IPEC Solver is a Windows program designed to analyze the stability of core–shell inter-polyelectrolyte complexes formed by complexation of oppositely charged block copolymers. The two-dimensional size distribution of the complexes composed by different numbers of positively or negatively charged polyelectrolytes is calculated based on the scaling model of block copolymer aggregation and Poisson–Boltzmann theory for electrostatic interactions [V.A. Baulin, E. Trizac, Self-assembly of spherical inter-polyelectrolyte complexes from oppositely charged polymers, Soft Matter 8 (25) (2012) 6755–6766]. Salt effects, charge distribution and distributions of labile cations and anions around the complexes are calculated numerically as a function of chain composition and solvent properties. Program summaryProgram title: IPEC SolverCatalogue identifier: AEPM_v1_0Program summary URL:http://cpc.cs.qub.ac.uk/summaries/AEPM_v1_0.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.: 53802No. of bytes in distributed program, including test data, etc.: 4582576Distribution format: tar.gzProgramming language: C++ (VS.10)Computer: PCOperating system: WinXP, Win7Has the code been vectorized or parallelized?: Supports parallel computation through OpenMPRAM: 1 GBClassification: 16.11Nature of problem:This free software is designed to illustrate physical mechanisms of self-assembly of inter-polyelectrolyte complexes in the presence of salt. It can suggest an optimal structure of the aggregates as a function of the structure of block copolymers.Solution method:The distribution of the complexes composed by a different number of positively or negatively charged polyelectrolytes is calculated based on a scaling model of block copolymer aggregation and Poisson–Boltzmann theory for electrostatic interactions [V.A. Baulin, E. Trizac, Self-assembly of spherical inter-polyelectrolyte complexes from oppositely charged polymers, Soft Matter, 8 (25) (2012) 6755–6766]. Salt effects, charge distribution and distributions of labile ions around the complexes are provided.Restrictions:IPEC complex is assumed to have a core–shell structure: a spherical core containing charged polymers is surrounded by a neutral corona. To insure stability of the solution, salt concentration, ka>0.2Additional comments:Solves the Poisson–Boltzmann equation in a broad range of parameters.Running time:From seconds to several hours.