Boundary conditions electron nuclear dynamics

In order to overcome the limitations of currently available empirical force field param-eterizations, we performed Car-Parrinello (CP) Molecular Dynamic simulations [36]. In the framework of DFT, the Car-Parrinello method is well recognized as a powerful tool to investigate the dynamical behaviour of chemical systems. This method is based on an extended Lagrangian MD scheme, where the potential energy surface is evaluated at the DFT level and both the electronic and nuclear degrees of freedom are propagated as dynamical variables. Moreover, the implementation of such MD scheme with localized basis sets for expanding the electronic wavefunctions has provided the chance to perform effective and reliable simulations of liquid systems with more accurate hybrid density functionals and nonperiodic boundary conditions [37]. Here we present the results of the CPMD/QM/PCM approach for the three nitroxide derivatives sketched above details on computational parameters can be found in specific papers [13]. 

Another issue is development of methods suitable for very large systems. The challenges here are associated with necessity for more efficient and accurate representation of orbitals, more robust elimination of finite size errors for solids and surface calculations with periodic boundary conditions. Advancement in this area also imply certain progress in QMC-based dynamical methods. The issue of QMC approaches that treat nuclear and electronic problem on the same footing remains open. 

With these benchmark calculations in hand, Laasonen et al. went on to perform ab initio molecular dynamics simulations on liquid water using the CP approach. The simulation had 32 D2O molecules at 300 K in a cubic cell of length 9.6 A with periodic boundary conditions. Deuterated water was used instead of ordinary water to slow down nuclear motion. This decrease makes it easier for the electronic parameters to adjust to nuclear motion and have the... 

---