Jdpd: an open java simulation kernel for molecular fragment dissipative particle dynamics
© The Author(s) 2018
Received: 5 January 2018
Accepted: 28 April 2018
Published: 21 May 2018
Mesoscopic simulation aims at describing supramolecular phenomena at the nanometer (length) and microsecond (time) scale for large interacting physical ensembles (representing millions of atoms) within comparatively short computational time frames (hours) by “coarse grained” neglect of uninteresting degrees of freedom. Dissipative Particle Dynamics (DPD) is a mesoscopic simulation technique for isothermal complex fluids and soft matter systems that combines features from Molecular Dynamics (MD), Langevin Dynamics and Lattice-Gas Automata [1–5]. It satisfies Galilean invariance and isotropy, conserves mass and momentum and achieves a rigorous sampling of the canonical ensemble due to soft particle pair potentials that diminish entanglements or caging effects. DPD is expected to show correct hydrodynamic behavior and to obey the Navier–Stokes equations.
Whereas DPD particles in general may be arbitrarily defined “fluid packets”  the Molecular Fragment DPD variant [5–12] identifies each particle with a distinct small molecule of molar mass in the order of 100 Da. Larger molecules are composed of adequate “molecular fragment” particles that are bonded by harmonic springs to mimic covalent connectivities and spatial 3D conformations. Thus this variant may be regarded as a chemically intuitive and molecular accurate “fine grained” version of the intrinsically “coarse grained” DPD technique.
A simulation kernel software comprises the fundamental data structures and numerical calculation algorithms that are necessary to approximate the temporal evolution of a defined particle ensemble. DPD (as well as similar MD) code consists basically of a main loop over (non-parallelizable) successive simulation steps in which (parallelizable) particle pair force evaluations are the most time-consuming part [13, 14]. Thus parallelization efforts focus mainly on these force calculations in order to speed up simulations.
Computational methods, implementation details and performance results
The sketched forces and corresponding potentials are implemented in calculation classes of the packages harmonicBonds, dpdCutoff1 and electrostatics. An additional gravitational acceleration that acts on particle masses may be defined for every direction. All calculation classes for DPD and electrostatics forces and potentials extend abstract class ParticlePairInteractionCalculator of package interactions which itself implements a cut-off length based simulation box cell partitioning : The resulting cell linked-list method allows a (near) linear scaling detection of interacting particle pairs within neighbored cells—a task that otherwise would be quadratically scaling with the number of particles. In addition the simulation box cells are grouped in (3*3*2=)18 parallelizable chunks (each cell is surrounded by 26 neighbor cells, every third cell in every of the three spatial directions has no common neighbor cell but due to symmetric forces every second cell can be used in one of the three directions, see method getParallelisationSafeCellChunks of class CellBox in package utilities): These chunks guarantee a separated non-overlapping access of internal force array elements from parallelized calculation threads so that fast lock-free array manipulations become possible (all particle related arrays are located in class ParticleArrays of package parameters). An analogue lock-free parallelization feature with separated parallelizable bond chunks is realized within the bond related calculation classes (see package harmonicBonds).
Since (unlike MD) dissipative DPD forces depend on relative particle velocities the common Velocity-Verlet (VV) integration of the equations of motion [13, 14] has to be modified (abbreviated MVV). Jdpd consists of optimized implementations of four DPD integration schemata (located in package integrationType) that cover different integration techniques: (1) The original Groot-Warren scheme (GWMVV) [5, 28, 29] which depends on a tuning parameter where GWMVV equals VV integration for a value of 0.5, (2) the self-consistent scheme (SCMVV) [28–30] with an adjustable number of self-consistent dissipative force iterations where a single iteration leads to the DPDMVV variant, (3) Shardlow’s S1 scheme (S1MVV) [31, 32] and (4) the Nonsymmetric Pairwise Noose-Hoover-Langevin thermostat (PNHLN)  that requires the definition of an additional coupling parameter.
