How to understand "conserved quantity"

[Ambrose Phillips]

Hello:

       I'm new to iPi. I'm confused what is the  "conserved quantity " in output files and how to extract it from the simulations. I didn't find its definition in the manual. Is there anyone can help me to understand it.

Thanks!

Ambrose

[Michele Ceriotti]

The "conserved quantity" is basically the total energy, supplemented with whatever additional term coming from an extended Lagrangian (e.g. barostat degrees of freedom) and whatever accumulated term coming from thermostats (see e.g. Bussi, G, and M Parrinello, 2007, “Accurate Sampling Using Langevin Dynamics,” Phys. Rev. E (APS) 75, 56707.) 

From a practical PoV, the idea is to help you make sure that the integration error is not too severe, by checking that fluctuations and drift in that quantity are not too large.

Hope this helps,

Michele

[Ambrose Phillips]

Thank u, Michele !  

What you mean here is that this quantity is just a auxiliary tool to help me check the convergence of my calculations. So if I am just concerned with the system's physical property I calculated, the output  quantities, " quantum kinetic energy" and "potential energy", is much useful to me. Just like what u listed in paper (M. Ceriotti and D. E. Manolopoulos, "Efficient First-Principles Calculation of the Quantum Kinetic Energy and Momentum Distribution of Nuclei", Phys. Rev. Lett. 109, 100604 (2012).)

Is it correct for me to understand your words in this way? Looking forward to your reply!

Ambrose

[Michele Ceriotti]

Yes, correct. The conserved quantity is more of a diagnostic tool. Incidentally, to preempt possible doubts, the "temperature" is another quantity that is not very meaningful if not as a diagnostic, see e.g. the discussion here http://gle4md.org/index.html?page=docs