A new program, PHI, with the ability to calculate the magnetic properties of large spin systems and complex orbitally degenerate systems, such as clusters of d‐block and f‐block ions, is presented. The program can intuitively fit experimental data from multiple sources, such as magnetic and spectroscopic data, simultaneously. PHI is extensively parallelized and can operate under the symmetric multiprocessing, single process multiple data, or GPU paradigms using a threaded, MPI or GPU model, respectively. For a given problem PHI is been shown to be almost 12 times faster than the well‐known program MAGPACK, limited only by available hardware. © 2013 Wiley Periodicals, Inc. A new and extensively parallelized code for the calculation of the magnetic properties of large spin systems or complex orbitally degenerate compounds is presented. The program can simulate theoretical systems or fit experimental data with a specific Hamiltonian. PHI is much faster than other commonly employed codes, and its use is very intuitive and approachable for novice users.