The GREENS package is a collection of electronic structure codes that are based on the Green's function formalism. Right now it contains two codes, BGFM and BSTR. BGFM (Bulk Green's Function Method) is an implementation of the KKR-ASA Green’s function technique [1,2]. BSTR (Bulk STRucture constants) has to be run before BGFM and calculates the transfer and multipole Madelung matrices for the underlying crystal structure.
BSTR has been written by H. L. Skriver, BGFM has been written by I. A. Abrikosov, A. V. Ruban, and H. L. Skriver. The GREENS codes are written in old Fortran-style with some elements of Fortran90.
 I. A. Abrikosov and H. L. Skriver, Self-consistent linear-muffin-tin-orbitals coherent-potential technique for bulk and surface calculations: Cu-Ni, Ag-Pd, and Au-Pt random alloys, Phys. Rev. B 47, 16532 (1993)
 A. V. Ruban and H. L. Skriver, Calculated surface segregation in transition metal alloys, Compt. Mat. Sci. 15, 119 (1999)
Some brief documentation:
Will be updated with more information later on.