1. Introduction

CO5BOLD - nickname COBOLD - is the short form of ``COnservative COde for the COmputation of COmpressible COnvection in a BOx of L Dimensions with l=2,3''. It is used to model solar and stellar surface convection (Freytag et al., 2012). For solar-type stars, only a small fraction of the stellar surface layers are included in the computational domain(Wedemeyer et al., 2004). In the case of red supergiants (Chiavassa et al., 2011; Freytag et al., 2002) or AGB-type stars (Freytag & Höfner, 2008), the computational box contains the entire star. The model range has been extended to sub-stellar objects like brown dwarfs (Freytag et al., 2010) and to white dwarfs (Tremblay, 2013) CO5BOLD solves the coupled non-linear equations of compressible hydrodynamics in an external gravity field together with non-local frequency-dependent radiation transport. Operator splitting is applied to solve the equations of hydrodynamics (including gravity), the radiative energy transfer (with a long-characteristics or a short-characteristics ray scheme), and possibly additional 3D (turbulent) diffusion in individual sub steps. The 3D hydrodynamics step is further simplified with directional splitting (usually). The 1D sub steps are performed with a Roe solver (Roe, 1986), accounting for an external gravity field and an arbitrary equation of state from a table. The radiation transport is computed with either one of three modules:

• MSrad module: It uses long characteristics. The lateral boundaries have to be periodic. Top and bottom can be closed or open (``solar module'').
• LHDrad module: It uses long characteristics and is restricted to an equidistant grid and open boundaries at all surfaces (old ``supergiant module'').
• SHORTrad module: It uses short characteristics and is restricted to an equidistant grid and open boundaries at all surfaces (new ``supergiant module'').

The code was supplemented with an (optional) MHD version (Schaffenberger et al., 2005) that can treat magnetic fields. There are also modules for the formation and advection of dust available. The current version now contains the treatment of chemical reaction networks, mostly used for the formation of molecules (Wedemeyer-Böhm et al., 2005), and hydrogen ionization (Leenaarts & Wedemeyer-Böhm, 2005), too. CO5BOLD is written in Fortran90. The parallelization is done with OpenMP directives.

To get a brief overview the ``Quickstart Sections'' might be helpful:
``How to compile CO5BOLD'' (Sect.4.1),
``Introduction to the UIO data format'' (Sect.6.2.1),
``How to make a proper parameter file'' (Sect.7.1.1),
``How to run CO5BOLD'' (Sect.9.1).