  These kinds of fictional components of models are paradigmatically exemplified in certain computer simulations. Two of his examples are the “silogen atom” and “artificial viscosity.” Silogen atoms appear in certain nanomechanical models of cracks in silicon—a species of the kind of multiscale models that blend quantum mechanics and molecular mechanics mentioned in section 2.3. The silogen containing models of crack propagation in silicon work by describing the crack itself using quantum mechanics and the region immediately surrounding the crack using classical molecular dynamics. To bring together the modeling frameworks in the two regions, the boundary gets treated as if it contains ‘silogen’ atoms, which have a mixture of the properties of silicon and those of hydrogen. Silogen atoms are fictions. They are not offered as even a ‘good enough’ description of the atoms at the boundary—their prima facie representational targets. But they are used so that the overall model can be hoped to get things right. Thus the overall model is not a fiction, but one of its components is. Artificial viscosity is a similar sort of example. Fluids with abrupt shocks are difficult to model on a computational grid because the abrupt shock hides inside a single grid cell, and cannot be resolved by such an algorithm. Artificial viscosity is a technique that pretends that the fluid is highly viscous—a fiction—right were the shock is, so that he shock becomes less abrupt, and blurs over several grid cells. Getting the viscosity, and hence the thickness of the shock, wrong, helps to get the overall model to work “well enough.” Again, the overall model of the fluid is not a fiction, it is a reliable enough guide to the behavior of the fluid. But the component called artificial viscosity is a fiction—it is not being used to reliably model the shock. It is being incorporated into a larger modeling framework so as to make that larger framework, “reliable enough.”