Writing friendly programs in modular form

Abstract

Friendly software is defined as computer programs which can be run interactively and profitably without incurring fatal errors causing execution termination. We discuss a modular library and a simple program structure providing for input validation, help, status, overview and change commands in a standard FORTRAN environment. As an illustration we show how program WONPSE [ Computers Chem. 13, 201 (1989)] was easily transformed from a conventional program into a friendly tool for frontier research and student training on N-electron symmetry-eigenfunctions.