vegt and Erman report on peptides and proteins; the self assembled structure of lipids, be it membranes or vesicles, are discussed by Risselada or Tieleman. Additionally, several other areas are covered including amphiphiles by Bates and Hyde and clays by Jardat. Karttunen actually reviews several recent developments in a perspective on “Multiscale modeling of emergent materials: biological and soft matter”. This cross-pollination covering a wide variety of fields is something we have to foster as we should try not to reinvent the wheel but make it go further.

I would like to thank the authors of all the papers as well as the PCCP team, especially Nicola Nugent who handled all the administrative problems around this Themed Issue. Without their efforts you, the readers, could not hold this issue in your hands (or have it on your screen). I hope that it stimulates you as much as working on this issue has stimulated me.

Roland Faller, University of California, Davis, USA

Papers in this issue

1	H.-J. Qian et al., DOI: 10.1039/b817584e
2	R. B. Pandey et al., DOI: 10.1039/b816187a
3	W. Paul et al., DOI: 10.1039/b818271j
4	M. S. Shell et al., DOI: 10.1039/b818512c
5	B. M. Mognetti, K. Binder et al., DOI: 10.1039/b818020m
6	J. J. K. Kirkensgaard and S. Hyde, DOI: 10.1039/b818032f
7	M. Jardat, J.-F. Dufrêche et al., DOI: 10.1039/b818055e
8	M. P. Allen et al., DOI: 10.1039/b818067a
9	H. J. Risselada and S. J. Marrink, DOI: 10.1039/b818782g
10	N. F. A. van der Vegt et al., DOI: 10.1039/b818144f
11	C. Peter et al., DOI: 10.1039/b818146m
12	M. Müller et al., DOI: 10.1039/b818111j
13	D. Mukherji and C. F. Abrams, DOI: 10.1039/b818039c
14	J. S. Lintuvuori and M. R. Wilson, DOI: 10.1039/b818616b
15	D. Chakrabarti and D. J. Wales, DOI: 10.1039/b818054g
16	G. A. Voth et al., DOI: 10.1039/b819182d
17	X. Chen et al., DOI: 10.1039/b817895j
18	D. J. Wales et al., DOI: 10.1039/b818062h
19	D. L. Cheung and A. Troisi, DOI: 10.1039/b818428c
20	B. Erman et al., DOI: 10.1039/b820801h
21	W. F. van Gunsteren et al., DOI: 10.1039/b818713d
22	M. A. Bates and M. Walker, DOI: 10.1039/b818926a
23	J. B. Hooper et al., DOI: 10.1039/b818971d
24	D. P. Tieleman et al., DOI: 10.1039/b819767a
25	M. Karttunen et al., DOI: 10.1039/b818051b

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