Path Integral MD Simulation

Now decide what kind of information you want to get from the molecular dynamics simulation. With standard ``primitive coordinates'' propagator you will get the proper averages (e.g. bond lengths) over the PI-MD trajectory, provided the trajectory is ergodic. This means that the averages calculated over ergodic trajectory are equal to the averages over the canonical ensemble. In this case the trajectory does not contain any time correlations and should not be treated as causal-related sequence of events. This option does not require any additional keywords in the input. More likely you will be interested not only in the averages but also in the trajectory itself. There are two options here: the STAGING or the NORMAL MODES propagator. Both of them work in transformed coordinates of the fictitious PI-beads and their use together with CENTROID DYNAMICS recovers physical significance of the path integral trajectory of the centroids. With the NORMAL MODES propagator you can decouple the centroid mode from the other modes using an adiabaticity parameter given in the line following the propagator specification. The example input for a PI-MD run would look like the following:

&CPMD
  PATH INTEGRAL
  MOLECULAR DYNAMICS CP
  RESTART WAVEFUNCTION COORDINATES LATEST
  [...]
&END

&PIMD
  CENTROID DYNAMICS
  NORMAL MODES
    25.D0
  FACMASS
    1.D0
  TROTTER DIMENSION
    16
&END