In Inelastic Neutron Scattering Spectroscopy, the neutron scattering intensity is plotted versus neutron energy loss giving a spectrum that looks like an infrared or a Raman spectrum. Unlike IR or Raman, INS does not have selection rules, i.e. all transitions are in principle observable. This particular characteristic makes INS a test bed for Density Functional Theory calculations of vibrational modes. aCLIMAX is the first user friendly program, within the Windows environment, that uses the output of normal modes to generate the calculated INS of the model molecule, making a lot easier to establish a connection between theory and experiment. Program summary Title of program: aCLIMAX 4.0.1 Catalogue identifier: ADSW Program summary URL: http://cpc.cs.qub.ac.uk/summaries/ADSW Program obtainable from: CPC Program Library, Queen's University of Belfast, N. Ireland Operating systems: Windows 95 onwards, except Windows ME where it does not work Programming language used: Visual Basic Memory requirements: 64 MB No. of processors: 1 Has the code been parallelized: No No. of bytes in distributed program, including test data, etc.: 2 432 775 No. of lines in distributed program, including test data, etc.: 17 998 Distribution format: tar gzip file Nature of physical problem: Calculation of the Inelastic Neutron Scattering Spectra from DFT calculations of the vibrational density of states for molecules. Method of solution: INS spectral intensity calculated from normal modes analysis. Isolated molecule approximation. Typical time of running: From few seconds to few minutes depending on the size of the molecule. Unusual features of the program: Special care has to be taken in the case of computers that have different regional options than the English speaking countries, the decimal separator has to be set as “.” (dot) instead of the usual “,” (comma) that most countries use.