OpenMX (Open source package for Material eXplorer) is a software package for nano-scale material simulations based on density functional theories (DFT), norm-conserving pseudopotentials, and pseudo-atomic localized basis functions. The methods and algorithms used in OpenMX and their implementation are carefully designed for the realization of large-scale ab initio electronic structure calculations on parallel computers based on the MPI or MPI/OpenMP hybrid parallelism. The efficient implementation of DFT enables us to investigate electronic, magnetic, and geometrical structures of a wide variety of materials such as biological molecules, carbon-based materials, magnetic materials, and nanoscale conductors. Systems consisting of 1000 atoms can be treated using the conventional diagonalization method if several hundreds cores on a parallel computer are used. Even ab initio electronic structure calculations for systems consisting of more than 10000 atoms are possible with the O(N) method implemented in OpenMX if several thousands cores on a parallel computer are available. Since optimized pseudopotentials and basis functions, which are well tested, are provided for many elements, users may be able to quickly start own calculations without preparing those data by themselves. Considerable functionalities have been implemented for calculations of physical properties such as magnetic, dielectric, and electric transport properties. Thus, we expect that OpenMX can be a useful and powerful theoretical tool for nano-scale material sciences, leading to better and deeper understanding of complicated and useful materials based on quantum mechanics. WWW: http://www.openmx-square.org/