# 2. Getting, compiling and running ESPResSo¶

This chapter will describe how to get, compile and run the software.

ESPResSo releases are available as source code packages from the homepage [1]. This is where new users should get the code. The code within release packages is tested and known to run on a number of platforms. Alternatively, people that want to use the newest features of ESPResSo or that want to start contributing to the software can instead obtain the current development code via the version control system software [2] from ESPResSo ’s project page at Github [3]. This code might be not as well tested and documented as the release code; it is recommended to use this code only if you have already gained some experience in using ESPResSo.

Unlike most other software, no binary distributions of ESPResSo are available, and the software is usually not installed globally for all users. Instead, users of ESPResSo should compile the software themselves. The reason for this is that it is possible to activate and deactivate various features before compiling the code. Some of these features are not compatible with each other, and some of the features have a profound impact on the performance of the code. Therefore it is not possible to build a single binary that can satisfy all needs. For performance reasons a user should always activate only those features that are actually needed. This means, however, that learning how to compile is a necessary evil. The build system of ESPResSo uses cmake [4] to compile software easily on a wide range of platforms.

## 2.1. Requirements¶

The following tools libraries, including header files, are required to be able to compile and use ESPResSo:

CMake
The build system is based on CMake
C++ Compiler
C++11 capable C++ compiler (e.g., Gcc 4.8.1 or later)
Boost
A number of advanced C++ features used by ESPResSo is provided by Boost.
FFTW
For some algorithms (P:math:^3M), ESPResSo needs the FFTW library version 3 or later [5] for Fourier transforms, including header files.
MPI
Because ESPResSo is parallelized with MPI, you need a working MPI environment that implements the MPI standard version 1.2.
Python
ESPResSo’s main user interface is via the Python scripting interface. Both, Python 2 and 3 are supported.
Cython
Cython is used for connecting the C++ core to Python

### 2.1.1. Installing Requirements on Ubuntu 16.04 LTS¶

To make ESPResSo run on Ubuntu 16.04 LTS, its dependencies can be installed with:

sudo apt install build-essential cmake cython python-numpy \
libboost-all-dev openmpi-common

Optionally the ccmake utility can be installed for easier configuration:

$sudo apt install cmake-curses-gui ### 2.1.2. Installing Requirements on Mac OS X¶ To make ESPResSo run on Mac OS X 10.9 or higher, its dependencies can be installed using MacPorts. First, download the installer package appropriate for your Mac OS X version from https://www.macports.org/install.php and install it. Then, run the following commands: sudo xcode-select --install sudo xcodebuild -license accept sudo port selfupdate sudo port install cmake python27 python27-cython python27-numpy \ openmpi-default fftw-3 +openmpi boost +openmpi +python27 sudo port select --set cython cython27 sudo port select --set python python27 sudo port select --set mpi openmpi-mp-fortran Alternatively, you can use Homebrew. sudo xcode-select --install sudo xcodebuild -license accept /usr/bin/ruby -e "$(curl -fsSL https://raw.githubusercontent.com/Homebrew/install/master/install)"
brew install cmake python cython boost boost-mpi fftw
brew install numpy --without-python3
ln -s /usr/local/bin/python2 /usr/local/bin/python

Note: If both MacPorts and Homebrew are installed, you will not be able to run ESPResSo. Therefore, if you have both installed, please uninstall one or the other by running one of the following two commands:

