Computer Physics Communications 127 (2000) 327–342 www.elsevier.nl/locate/cpc Green’s function calculation of Ballistic Electron Emission Microscopy currents (BEEM v2.1) K. Reuter a,c , P.L. de Andres a , F.J. Garcia-Vidal b , D. Sestovic b , F. Flores b , K. Heinz c a Instituto de Ciencia de Materiales (CSIC), Cantoblanco, E-28049 Madrid, Spain b Department de Fisica Teorica de la Materia Condensada (UAM), Universidad Autonoma de Madrid, E-28049 Madrid, Spain c Lehrstuhl für Festkörperphysik, Universität Erlangen-Nürnberg, D-91058 Erlangen, Germany Received 24 February 1999; accepted 25 October 1999 Abstract We describe the FORTRAN-90 program BEEM v2.1 for the computation of real and reciprocal space currents in Ballistic Electron Emission Microscopy (BEEM), and Spectroscopy (BEES). Based on a LCAO Hamiltonian, the equilibrium one- particle Green’s functions of a semi-inﬁnite crystal are obtained using a decimation technique. By means of Keldysh’s formalism, the STM tip is coupled to this crystal, and expressions for BEEM currents in real and reciprocal space are obtained. Finally, a Dyson-like equation and a self-consistent charge neutrality condition are applied to incorporate reconstructed or relaxed layers in the surface region. Running demos for Au and CoSi 2 are provided.  2000 Published by Elsevier Science B.V. All rights reserved. PROGRAM SUMMARY Title of program: BEEM Catalogue identiﬁer: ADLR Program Summary URL: http://cpc.cs.qub.ac.uk/summaries/ADLR Program obtainable from: CPC Program Library, Queen’s Univer- sity of Belfast, N. Ireland Computer: Alpha DEC3000/400 Operating system: OSF1 DEC-Unix v4.0 Programming language: FORTRAN 90 No. of bytes in distributed program including test data, etc.: 834 471 bytes High-speed storage required for the test run: 32 Mbytes Disk storage required: a scratch area of at least 300 Mbytes is used for the test run No. of bits in a word: 64 Keywords: Ballistic Electron Emission Microscopy, Green’s func- tions, decimation, layer-doubling, surface electronic structure Nature of problem We describe the FORTRAN-90 program BEEM v2.1 for the com- putation of real and reciprocal space currents in Ballistic Electron Emission Microscopy (BEEM), and Spectroscopy (BEES) [1–4]. Method of solution Based on a LCAO Hamiltonian, the equilibrium one-particle Green’s Functions of a semi-inﬁnite crystal are obtained using a decimation technique. By means of Keldysh’s formalism, the STM 0010-4655/00/$ – see front matter  2000 Published by Elsevier Science B.V. All rights reserved. PII:S0010-4655(99)00502-0