The Hybrid Reverse Monte Carlo (HRMC) code models the atomic structure of materials via the use of a combination of constraints including experimental diffraction data and an empirical energy potential. In this version 2.1 update, an empirical potential for silicon-carbide has been added to the code along with an experimentally motivated constraint on the bond type fraction applicable to systems containing multiple elements. New version program summaryProgram title: HRMC version 2.1Catalogue identifier: AEAO_v2_1Program summary URL:http://cpc.cs.qub.ac.uk/summaries/AEAO_v2_1.htmlProgram obtainable from: CPC Program Library, Queen’s University, Belfast, N. IrelandLicensing provisions: Standard CPC licence, http://cpc.cs.qub.ac.uk/licence/licence.htmlNo. of lines in distributed program, including test data, etc.: 114084No. of bytes in distributed program, including test data, etc.: 1289369Distribution format: tar.gzProgramming language: FORTRAN 90.Computer: any computer capable of running executables produced by the Fortran 90 compiler. For example, the code runs in Windows 7, once compiled with the GNU Fortran 95 compiler.Operating system: Unix, Windows.Classification: 7.7.Catalogue identifier of previous version: AEAO_v2_0Journal reference of previous version: Comput. Phys. Comm. 184 (2013) 1946Does the new version supersede the previous version?: yesNature of problem:atomic modelling using a combination of empirical potentials, fits to experimental data and other chemically or physically motivated constraints.Solution method:single move Metropolis Monte Carlo method used to minimize the total energy and discrepancy between simulation and experimental data.Reasons for new version:extension of capabilities from old version.Summary of revisions:1.Inclusion of a bond-order potential for silicon carbide (P. Erhart, K. Albe, Phys. Rev. B 71 (2005) 035211). The selection of this potential is via the POT_TYPE parameter in the HRMC.inp file and the number of carbon and silicon atoms needs to be specified via the configA and configB parameters.2.Inclusion of a constraint on the bond type fraction for multi-elemental systems. If a simulation has elements A, B and C, this constraint allows one to fix for element A, the fraction of A–A, A–B and A–C bonds (which should sum to unity) and similarly for elements B and C. The constraint is turned on via the BT_OPTION in the HRMC.inp file and it has its constraint weightings labelled in each stage alongside the other constraint weightings. The BT_OPT(1), BT_OPT(2) and BT_OPT(3) parameters are used to select which elements have their bond type fractions fitted via this constraint.3.A new file is output called data_xyzdetail.dat. This file contains lists of the neighbour separations, bond angles and dihedral angles.4.The header file hrmc.h has been replaced with hrmc_data.f90 which replaces the common block with a module to define global variables.Running time:1000 s for test run on a Intel Xeon 2.93 GHz—Nehalem series processor