A. F. Wright, N. A. Modine, S. M. Foiles, A. E. Mattsson; Sandia National Laboratories
A. Tackett, R. Hatcher; Vanderbilt University
N. A. W. Holzwarth, Wake Forest University

Electronic-structure calculations based on density-functional theory (DFT) are widely used to determine and understand the behavior of materials. While the theoretical foundations underlying DFT and related techniques are quite well developed, computer codes that implement these techniques generally do not conform to modern software engineering standards. This makes them difficult to modify, thereby impeding the implementation and utilization of new electronic-structure techniques and decreasing the productivity of researchers. Socorro is our effort to develop a flexible, easily maintained, open-source electronic-structure code. The code is written mainly in Fortran 90/95 + MPI, and widespread use has been made of  object-oriented features of Fortran 90/95 such as modules, derived types, data-hiding, and operator overloading. A Socorro module encapsulates a data structure, whose details are kept private, along with public routines used by code outside the module to manipulate the data structure. Since outside code does not depend on details of the module's data structure, the implementation can be changed independently of the outside code. The flexibility inherent in this approach is greatly enhanced by the use of a specification- driven methodology wherein the focus is on careful design of interfaces between modules.
Socorro is organized as a hierarchy of modules where upper level modules group and enforce constraints between objects defined within lower level modules. Details of this structure and how it facilitates extensibility will be discussed. Considerable effort has also been devoted to maintaining high performance in an object-oriented framework. The tools that we have found useful to achieve this include lazy copying, data polymorphism, and light-weight comparisons.
Socorro is available (under a GNU public license) for use as a platform on which to develop new electronic-structure techniques and as a high-performance tool for studying the behavior of complex materials. The current version of Socorro implements the local-density approximation for exchange and correlation, three forms of the generalized gradient approximation (PW91, PBE, and BLYP), molecular dynamics, the dimer method for finding transition states, and the projector-augmented wave technique.