Discontinuities in the conserved energy between successive NVT runs

Questions and discussions regarding the use of Qbox
Forum rules
You must be a registered user to post in this forum. Registered users may also post new topics if they consider that their subject does not correspond to any topic already present on the forum.
Post Reply
fgygi
Site Admin
Posts: 167
Joined: Tue Jun 17, 2008 7:03 pm

Discontinuities in the conserved energy between successive NVT runs

Post by fgygi »

It is sometimes observed that when using the BDP thermostat in a sequence of runs, the plot generated by the econste.plt script has discontinuities at the beginning of each run. The reason is the following:

When using no thermostat (NVE simulation) the constant of the motion <econst> is conserved (if the time step is small enough and if the ground state is reached with sufficient accuracy at each MD step. The value of <econst> is <ekin_ion> + <etotal> where <ekin_ion> is the kinetic energy of the nuclei, and <etotal> is the total Kohn-Sham energy.

When using a thermostat, the quantity <ekin_ion>+ <etotal> is not constant since the thermostat modifies the ionic velocities and this changes <ekin_ion>.

In the case of the BDP thermostat, the amount of kinetic energy that is given to (or taken from) the ions can be computed, and in that case the quantity <econst> includes that contribution from the thermostat. This means that in the case of the BDP thermostat, <econst> should be conserved.

This is done by keeping track (i.e. integrating) all the contributions to <ekin_ion> from the thermostat during the run. However, this information is not carried from one run to the next, so at the beginning of each run, it starts from a zero contribution from the thermostat. This causes the discontinuity in the plot of <econst> across multiple runs.

The only information that should be extracted from the econste.plt plot is whether the value of <econst> is reasonably constant during a run. This would indicate that the time step is small enough and the ground state calculations are converged after taking into account the contributions from the thermostat.
Post Reply