A perspective on multi-scale simulation: Toward understanding water-silica

We discuss the conceptual and practical developments that evolved over the past seven-plus years when our multidisciplinary team took on the challenge of understanding a single complex system - water and silica - through multi-scale modeling and simulation. The discussion provides the context for the ten contributions, from various groupings of the team, that make this coordinated special issue. In the evolution of our project, we have come to appreciate the need for a framework that essentially defines the intellectual basis of computational science. We have found that the usual utilitarian notion of simulation is lacking a conceptual counterpart: by itself, it does not address the scientific challenge of analyzing complex phenomena, such as chemo-mechanical processes, across various length scales. The problem of water and silica is prototypical with regard to many complex systems of current interest. In them, the effects of chemical activity and dynamical stress are involved simultaneously and essentially. This essential dependence presents opportunities for a multi-scale strategy that combines quantum and classical methods of simulation. As often is the case with "obvious" approaches, one encounters many subtle aspects. We summarize the issues we have encountered, thus laying the ground work for the detailed topical papers that follow.