Total energy not conversed in NVE

Hi all,

I have a question about the conservative of total energy in NVE.

I am working on a simulation of rigid TIP3P water under nve ensemble. As I think, the total energy should have very small drift duing a long simulation time because of the numerical integration uncertainty.

However, from the result I had(attached, NVEvstimestep.bmp), the total energy increased with the timestep of 1fs and even decreased with timestep 2fs.

The simulation was divided into three steps:
1)From a initial configration, water box with 1969 water molecules was equilibrated for 1 ns in NPT.timestep=1fs
2)After that, the box was equilibrated for another 0.5 ns in NVT. timestep=1fs
3)Lastly, have the box equilibrated in NVE with different timesteps :2fs, 1fs, 0.5fs and 0.1fs.

During the simulation, water bond and angle are hold as rigid by SHAKE method. Vdw interaction larger than cutoff was shifted.

I don’t understand :

1)For timestep of 2fs, why the total energy decrease?

2)Why the energy increase with the timestep 1fs. Is it resulted from the numerical integration uncertainty?

The initial input file was also attached.

I appreciate any help.



NVEvstimestep.bmp (1.23 MB)

input.txt (875 Bytes)