ASEP/MD is a computer program designed to implement the Averaged Solvent Electrostatic Potential/Molecular Dynamics (ASEP/MD) method developed by our group. It can be used for the study of solvent effects and properties of molecules in their liquid state or in solution. It is written in the FORTRAN90 programming language, and should be easy to follow, understand, maintain and modify. Given the nature of the ASEP/MD method, external programs are needed for the quantum calculations and molecular dynamics simulations. The present version of ASEP/MD includes interface routines for the GAUSSIAN package, HONDO, and MOLDY, but adding support for other programs is straightforward. This article describes the program and its usage. Program summary Title of program: ASEP/MD Catalogue identifier:ADSF Program Summary URL: http://cpc.cs.qub.ac.uk/summaries/ADSF Program obtainable from: CPC Program Library, Queen's University of Belfast, N. Ireland Computer for which the program is designed: it has been tested on Intel-based PC and Sun Operating systems under which the program has been tested: Red Hat Linux 7.2 and SunOS 5.6 Programming language used: FORTRAN90 Memory required to execute with typical data: greatly depends on the system No. of processors used: 1 Has the code been vectorized or parallelized?: no No. of bytes in distributed program, including test data, etc.: 44 544 Distribution format: tar gzip file Keywords: Solvent effects, QM/MM methods, mean field approximation, geometry optimization Nature of physical problem: The study of molecules in solution with quantum methods is a difficult task because of the large number of molecules and configurations that must be taken into account. The quantum mechanics/molecular mechanics methods proposed to date either require massive computational power or oversimplify the solute quantum description. Method of solution: A non-traditional QM/MM method based on the mean field approximation was developed where a classical molecular dynamics simulation is coupled with a quantum calculation. The average electrostatic potential generated by the solvent over the solute is calculated from the simulation and introduced into the quantum calculation as an external field. This process can be performed iteratively. Standard external programs are used for the molecular dynamics simulations and for the quantum calculations. The present program acts as an interface and controls the flow of the calculation. Restrictions on the complexity of the problem: At present, only pure liquids and binary dilute solutions (a single solute molecule) can be studied. For the molecular dynamics only MOLDY is implemented, while GAUSSIAN and HONDO are available for the quantum calculations. Restrictions of the aforementioned programs apply. Typical running time: Running time depends on the nature of the chemical system and the options passed to the external programs, which are usually by far the longest part of the calculations. Unusual features of the program: Uses SYSTEM and GETARG calls.