sudo port -f uninstall installed && rm -r /opt/local
ruby -e "$(curl -fsSL https://raw.githubusercontent.com/Homebrew/install/master/uninstall)" ### 2.1.3. Installing python dependencies¶ There are a few python packages needed to e.g. build the documentation. To install the required packages as a non-root user execute the following command in ESPResSo ‘s source directory: pip install -r requirements.txt --user --upgrade ## 2.2. Quick installation¶ If you have installed the requirements (see section Requirements ) in standard locations, to compile, it is usually enough to create a build directory and call cmake and make (optional steps which modify the build process are commented out): mkdir build cd build #cp myconfig-default.hpp myconfig.hpp # use the default configuration as template #nano myconfig.hpp # edit to add/remove features as desired cmake .. #ccmake . // in order to add/remove features like SCAFACOS or CUDA make This will build ESPResSo with a default feature set, namely src/core/myconfig-default.hpp. This file is a c++ header file, which defines the features that should be compiled in. You may want to adjust the feature set to your needs. This can be easily done by copying the myconfig-sample.hpp which has been created in the build directory to myconfig.hpp and only uncomment the features you want to use in your simulation. The cmake command looks for libraries and tools needed by ESPResSo. So ESPResSo can only be built if cmake reports no errors. The command make will compile the source code. Depending on the options passed to the program, make can also be used for a number of other things: • It can install and uninstall the program to some other directories. However, normally it is not necessary to actually install to run it: make install • It can invoke code checks: make check • It can build this documentation: make sphinx When these steps have successfully completed, ESPResSo can be started with the command: ./pypresso <SCRIPT> where is <SCRIPT> is a python script which has to be written by the user. You can find some examples in the samples folder of the source code directory. If you want to run in parallel, you should have compiled with Open MPI, and need to tell MPI to run in parallel. The actual invocation is implementation dependent, but in many cases, such as Open MPI, you can use mpirun -n <N> ./pypresso <SCRIPT> where <N> is the number of prcessors to be used. ## 2.3. Configuring¶ ### 2.3.1. myconfig.hpp: Activating and deactivating features¶ ESPResSo has a large number of features that can be compiled into the binary. However, it is not recommended to actually compile in all possible features, as this will slow down significantly. Instead, compile in only the features that are actually required. A strong gain in speed can be achieved, by disabling all non-bonded interactions except for a single one, e.g. . For the developers, it is also possible to turn on or off a number of debugging messages. The features and debug messages can be controlled via a configuration header file that contains C-preprocessor declarations. Appendix lists and describes all available features. The file myconfig-sample.hpp that configure will generate in the build directory contains a list of all possible features that can be copied into your own configuration file. When no configuration header is provided by the user, a default header, found in src/core/myconfig-default.hpp, will be used that turns on the default features. When you distinguish between the build and the source directory, the configuration header can be put in either of these. Note, however, that when a configuration header is found in both directories, the one in the build directory will be used. By default, the configuration header is called myconfig.hpp. The configuration header can be used to compile different binary versions of with a different set of features from the same source directory. Suppose that you have a source directory$srcdir and two build directories $builddir1 and$builddir2 that contain different configuration headers:

• $builddir1/myconfig.hpp: #define ELECTROSTATICS #define LENNARD-JONES •$builddir2/myconfig.hpp:
#define LJCOS

Then you can simply compile two different versions of via:

cd builddir1
cmake ..
make

cd builddir2
cmake ..
make

To see, what features were activated in myconfig.hpp, run::

./pypresso

and then in the Python interpreter:

import espressomd
print(espressomd.features())

## 2.4. cmake¶

In order to build the first step is to create a build directory in which cmake can be executed. In cmake, the source directory (that contains all the source files) is completely separated from the build directory (where the files created by the build process are put). cmake is designed to not be executed in the source directory. cmake will determine how to use and where to find the compiler, as well as the different libraries and tools required by the compilation process. By having multiple build directories you can build several variants of ESPResSo, each variant having different activated features, and for as many platforms as you want.

Example:

When the source directory is srcdir (the files where unpacked to this directory), then the user can create a build directory build below that path by calling mkdir srcdir/build. In the build direcotry cmake is to be executed, followed by a call of make. None of the files in the source directory is ever modified when by the build process.

$cd build$ cmake ..
$make Afterwards Espresso can be run via calling ./pypresso from the command line. ## 2.5. ccmake¶ Optionally and for easier use the curses interface to cmake can be used to configure ESPResSo interactively. Example: Alternatively to the previous example instead of , the executable is called in the build direcotry to configure ESPResSo previous to its compilation followed by a call of make:$ cd build
$ccmake ..$ make

Fig. ccmake interface shows the interactive ccmake UI.

ccmake interface

### 2.5.1. Options and Variables¶

The behaviour of ESPResSo can be controlled by the means of options and variables in the CMakeLists.txt file. Also options are defined there. The following options are available:

• WITH_CUDA: Build with GPU support
• WITH_HDF5: Build with HDF5
• WITH_TESTS: Enable tests
• WITH_SCAFACOS: Build with Scafacos support
• WITH_VALGRIND_INSTRUMENTATION: Build with valgrind instrumentation markers

When the value in the CMakeLists.txt file is set to ON the corresponding option is created if the value of the opition is set to OFF the corresponding option is not created. These options can also be modified by calling cmake with the command line argument -D:

cmake -D WITH_HDF5=OFF srcdir

In the rare event when working with cmake and you want to have a totally clean build (for example because you switched the compiler), remove the build directory and create a new one.

