Energy conservation NVT/NVE simulation

[marco masia]

Dear all,

recently I have noticed the same problem with FIST simulations. I took the 
H2O-32_ewald.inp input, equilibrated it for 1 ns and run a 1ns NVE 
simulation from the thermalized configuration. I have tried to refine many 
simulation parameters (e.g. time step, RCUT, Ewald pars) and also (thinking 
that there was a bad implementation of constraints) I run the simulations 
for a flexible model. I always get a non-negligible drift (see attachment); 
the graph shows block averaged energies of 1 ns simulations performed with 
cp2k and obeli.x (a program of mine). The values are shifted for a better 
comparison. I have been looking into this, thinking that the problem was in 
the input file...but since other people are experiencing it with QS, I 
wonder if there is some bug in the velocity verlet routines. The results 
are platform independent (I tried both on mac and linux).

best

marco

On Wednesday, September 5, 2012 3:32:26 AM UTC-4, albe... at virgilio.it 
wrote:
>
> Dear all,
>
> I'm carrying out some tests to reproduce the VDOS of water and I used as
> starting points the input files 'H2O-32.inp' and 'H2O-64.inp' in
> tests/QS/benchmark directory.
>
> In both cases I ran a NVT simulation at 300K for 15 ps followed by an NVE
> simulation on the equilibrated system.
>
> I'm puzzled about the results in the energies: in both NVT and NVE  the 
> plot of
> "Cons Qty" show a constant and linear drift while temperature, kinetic and
> potential energy are equilibrated and stable.
>
> Since "Cons Qty" should be conserved and this is not the case I think that
> there are some problems in the setup of the simulation.
> I did not modify the parameters of the benchmark inputs but I just added 
> the
> thermostat section in NVT and I modified only the printing option.
>
> Thank you very much for your help and suggestions,
>
> Alberto
>
>
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