TopoGromacs: Automated Topology Conversion from CHARMM to GROMACS within VMD.

Molecular dynamics (MD) simulation engines use a variety of different approaches for modeling molecular systems with force fields that govern their dynamics and describe their topology. These different approaches introduce incompatibilities between engines, and previously published software bridges the gaps between many popular MD packages, such as between CHARMM and AMBER or GROMACS and LAMMPS. While there are many structure building tools available that generate topologies and structures in CHARMM format, only recently have mechanisms been developed to convert their results into GROMACS input. We present an approach to convert CHARMM-formatted topology and parameters into a format suitable for simulation with GROMACS by expanding the functionality of TopoTools, a plugin integrated within the widely used molecular visualization and analysis software VMD. The conversion process was diligently tested on a comprehensive set of biological molecules in vacuo. The resulting comparison between energy terms shows that the translation performed was lossless as the energies were unchanged for identical starting configurations. By applying the conversion process to conventional benchmark systems that mimic typical modestly sized MD systems, we explore the effect of the implementation choices made in CHARMM, NAMD, and GROMACS. The newly available automatic conversion capability breaks down barriers between simulation tools and user communities and allows users to easily compare simulation programs and leverage their unique features without the tedium of constructing a topology twice.

Surfactin at the Water/Air Interface and in Solution.

The lipopeptide surfactin produced by certain strains of Bacillus subtillis is a potent biosurfactant with high amphiphilicity and a strong tendency for self-aggregation. Surfactin possesses a number of valuable biological properties such as antiviral, antibacterial, antifungal, and hemolytic activities. Owing to these properties, in addition to the general advantages of biosurfactants over synthetic surfactants, surfactin has potential biotechnological and biomedical applications. Here, the aggregation properties of surfactin in solution together with its behavior at the water/air interface were studied using classical molecular dynamics simulations (MD) at three different pH values. Validation of the MD structural data was performed by comparing neutron reflectivity and volume fraction profiles computed from the simulations with their experimental counterparts. Analysis of the MD trajectories supported conclusions about the distribution, conformations, and interactions of surfactin in solution and at the water-air interface. Considering altogether, the work presented provides atomistic models for the rationalization of some of the structural and dynamic characteristics as well as the modes of action of surfactin at different pH values.

Type Label Framework for Bonded Force Fields in LAMMPS.

New functionality is added to the LAMMPS molecular simulation package, which increases the versatility with which LAMMPS can interface with supporting software and manipulate information associated with bonded force fields. We introduce the "type label" framework that allows atom types and their higher-order interactions (bonds, angles, dihedrals, and impropers) to be represented in terms of the standard atom type strings of a bonded force field. Type labels increase the human readability of input files, enable bonded force fields to be supported by the OpenKIM repository, simplify the creation of reaction templates for the REACTER protocol, and increase compatibility with external visualization tools, such as VMD and OVITO. An introductory primer on the forms and use of bonded force fields is provided to motivate this new functionality and serve as an entry point for LAMMPS and OpenKIM users unfamiliar with bonded force fields. The type label framework has the potential to streamline modeling workflows that use LAMMPS by increasing the portability of software, files, and scripts for preprocessing, running, and postprocessing a molecular simulation.

Self-assembly of charged surfactants: full comparison of molecular simulations and scattering experiments.

We present a study of the self-assembly of charged surfactants by a combination of molecular simulations and anomalous small-angle X-ray scattering (ASAXS). Solvent-free grand canonical Monte Carlo simulations are used to obtain the equilibrium structure of tetradecyltrimethylammonium bromide (TTAB) micelles. Subsequent molecular dynamics simulations of multiple micelles were used to calculate the scattering intensity obtained at low surfactant concentrations (17 mM). In particular, the partial intensities of the macroion formed by the surfactant and the counterions were derived and compared directly to experimental data obtained from ASAXS, which revealed reasonably good agreement. Emphasis is put on the fluctuations of the spatial distributions of the surfactant molecules and the counterions, respectively. Criteria for the assessment of these fluctuations are given and compared to simulations and experiments. We found that fluctuations are mainly caused by counterions and need to be accounted for the correct interpretation of scattering data. It is demonstrated that the combination of molecular simulations with ASAXS leads to a comprehensive understanding of self-assembly in systems of charged surfactants.

Ab initio molecular dynamics study of supercritical carbon dioxide including dispersion corrections.

Density functional theory based ab initio molecular dynamics simulations with explicit inclusion of empirical van der Waals (vdW) corrections of supercritical carbon dioxide have been performed. Significant changes in the intermolecular pair correlation functions of the fluid modeled with the added vdW interactions are observed, particularly at lower densities. The closest neighbor to a molecule is seen to be oriented in a distorted T-shaped geometry, consistent with earlier experiments and simulations.

