Developer and Contributor Guide

This page is designed to give new developers general guidelines for implementing code consistent with the VOTCA and cpp style and standard.

Reporting Bugs

To report a bug, please create an issue on the appropriate GitHub repo. Please be sure to provide as much information as possible such as:

  • The error messages

  • The operating system

  • What compiler was used

  • What dependencies were installed

  • The calculation that was being run

Issues can be directly created on the GitHub repo.

Formatting code

VOTCA uses clang-format to format the code, code that is not properly formatted is automatically rejected. The style files can be found in each repo. CMake provides a format target which you can run to format your code. The easiest way to format your code is just a @votca-bot format comment in the PR, which then will automatically format your code.

Doxygen documentation

A complete overview of all C++ classes and code can be found on

VOTCA dev-tools

Running clang-format on every commit can be a drag, as can changing the copyright in every header. Fortunately, you will find small scripts in the dev-tools repo, which can automate this.

VOTCA Continuous Integration (GitHub Actions)

Each pull request to master in the votca repository is built on a machine in the cloud using GitHub actions

VOTCA can be built on various linux distributions, which are not all natively supported by GitHub actions. For non natively supported distributions, instead of using the default virtual machines, VOTCA first builds and then runs a docker container for each Pull Request. The container contains all the necessary dependencies of VOTCA (see buildenv below)

The docker images can be found at Docker Hub. The votca/buildenv containers are the basic containers, which contain all the dependencies VOTCA requires; VOTCA code itself is not included. The votca/buildenv can be found on VOTCA’s GitHub Container registry. The actual containers used for running the test builds are built on top of the votca/buildenv containers, the resulting votca/votca container can be found on Docker Hub as well as VOTCA’s GitHub Container registry.

More information can be found in the GitHub workflow files.

Making a Release

Similar to the VOTCA containers, releases are also handled by GitHub actions. votca/votca has a release workflow that can only be triggered manually. To trigger it go this GitHub Action. The release can only be made from the master branch, but testing the creation of a release can be triggered on any branch. To make a release, trigger the action from the master branch, pick a new release tag in the release tag box (all CHANGELOG files should already contain a section with the tag, but the date will be updated) and type yesyesyes into the deploy box. A new release will trigger the creation of the release tag.

Release names

Some releases have names, so far we have:

  • 1.1: SuperAnn - named after the spouse of a core developer

  • 1.2: SuperDoris - named after the administrator at MPI-P (VOTCA’s birthplace)

  • 1.3: SuperUzma - named after the spouse of a core developer

  • 1.4: SuperKurt - in occasion of Kurt Kremer’s 60th birthday

  • 1.5: SuperVictor - named after Victor Rühle, one of the original core developers

  • 1.6: SuperPelagia - named after the spouse of a core developer

  • 1.6.2: SuperGitta - in memory of the grandmother of a core developer

CPP Resources

A good starting point, is to take a look at the cpp standard. Though the code has not always consistently followed the cpp standard we now make an effort to really enforce it and follow best practices.

CPP Coding Rules

Here are a few general rules that should be followed:


  • Each class goes into a separate file.

  • Each filename should be the the name of the class it contains written in lowercase.


  • When including a header file from within the same repo that you are working use the relative includes. This consists of using quotation marks i.e.

    #include “molecule.h”

  • When including from another module, for instance you are working in the csg module and want to include a file from the tools repo use the anglular brackets i.e.

    #include <votca/tools/molecule.h>

Header Files

  • One class, one header.

  • When creating header guards use the template: VOTCA_VOTCA-REPO-NAME_CLASS-NAME_H. Where “VOTCA-REPO-NAME” is replaced by whichever repo the header file is in, this could be tools, csg or xtp. The “CLASS-NAME” component should also be replaced, but by the name of the class described in the header file:


  • Never use the “using namespace” in a header file.

  • Avoid using includes in header files. If possible forward declare a class instead.


  • Avoid using auto unless the type is very long, the reason being auto obscures the underlying type and can make it difficult to discern what a variable is meant to be used for.


  • Normally class names in upper case.

  • Order of access modifiers in class definitions should be as follows: - first public all functions - then private/protected all member variables - then private/protected member functions

  • There is no rule as to where to define a public typedef in the class.

  • All member variables are private/public.

  • The body of class methods should be placed in a source file or inlined at the end of the header if it exceeds a single line.

Naming in Classes

  • All member variables should be in lower case and end with _.

  • All functions should start with upper case, no _ should exist in their names.

  • Only get/set methods can begin with lower case letters.

  • For consistency all Ids should start at 0 not 1.

get/set Functions

  • get/set functions should start with a lowercase get/set (these are the only functions which should directly set/get a private member variable)

  • get must return a constant reference and keep the class const: const int &getId() const;

  • set only sets the member, e.g. void setId(const int &id) { _id = id; }


  • Functions should remain short.

  • Functions should not have more than one use, so use boolean arguments sparingly.


  • In general, use pointers sparringly. Most objects are small and a copy does not change performance. Use references if you want to avoid copies.

  • If your pointer owns an object (i.e. it has to delete it later) use a unique_ptr to it, so you do not have to call delete on it yourself.

  • If multiple objects own an object and the last object alive should delete it, use a shared_ptr.

  • If your object does not have ownership but just wants to visit, you can use a raw pointer, but if you can a reference is better.

  • If you ever have to explicitly call delete, you did something very wrong.


