Computer Physics Communications 178 (2008) 724–731 www.elsevier.com/locate/cpc An investigation of soft-core potentials for the simulation of mesogenic molecules and molecules composed of rigid and flexible segments Zak E. Hughes a , Lorna M. Stimson b,c , Henk Slim a , Juho S. Lintuvuori a , Jaroslav M. Ilnytskyi d , Mark R. Wilson a,∗ a Department of Chemistry, University of Durham, South Road, Durham, DH1 3LE, UK b Biophysics and Statistical Mechanics Group, Department of Applied Maths, University of Western Ontario, 1151 Richmond Street North, London (ON), Canada c Laboratory of Physics, Helsinki University of Technology, P.O. Box 9203, Espoo 02170, Finland d Institute for Condensed Matter Physics, Nat. Acad. Sci. of Ukraine, 1 Svientsitskii Street, 79011 Lviv, Ukraine Received 5 September 2007; received in revised form 21 December 2007; accepted 16 January 2008 Available online 2 February 2008 Abstract The phase behaviour of three soft core spherocylinder models is investigated with a view to producing an effective potential for use in coarse- grained simulations of liquid crystal phases and polymers composed of rigid and flexible segments. Provided potentials are not made too soft, two of the soft core models are found to work well in terms of successfully reproducing mesophases and in providing considerable improvements in computational speed over other commonly used coarse-grained models. In Monte Carlo simulations a soft-core spherocylinder model in which a cut and shifted Lennard–Jones potential is truncated with a linear tangential potential is found to be particularly effective; while for molecular dynamics a better model is provided by a DPD-like quadratic potential. Here, computational speed-ups of 20–30× are seen in equilibration times in comparison to the well-known soft repulsive spherocylinder (SRS) model. The quadratic potential is used in an additional set of coarse-grained simulations of a liquid crystal with a flexible chain, which exhibits spontaneous formation of a nematic phase. The use of different types of interaction sites is also illustrated by the simulation of a spherocylinder with two “tails” formed from spheres. Here, varying the hardness of the sphere-spherocylinder interaction potential allows the formation of a smectic-A phase which exhibits microphase separation.  2008 Elsevier B.V. All rights reserved. PACS: 07.05.Tp; 02.70.Ns; 61.30.-v; 64.70.M- Keywords: Liquid crystals; Spherocylinder; Soft-core potential; Molecular dynamics; Monte Carlo; Phase diagram; Phase transition 1. Introduction Rapid increases in computer power have led in recent years to the simulation of a whole range of complex fluids at a mole- cular level: including thermotropic and lyotropic liquid crystals [1,2], lipid bilayers and vesicles [3–6] and mesophases formed by block co-polymers [7]. For many such systems, atomistic simulation is still prohibitively slow; and would require simu- lations of many thousands (or even millions) of atoms simply to see the formation of complex mesophases. Moreover, the * Corresponding author. E-mail address: mark.wilson@durham.ac.uk (M.R. Wilson). equilibration times required for such models are sometimes unfeasibly long, with molecular dynamics simulations requir- ing equilibration times of 100 ns or greater [8,9], beyond what can reasonably be achieved using atomistic modelling. Hence, a range of coarse-grained models have been developed, which keep some of the molecular details but reduce the complexity of the system studied. In the field of liquid crystals, hard non- spherical potentials such as hard-spherocylinders [10,11] and soft-nonspherical models such as the Gay–Berne potential [12– 15] are most prominent; while for polymers and lipids, varieties of bead-spring models employing hard or soft beads have been widely used [16,17]. Despite the success of the above models, the computer time needed for studying mesophases is still huge and for large mole- 0010-4655/$ – see front matter  2008 Elsevier B.V. All rights reserved. doi:10.1016/j.cpc.2008.01.047