## Section: New Results

Keywords : quantum chemistry, molecular dynamics, hierarchical methods, coupling model.

### Schemes and algorithms for computational chemistry

Participants : Guillaume Anciaux, Olivier Coulaud, Pierre Fortin, Jean Roman.

#### Crack simulation

A study of crack propagation in silica glasses with a coupling method between molecular dynamics and elasticity begun in collaboration with the CEA Ile-de-France in December 2003. Simulations which follow crack propagation at atomistic level lead to huge number of atoms on a small domain. The coupling between two length scales allows us to treat larger domains with smaller number of atoms. Nevertheless 3D atomistic simulations involve several million atoms; they must be parallel and use a coupling with elasticity codes based on finite element approximation.

Our algorithm to couple such models is based on the Bridging method introduced by T. Belytschko. We have extended our previous work on 1D analysis of the model to higher dimension and we have developed a parallel framework to compute and visualize the coupling algorithms. This framework allows us to couple finite element technique with molecular dynamics. We validated the approach based on the Bridging Method on several multi-dimensional cases like wave propagation and crack propagation. The coupling algorithm solves a coupling linear equation and redistributes the corrections among degrees of freedom (atoms and finite elements nodes). Optimized data structures have been used in several parts of the coupling process. For example we build an efficient algorithm based on an initial computing of the finite element shape functions in order to accelerate the field's interpolation at atom positions. One other crucial service of the framework is the ability to control and forward the information on dynamic load balance strategies. Those strategies migrate atoms between processors; thus the communication scheme to update the variables attached to the coupling system (like dofs) needs to be updated [2] . Moreover, this framework integrates EPSN that allows a powerful monitoring. An article on the description of that framework and justifying all the choices that have been made is under writing.