[gmx-developers] Conserved energy drifts more with shorter time steps

[David van der Spoel]

Hi,

this is in between a user and a developer query. We find that for 
flexible liquid simulations using gromacs 5.0.4/single precision the 
energy conservation gets WORSE with decreasing time step. A plot showing 
this is in
http://folding.bmc.uu.se/images/Econserved-timestep.xvg or
http://folding.bmc.uu.se/images/Econserved-timestep.pdf

MDP settings
    coulombtype                    = PME
    coulomb-modifier               = Potential-shift
    rcoulomb-switch                = 0
    rcoulomb                       = 1.1
    epsilon-r                      = 1
    epsilon-rf                     = inf
    vdw-type                       = PME
    vdw-modifier                   = Potential-shift
    rvdw-switch                    = 0
    rvdw                           = 1.1
    tcoupl                         = Nose-Hoover
    nsttcouple                     = 10
    nh-chain-length                = 1
    pcoupl                         = No
    ref-t:      298.15
    tau-t:         0.5

Any clues whether we are doing something wrong? Or is there a bug?

Cheers,
-- 
David van der Spoel, Ph.D., Professor of Biology
Dept. of Cell & Molec. Biol., Uppsala University.
Box 596, 75124 Uppsala, Sweden. Phone:	+46184714205.
spoel at xray.bmc.uu.se    http://folding.bmc.uu.se