Structured programming is no longer enough for dealing with the large software projects allowed by today's computer hardware. An object-oriented computational model has been developed in order to achieve reuse, rapid prototyping and easy maintenance in large scale materials science calculations. The exclusive use of an object-oriented language is not mandatory for implementing the model. On the contrary, embedding Fortran code in an object-oriented language can be a very efficient way of fulfilling these goals without sacrificing the huge installed base of Fortran programs. Reuse can begin from one's old Fortran programs. These claims are substantiated with practical examples from a professional code for the study of the electronic properties of atomic clusters. Out of the about 20,000 lines of the original Fortran program, more than 70% of them could be reused in the C++ objects of the new version. Facilities for dealing with periodic systems and for scaling linearly with the number of atoms have been added without any change in the computational model.