libgeant321-2-dev software package provides [Physics] Library for GEANT 3.21 (development files), you can install in your Debian 8 (Jessie) by running the commands given below on the terminal,
$ sudo apt-get update $ sudo apt-get install libgeant321-2-dev
libgeant321-2-dev is installed in your system.
Make ensure the libgeant321-2-dev package were installed using the commands given below,
$ sudo dpkg-query -l | grep libgeant321-2-dev *
You will get with libgeant321-2-dev package name, version, architecture and description in a table
| Distro | Debian 8 jessie |
| Section | Library development |
| Package | libgeant321-2-dev |
| Version | 1:3.21.14.dfsg-11+b1 [amd64, a |
| Description | [Physics] Library for GEANT 3.21 (development files) |
| Sub Section | Libdevel |