  • Do not comment out code, if you do not use it delete it.

  • Variables should have clear and explicit names.

  • Do not duplicate code.

  • Functions should have no more than 3 arguments. Otherwise create a class.

  • XYZ positions should be Eigen::Vector3d from the eigen library.

  • Readability is more important than elegant design.

  • Leave the code better than you found it.

  • Use pointers sparingly and especially try not to pass them around objects. Prefer references.

  • Do not write code, which you may use in the future. Only write code you will use now. Write code, you need later, later. This avoids cluttering the codebase with unused “at some point we will need this functions”.

VOTCA specifics (indexing, ids, units)

This can all be found here VOTCA_LANGUAGE_GUIDE


Unit Testing

Each module contains a src folder. Within the src folder exists a library folder: libtools, libcsg etc… and a tools folder. A tests folder should also exist in the src folder. If it does not you should create one.

For every new object and algorithm created there should exist a test. We use the Boost libraries testing framework. Good documentation can be found here:

We will outline the general workflow here using the vec object in votca::tools. This object only has a header file it is in: tools/include/votca/tools/vec.h.

Determine if a tests folder has already been created or not in /src. If it has not, take a look at what was done in the votca-tools repo.

  1. Create a test file in tools/src/tests/ must have the same name as what appears in the foreach in the CMakeLists.txt file. And place the following contents:

    #define BOOST_TEST_MAIN
    #define BOOST_TEST_MODULE vec_test
    #include <boost/test/unit_test.hpp>
    #include <exception>
    #include <votca/tools/vec.h>
    using namespace std;
    using namespace votca::tools;

Replace the ‘…’ and ‘:’ with the appropriate syntax. For more info on which boost test macros to use refer to the boost documentation

  1. To run the test involve following command:

    make test

Ensure you have an up to date version of cmake or use cmake3.

CPP Coding Style Guide

VOTCA uses a few auto formatting tools to help enforce the rules.


Automatically ensures consistent formatting for .cc and .h files. The style follows the google style fomatting rules. Have a look at the .clang-format file in the main votca repository for details.

To run the clang-format function on

clang-format -i -style=file

‘-i’ ensures it will make changes to, omitting the ‘-i’ will display the changes without implementing them. ‘-style=file’ ensures the format is read from the .clang-format file otherwise it will use a default style guide.

By default tabs should not be used to indent, avoid inserting ‘\t’, it is preferable that spaces be used instead.


Automatically formats python .py files. We are useing the default format rules of autopep8. To run on and update the file run:

autopep8 -i

Automating Formatting

The above formatters can be automated at every commit using the script found in the dev-tools repository. To use it copy the file pre-commit to your local .git subfolder to the hooks folder. E.g.

chmod 777 dev-tools/pre-commit
cp dev-tools/pre-commit votca/.git/hooks/

The above will make the script executable and then copy it to the local .git/hooks directory in the votca repository. The script not only updates the file format of every file staged during a commit it will also update the license date.

CPP Comment Guide

It is preferential that the following guidelines be followed when adding comments to code:

  1. The /* */ comment blocks should be avoided and the // used in their place. This is so that the /* */ comment blocks can be easily used for debugging.

  2. It would be preferential that the following doxygen commenting stencil be used in the header files above each class and function description.

    * \brief function/class summary
    * Detailed function/class description if needed
    * @param[in] - description of parameter 1
    * @param[out] - description of parameter 2
    * @param[in,out] - description of parameter 3
    * :
    * @return - description of return type

Doxygen commenting will help future developers maintain the code, in its fully compiled state. It may be found at:

NOTE: Compilation of the doxygen documentation is automated when code is merged into the master votca branch!

Failed Release Builds

To prepare votca for distribution on different linux flavors there are different requirements from the package managers. Some of the architectures that the package managers support can be quite varied. In the case that a failure occurs on an architecture, that is not available to you, there are different approaches for debugging the problem. As an example, fedora dnf has extended support to the pcc64le architecture. Assuming you have access to fedora you can run the following commands to simulate the build process on the pcc64le architecture:

dnf update
dnf install qemu-user-static dnf-utils
usermod -a -G mock <username>
mock -r epel-7-ppc64le --forcearch ppc64le --dnf --init
spectool -g votca.spec
rpmbuild -D"_sourcedir ${PWD}" -D"_srcrpmdir ${PWD}" -bs votca.spec
mock -r epel-7-ppc64le --forcearch ppc64le --dnf --no-clean votca-1.5-1.*.src.rpm

Here, votca-1.5-1 should be replaced with the correct version. The above commands would setup and run the dnf installation process on the pcc64le enviroment. If a bug was found and the build crashes one can interactively intervene by issuing the following command:

mock -r epel-7-ppc64le --forcearch ppc64le --shell

You will also need to install a text editor if you want to change the source files before running the interactive instance.

mock -r epel-7-ppc64le --forcearch ppc64le --install vim

Note: we have used this process with the ppc64le architecture as an example, but the same procedure can be extended with different architectures and diferent operating systems. For example, you could use the aarch64 or armv7hl architecture in place of pcc64le. You could also replace the epel-7-ppc64le os-architecure to fedora-28-ppc64le, fedora-27-aarch64 or some other combination. A final point, if you simply want to build natively, for instance if you are running fedora on an x86_64 machine, the frocearch pcc64le in the above case could just be dropped.