Very long computational time is the main obstacle in first principles Molecular Dynamics (MD). High frequency modes of vibrations in molecular solids (bond stretching and bond bending) cause this expense as they require very small time-step (of integration), but they are (a) fairly decoupled from the low frequency modes and (b) should be treated with quantum rather than classical ionic motion. We can increase the time-step by constraining the detailed dynamics of the high frequency modes which do not affect the physics and are, in any case, misdescribed by classical ionic MD. I will give a general review of first principle/ab-initio Molecular dynamics and than will discuss different CONSTRAIN ALGORITHMS, specially SHAKE and RATTLE; finally I will explain their (first ever!!) implementation on first principle MD. I will show some movies of Ammonia Crystal MD simulation and compare how CONSTRAINED dynamics helps to get quicker result still keeping the physics right.
This is a weekly series of informal talks given primarily by members of the soft condensed matter and statistical mechanics groups, but is also open to members of other groups and external visitors. The aim of the series is to promote discussion and learning of various topics at a level suitable to the broad background of the group. Everyone is welcome to attend..