CalcHEP is a package for automatic calculations of elementary particle decay and collision properties in the lowest order of perturbation theory (the tree approximation). The main idea prescribed into the CalcHEP is to make available passing on from the Lagrangian to the final distributions effectively with a high level of automation.

Two variants are provided:

• calchep builds in "blind" (non-interactive) mode, with no X11 dependency. The symbolic engine, numerical integration, batch operation, and matrix-element generation all work; only the interactive graphical menu is unavailable.
• calchep-gui adds the X11-enabled interactive calchep-gui front-end (it depends on calchep for the shared data and libraries).

CalcHEP is a run-in-place package installed under $CONDA_PREFIX/share/calchep; activation exports the CALCHEP environment variable. Generating a new process compiles C code on demand at run time, so the package depends on a C compiler (gcc on Linux, clang on macOS) and works out of the box. (A Fortran compiler is only needed for models that use user-defined Fortran functions; standard process generation is C only.)