OpenMPCD
OpenMPCD -- Simulation and Data Analysis Package for Multi-Particle Collision Dynamics

# Introduction

OpenMPCD is a software package that implements Multi-Particle Collision Dynamics [14] [15] [4] – in particular, the Stochastic Rotation Dynamics [6] [7] [8] variant that does not conserve angular momentum – on GPUs via CUDA, and also comes with a Python toolkit to analyze the data produced by the simulations.

# List of features

The following is an incomplete list of features and techniques implemented in the package at the moment:

• Molecular Dynamics (MD) code that couples to MPC
• Grid Shift [7] in order to guarantee Galilean invariance
• Maxwell-Boltzmann-Scaling Thermostat [5]
• Viscoelastic MPC fluids, modeled as ideal linear polymers with harmonic spring potentials [17] [19] [10] [20]
• Various solutes (such as star polymers) and pre-defined pair potentials for particle interactions during the streaming step
• Lees-Edwards boundary conditions [12] [1], or, as a special case, ordinary periodic boundary conditions.

# Building the executable

Currently, most parts of the OpenMPCD simulation package require a CUDA-capable graphics card to be present for calculations, and consequently requires that a CUDA-capable compiler (such as nvcc) is present.

To build OpenMPCD, change into its root directory, where there is a Makefile. Typing make should produce, relative to the root directory, the openmpcd executable, which can be used to run simualations, and test/runtests that executes the unit and integration tests that come with the simulation software. (To test the analysis suite, run python -m pytest Analysis/MPCDAnalysis/test.)

# The main configuration file

Most of the behavior of OpenMPCD is configured with the file config.txt, which will be looked for in the current working directory when executing openmpcd. This file is in libconfig [13] format. While it is recommended that one reads that library's documentation in order to understand the configuration syntax, most of it will probably be considered natural after the following examples. It is important to know, however, that libconfig strictly distinguishes integers from floating-point values, and does not by default convert one to the other; as such, when a configuration option expects a floating-point value, it is not permissible to specify, say, 1; rather, one has to write 1.0.

The following would correspond to the simplest configuration permissible:

initialization:
{
particleDensity = 10
particleVelocityDistribution:
{
mean = 0.0
standardDeviation = 1.0
}
}
mpc:
{
fluid:
{
simple: {}
}
simulationBoxSize:
{
x = 30
y = 30
z = 30
}
timestep = 0.1
srdCollisionAngle = 2.27 //corresponds to ~130°
warmupSteps = 1000
sweeps = 10000
sweepSize = 1
gridShiftScale = 1.0
}
bulkThermostat:
{
type = "MBS"
targetkT = 1.0
}
boundaryConditions:
{
LeesEdwards:
{
shearRate = 0.0
}
}

The order in which the configuration settings are specified is irrelevant, except that they should not be given multiple times.

This configuration corresponds to a simulation with 30x30x30 MPC collision cells (mpc.simulationBoxSize.x, mpc.simulationBoxSize.y, and mpc.simulationBoxSize.z), each cell containing, on average, 10 particles, for a total number of 10x30x30x30 particles (initialization.particleDensity). The particles will be placed randomly in the primary simulation volume, and their initial velocitiy (Cartesian) components will be drawn from a normal distribution with mean and standardDeviation as given in initialization.particleVelocityDistribution. After the velocities have been drawn, all velocities are shifted such that the system as a whole has a net velocity of 0 (note however, that this condition is not necessarily perfectly fulfilled, due to inaccuracies in floating-point calculations and representation).