Inverse temperature transition of a biomimetic elastin model: reactive flux analysis of folding/unfolding and its coupling to solvent dielectric relaxation.

The inverse temperature transition (ITT) of a biomimetic model for elastin, capped GVG(VPGVG) in liquid water, is investigated by a comprehensive classical molecular dynamics study. The temperature dependence of the solvation structure and dynamics of the octapeptide are compared using three common force fields, CHARMM, GROMOS, and OPLS. While these force fields differ in quantitative detail, they all predict this octapeptide to undergo a "folding transition" to closed conformations upon heating and a subsequent "unfolding transition" to open conformations at still higher temperatures, thus reproducing the ITT scenario. The peptide kinetics is analyzed within the reactive flux formalism applied to the largest-amplitude mode extracted from principal component analysis, and the solvent's dielectric fluctuations are obtained from the total water dipole autocorrelations. Most importantly, preliminary evidence for an intimate coupling of peptide folding/unfolding dynamics, and thus the ITT, and dielectric relaxation of bulk water is given, possibly being consistent with a "slave mode" picture.

Reconsidering Dispersion Potentials: Reduced Cutoffs in Mesh-Based Ewald Solvers Can Be Faster Than Truncation.

Long-range dispersion interactions have a critical influence on physical quantities in simulations of inhomogeneous systems. However, the perceived computational overhead of long-range solvers has until recently discouraged their implementation in molecular dynamics packages. Here, we demonstrate that reducing the cutoff radius for local interactions in the recently introduced particle-particle particle-mesh (PPPM) method for dispersion [Isele-Holder et al., J. Chem. Phys., 2012, 137, 174107] can actually often be faster than truncating dispersion interactions. In addition, because all long-range dispersion interactions are incorporated, physical inaccuracies that arise from truncating the potential can be avoided. Simulations using PPPM or other mesh Ewald solvers for dispersion can provide results more accurately and more efficiently than simulations that truncate dispersion interactions. The use of mesh-based approaches for dispersion is now a viable alternative for all simulations containing dispersion interactions and not merely those where inhomogeneities were motivating factors for their use. We provide a set of parameters for the dispersion PPPM method using either ik or analytic differentiation that we recommend for future use and demonstrate increased simulation efficiency by using the long-range dispersion solver in a series of performance tests on massively parallel computers.

Fast Analysis of Molecular Dynamics Trajectories with Graphics Processing Units-Radial Distribution Function Histogramming.

The calculation of radial distribution functions (RDFs) from molecular dynamics trajectory data is a common and computationally expensive analysis task. The rate limiting step in the calculation of the RDF is building a histogram of the distance between atom pairs in each trajectory frame. Here we present an implementation of this histogramming scheme for multiple graphics processing units (GPUs). The algorithm features a tiling scheme to maximize the reuse of data at the fastest levels of the GPU's memory hierarchy and dynamic load balancing to allow high performance on heterogeneous configurations of GPUs. Several versions of the RDF algorithm are presented, utilizing the specific hardware features found on different generations of GPUs. We take advantage of larger shared memory and atomic memory operations available on state-of-the-art GPUs to accelerate the code significantly. The use of atomic memory operations allows the fast, limited-capacity on-chip memory to be used much more efficiently, resulting in a fivefold increase in performance compared to the version of the algorithm without atomic operations. The ultimate version of the algorithm running in parallel on four NVIDIA GeForce GTX 480 (Fermi) GPUs was found to be 92 times faster than a multithreaded implementation running on an Intel Xeon 5550 CPU. On this multi-GPU hardware, the RDF between two selections of 1,000,000 atoms each can be calculated in 26.9 seconds per frame. The multi-GPU RDF algorithms described here are implemented in VMD, a widely used and freely available software package for molecular dynamics visualization and analysis.

Micellization Studied by GPU-Accelerated Coarse-Grained Molecular Dynamics.

The computational design of advanced materials based on surfactant self-assembly without ever stepping foot in the laboratory is an important goal, but there are significant barriers to this approach, because of the limited spatial and temporal scales accessible by computer simulations. In this paper, we report our work to bridge the gap between laboratory and computational time scales by implementing the coarse-grained (CG) force field previously reported by Shinoda et al. [Shinoda, W.; DeVane, R.; Klein, M. L. Mol. Simul. 2007, 33, 27-36] into the HOOMD-Blue graphical processing unit (GPU)-accelerated molecular dynamics (MD) software package previously reported by Anderson et al. [Anderson, J. A.; Lorenz, C. D.; Travesset, A. J. Comput. Phys. 2008, 227, 5342-5359]. For a system of 25 750 particles, this implementation provides performance on a single GPU, which is superior to that of a widely used parallel MD simulation code running on an optimally sized CPU-based cluster. Using our GPU setup, we have collected 0.6 ms of MD trajectory data for aqueous solutions of 7 different nonionic polyethylene glycol (PEG) surfactants, with most of the systems studied representing ∼1 000 000 atoms. From this data, we calculated various properties as a function of the length of the hydrophobic tails and PEG head groups. Specifically, we determined critical micelle concentrations (CMCs), which are in good agreement with experimental data, and characterized the size and shape of micelles. However, even with the microsecond trajectories employed in this study, we observed that the micelles composed of relatively hydrophobic surfactants are continuing to grow at the end of our simulations. This suggests that the final micelle size distributions of these systems are strongly dependent on initial conditions and that either longer simulations or advanced sampling techniques are needed to properly sample their equilibrium distributions. Nonetheless, the combination of coarse-grained modeling and GPU acceleration marks a significant step toward the computational prediction of the thermodynamic properties of slowly evolving surfactant systems.