## 2.6. make: Compiling, testing and installing¶

The command make is mainly used to compile the source code, but it can do a number of other things. The generic syntax of the make command is:

\$ make [options] [target] [variable=value]

When no target is given, the target all is used. The following targets are available:

all
Compiles the complete source code. The variable can be used to specify the name of the configuration header to be used.
check
Runs the testsuite. By default, all available tests will be run on 1, 2, 3, 4, 6, or 8 processors.
clean
Deletes all files that were created during the compilation.
install
Install ESPResSo. Use make DESTDIR=/home/john install to install to a specific directory.
doxygen
Creates the Doxygen code documentation in the doc/doxygen subdirectory.
sphinx
Creates the sphinx code documentation in the doc/sphinx subdirectory.
tutorials
Creates the tutorials in the doc/tutorials subdirectory.
doc
Creates all documentation in the doc subdirectory (only when using the development sources).

A number of options are available when calling make. The most interesting option is probably -j num_jobs, which can be used for parallel compilation on computers that have more than one CPU or core. num_jobs specifies the maximal number of jobs that will be run. Setting num_jobs to the number of available processors speeds up the compilation process significantly.

## 2.7. Running ESPResSo¶

ESPResSo is implemented as a Python module. This means that you need to write a python script for any task you want to perform with . In this chapter, the basic structure of the interface will be explained. For a practical introduction, see the tutorials, which are also part of the distribution. To use , you need to import the espressomd module in your Python script. To this end, the folder containing the python module needs to be in the Python search path. The module is located in the src/python folder under the build directory. A convenient way to run python with the correct path is to use the pypresso script located in the build directory.

./pypresso simulation.py

The pypresso script is just a wrapper in order to expose our self built python modules to the systems python interpreter by modifying the PYTHONPATH. Please see the following chapters describing how to actually write a simulation script for ESPResSo.

### 2.7.1. Basic python simulation script¶

In this section, a brief overview is given over the most important components of the Python interface and their usage is illustrated by short examples. The interface is contained in the espressomd Python module, which needs to be imported, before anything related can be done.

import espressomd

Access to the simulation system is provided via the System class. As a first step, an instance of the class needs to be created

system=espressomd.System()

Note that only one instance of the System class can be created, due to limitations in the simulation core. Properties of the System class are used to access the parameters concerning the simulation system as a whole, , the box geometry and the time step

system.box_l =(10.0,10.0,15.0) print system.time_step

The particles in the simulation are accessed via the ParticleList class. It is used to retrieve individual particles of the simulation as well as for adding particles. An instance of the class is provided as the part attribute of the System class. Individual particles can be retrieved by their numerical id by using angular brackets

p=system.part[0]

It is also possible to loop over all particles

for p in system.part: ...

An individual particle is represented by an instance of ParticleHandle. The properties of the particle are implemented as Python properties:

p=system.part[0] p.pos=(0,0,0) print p.id,p.pos system.part[0].q=-1

Properties of several particles can be accessed by using Python ranges

v=system.part[:].v

Interactions between particles fall in three categories:

• Non-bonded interactions are short-ranged interactions between all pairs of particles of specified types. An example is the Lennard-Jones interaction mimicking overlap repulsion and van der Wals attraction.
• Bonded interactions act only between two specific particles. An example is the harmonic bond between adjacent particles in a polymer chain.
• Long-range interactions act between all particles with specific properties in the entire system. An example is the coulomb interaction.

Non-bonded interactions are represented as subclasses of espressomd.interactions.NonBondedInteraction, e.g. espressomd.interactions.LennardJonesInteraction. Instances of these classes for a given pair of particle types are accessed via the non_bonded_inter attribute of the System class. Parameters are set as follows

system.non_bonded_inter[0,0].lennard_jones.set_params(epsilon=1,sigma=1,cutoff=1.5,shift=“auto”)

Bonded interactions are represented by subclasses of BondedInteraction. To set up a bonded interaction, first an instance of the appropriate class is created with the desired parameters. Then, the bonded interaction is registered with the simulation core. Finally, the bond can be added to particles using the add_bond()-method of ParticleHandle with the instance of the bond class and the id of the bond partner particle.

from espressomd.interactions import HarmonicBond