The presence of the mpc.fluid.simple setting group signals that one wishes to simulate with an ordinary MPC fluid, i.e. a collection of point particles that stream independently of one another. mpc.timestep and mpc.srdCollisionAngle configure the streaming timestep and the SRD collision angle $$\alpha$$.

mpc.sweepSize configures how many streaming-collision-cycles should be performed for what is called a "sweep". Measurements on the system are performed once after each sweep, so this configuration option allows one to avoid carrying out expensive measurements more frequently than desired.

mpc.warmupSteps configures the number of streaming-collision-cycles that are to be performed before the first sweep (and hence the first measurement) is performed. This is useful to let the system reach a steady state from its initial (and quite possibly unphysical) state.

mpc.sweeps specifies the number of sweeps after which to terminate the simulation and save the resulting measurement data to disk.

mpc.gridShiftScale should in most cases be set to 1.0 to ensure Galilean invariance, but can be set to 0.0 to turn off the grid shift procedure.

bulkThermostat.type specifies the type of the bulk thermostat; currently, only "MBS", the Maxwell-Boltzmann-Scaling thermostat, is supported. bulkThermostat.targetkT configures the target temperature (multiplied by Boltzmann's constant) of the thermostat.

Finally, boundaryConditions defines the type of boundary conditions, with the only one currently supported being "LeesEdwards". Note, however, that this is equivalent to ordinary periodic boundariy conditions if boundaryConditions.LeesEdwards.shearRate is set to 0.0.

# Running the openmpcd executable

After the configuration file has been written, openmpcd can be run, either without options, or with the options given when running openmpcd --help. If the --rundir option is not given, the "run directory", i.e. the directory in which output data for this simulation run will be saved, will be created in the runs directory (relative to the current working directory), which must exist and be writable. Within that directory, a new directory with the current date and time will be created automatically.

The simulation ends after mpc.sweeps sweeps have been performed, or after a specified number of seconds have passed (see --maxRuntime; note that the value specified there does not include the time it takes to write data to the disk). Alternatively, the simulation terminates, after completing the current sweep and writing data to disk, in case the program receives the SIGINT or SIGTERM signals (which can be generated by pressing Ctrl+C). If another SIGINT or SIGTERM signal is received, openmpcd ends abruptly without saving data.

If the SIGUSR1 signal is received (see e.g. the kill(1) manpage), openmpcd will first complete the current sweep, then create a directory named EarlySave_<current date and time> within the run directory, and save the data accumulated so far in that directory. After that, it will continue the simulation as if nothing had happened. This feature may be useful if one wants to have a look at the simulation state, without aborting the simulation and having to start anew.

# Representation of particle positions

Particle positions are represented as a set of three Cartesian floating-point coordinates. Although during initialization, they are set to values within what is called the "primary simulation volume", i.e. in the range from 0 to mpc.simulationBoxSize.x for the x coordinate, and analogously for the y and z coordinates, the particle positions are in no way restricted to these ranges. This simplifies some aspects (e.g. displacement measurements, particle-particle-distances within one solute object (unless it is of comparable size as the whole system)), but complicates others (such as determining the minimal distance between a pair of particles that does not belong to the same solute, i.e. where one has to take mirror images into account).

# Organization of the OpenMPCD package.

OpenMPCD is organized in two parts: first, the C++/CUDA part that carries out simulations, and second, the Python part that can be used for data analysis (which also contains some C++ code in case improved performance was required).

The C++/CUDA files can be found in the include, src, and cudaDeviceCode directories, with unit and integration tests residing in the test directory.

The Python part can be found in the Analysis directory. The files in that directory are meant to be executed to perform analysis tasks, while the files and directories in the Analyis/MPCDAnalysis directory contain the underlying library of Python classes and functions.

The OpenMPCD C++/CUDA simulation code lies in the OpenMPCD namespace. Most of the actual simulation code currently relies on CUDA being present, and is thus declared and defined in the OpenMPCD::CUDA namespace; future versions of OpenMPCD might include support for CPU-only calculations.

The most important namespaces for users, or those who want to add functionality, are probably the OpenMPCD::CUDA::MPCFluid and OpenMPCD::CUDA::MPCSolute namespaces, and the Instrumentation namespaces therein. OpenMPCD currently distinguishes, for better or worse, between "MPC fluids", i.e. those parts of the systems that make up the majority of the MPC particles, and "MPC solutes", which typically consist of a number of particles orders of magnitude lower than the number of MPC fluid particles, but typically require more detailed simulation during the streaming step, via coupled Molecular Dynamics.

For further details on how the program is structured, see the ProgramOverview page .