Free energy landscape of a DNA-carbon nanotube hybrid using replica exchange molecular dynamics.

The DNA-carbon nanotube hybrid (DNA-CN) consists of a single-wall carbon nanotube (SWCN) coated with a self-assembled monolayer of single-stranded DNA (ssDNA). Recent experiments have demonstrated that this nanomaterial is ideal for numerous nanotechnological applications. Despite this importance, the structure of this material remains poorly understood. Molecular dynamics (MD) simulations have provided information about the self-assembly mechanisms and ssDNA conformations that characterize DNA-CN. However, MD simulations of biopolymers at low temperatures (T approximately 300 K) result in kinetic trapping that limits conformational sampling. Here, we present results of a large-scale replica exchange molecular dynamics (REMD) simulation that provides extensive sampling of the entire ensemble of oligonucleotide conformations in a (GT)(7)-SWCN hybrid. We calculate the free energy landscape and find minima corresponding to six distinct conformations, with a nonhelical loop structure as the global minimum. The hybrid contains significant structural disorder, with desorbed bases as an important structural feature. These results expand our understanding of DNA-CN and indicate the relevance of REMD for explorations of the physical properties of organic-inorganic multifunctional nanomaterials.

Probing peptide nanotube self-assembly at a liquid-liquid interface with coarse-grained molecular dynamics.

Self-assembly at a liquid-liquid interface is a powerful experimental route to novel nanomaterials. We report herein a computational study of peptide nanotube formation at an oil-water interface. We probe interfacial self-assembly and nanotube formation of the cyclic octapeptide, cyclo [(-L-Trp-D-Leu-)4] as an illustrative example. Individual peptide rings are rapidly adsorbed at the liquid-liquid interface where they self-assemble. Monomeric and dimeric peptide rings lie with their molecular planes mostly parallel to the interface. Longer oligomeric nanotubes are increasingly tilted at the interface and grow by an Oswald ripening mechanism to eventually align their tube axis parallel to the interface. The present results on nanotube assembly suggest that computation will be a useful complement to experiment in understanding the nature of self-assembly of nanomaterials at liquid-liquid interfaces.

Temperature-dependent conformational transitions and hydrogen-bond dynamics of the elastin-like octapeptide GVG(VPGVG): a molecular-dynamics study.

A joint experimental/theoretical investigation of the elastin-like octapeptide GVG(VPGVG) was carried out. In this article a comprehensive molecular-dynamics study of the temperature-dependent folding and unfolding of the octapeptide is presented. The current study, as well as its experimental counterpart (see companion article in this issue) find that this peptide undergoes an inverse temperature transition (ITT), leading to a folding at approximately 40-60 degrees C. In addition, an unfolding transition is identified at unusually high temperatures approaching the normal boiling point of water. Due to the small size of the system, two broad temperature regimes are found: the ITT regime at approximately 10-60 degrees C and the unfolding regime at approximately T > 60 degrees C, where the peptide has a maximum probability of being folded at T approximately 60 degrees C. A detailed molecular picture involving a thermodynamic order parameter, or reaction coordinate, for this process is presented along with a time-correlation function analysis of the hydrogen-bond dynamics within the peptide as well as between the peptide and solvating water molecules. Correlation with experimental evidence and ramifications on the properties of elastin are discussed.

Folding and unfolding of an elastinlike oligopeptide: "inverse temperature transition," reentrance, and hydrogen-bond dynamics.

The temperature-dependent behavior of a solvated oligopeptide, GVG(VPGVG), is investigated. Spectroscopic measurements, thermodynamic measurements, and molecular dynamics simulations find that this elastinlike octapeptide behaves as a two-state system that undergoes an "inverse temperature" folding transition and reentrant unfolding close to the boiling point of water. A molecular picture of these processes is presented, emphasizing changes in the dynamics of hydrogen bonding at the protein/water interface and peptide backbone librational entropy.