A single simulation task is performed by instantiating a DpdSimulationTask (which implements a Callable interface, see package jdpd) that may be submitted to an appropriate thread executor service to be invoked. The DpdSimulationTask constructor requires six configuration objects: (1) An (optional) RestartInfo instance that contains information about a possible simulation job restart (all Jdpd jobs may be restarted with altered settings which allows flexible job execution chains), (2) an input instance that implements the IInput interface with all simulation settings (e.g. particle types and their interactions, initial particle positions within the simulation box, additional properties like molecule boundaries etc.), (3) an output instance implementing the IOutput interface for all output data, (4) a progress monitor instance implementing the IProgressMonitor interface for real-time simulation progress information, (5) an ILogger instance for log-level dependent accumulation of detailed internal calculation progress information and (6) a ParallelizationInfo instance that describes internal settings for parallelized calculations. All interfaces are located in package interfaces and possess concrete sample implementations like classes FileInput and FileOutput of package samples that implement file-based I/O methods. The RestartInfo and ParallelizationInfo classes are found in package parameters.
An IInput object contains a Factory instance (located in package utilities) that implements enumerated type definitions for random number generation, DPD and electrostatics calculations, harmonic bonds or integration schemata. The factory pattern allows a simple extension or replacement of computational algorithms used throughout the whole simulation kernel, e.g. the implemented random numbers generators [34–36] may be supplemented by alternative methods with a few additional lines of code in the Factory class. An IInput object also provides detailed molecular information (see class MoleculeDescription in package utilities): These molecular descriptions differentiate between topological bonds of a molecules’ particles for an adequate description of covalent connectivity and bonds between backbone particles that maintain a defined spatial 3D structure, e.g. ring systems or secondary and tertiary structures of proteins. As a matter of course both bond types are treated equally throughout calculations.
For software programs with extensive parallelized numerical calculations the debugging facilities of current integrated development environments are often not sufficient to tackle subtle errors. Thus Jdpd comprises (extensible) loggers (see classes FileLogger and MemoryLogger in package logger) with different (extensible) log-levels to obtain detailed information about the simulation progress.
Jdpd efficiently calculates kinetic and potential energies, average kinetic temperatures , surface tensions along the simulation box axes [14, 37] or particle based radii of gyration . In addition a fast parallelized particle and molecule based nearest-neighbor analysis for the whole simulation process is implemented which allows a detailed temporal monitoring of changing averaged particle’s and molecule’s vicinities. Initial force steps for potential energy minimization to improve start geometries may be defined and velocities may be scaled to keep a desired temperature. Molecule specific position or velocity fixation is provided with the additional option to define molecule specific boundaries in form of reflective virtual walls within the simulation box to confine molecules to a desired subspace. Molecule specific periodic “force kicks” may be defined to “smoothly drive” molecules into a desired direction. Periodic boundary conditions as well as reflective box walls are supported. Implemented safeguards try to tackle common problems like unfavorable simulation box start configurations with unphysical (high or low) particle densities.
For testing purposes Jdpd contains two appropriate packages tests and tests.interactions that contain Unit test code e.g. for a command file driven Jdpd usage.
Jdpd is a new open DPD simulation kernel completely written in Java that complements the small available set of general purpose DPD kernels. It especially supports molecular fragment structures and offers parallelizable force calculation plus efficient caching options with an interface and factory-pattern driven design for comfortable and low expenditure code extensions, customizations or replacements. Detailed input/output communication, parallelization and process control as well as internal logging capabilities for debugging purposes are supported. The new kernel may be utilized in different simulation environments ranging from flexible scripting solutions up to fully integrated “all-in-one” simulation systems described in  where it may help to avoid polyglot programming.
The Jdpd library uses the Apache Commons RNG libraries  and is publically available as open source published under the GNU General Public License version 3 . The Jdpd repository on GitHub comprises the Java bytecode libraries (including the Apache Commons RNG libraries), the Javadoc HTML documentation  and the Netbeans  source code packages including Unit tests.
KvdB and AZ designed, implemented and tested the Jdpd related code. HK and AZ conceived the Jdpd approach and lead the project development. All authors read and approved the final manuscript.
The authors like to thank Matthias Epple for helpful discussions, kind support and continual encouragement as well as Marc Hamm and the reviewers for helpful suggestions. The support of GNWI – Gesellschaft für naturwissenschaftliche Informatik mbH, Oer-Erkenschwick, Germany, is gratefully acknowledged.
The authors declare that they have no competing interests.
Availability of data and materials
Project name: Jdpd, Project home page: Jdpd repository at https://github.com/zielesny/Jdpd, Operating system(s): Platform independent, Programming language: Java, Other requirements: Java 1.8 or higher, Apache Commons RNG libraries, License: GNU General Public License version 3.
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