  The interaction of electrons with quantized phonons and photons underlies the
ultrafast dynamics of systems ranging from molecules to solids, giving rise to
a plethora of physical phenomena experimentally accessible using time-resolved
techniques. Green's function methods offer an invaluable interpretation tool
since scattering mechanisms of growing complexity can be selectively
incorporated in the theory. cheers is a general-purpose nonequilibrium Green's
function code that implements virtually all known many-body approximations and
is designed for first principles studies of ultrafast processes in molecular
and model solid state systems. The aims of generality, extensibility,
efficiency, and user friendliness of the code are achieved through the
underlying theory development and the use of modern software design practices.
Here, we motivate the necessity for the creation of such a code and overview
its design and capabilities.
