# group energy increases while using NVE for a group

Dear all,

There are two parts in my system: metal substrate and liquid bulk. I am using metal to heat liquid, where "fix NVT" is used for metal and "fix NVE" for liquid. The initial temperature is 130 K for metal and 100 K for liquid. Periodic boundary condition is used for x and y dimensions, and reflection boundary is employed for the upper z dimension and fixed boundary condition for the lower z dimension.

During the simulation process, the liquid energy increases gradually as its temperature increases from 100 to 130 K. My question is why the liquid energy increases since I use "fix NVE" for the liquid. As far as I know, using NVE for a group means the group energy will not fluctuate significantly during simulation process.

Best wishes,

Xiaohui She

University of Wisconsin-Madison

Dear all,

There are two parts in my system: metal substrate and liquid bulk. I am using metal to heat liquid, where "fix NVT" is used for metal and "fix NVE" for liquid. The initial temperature is 130 K for metal and 100 K for liquid. Periodic boundary condition is used for x and y dimensions, and reflection boundary is employed for the upper z dimension and fixed boundary condition for the lower z dimension.

During the simulation process, the liquid energy increases gradually as its temperature increases from 100 to 130 K. My question is why the liquid energy increases

it increases, because it is in contact with the hotter metal and thus
kinetic energy is transferred.

since I use "fix NVE" for the liquid. As far as I know, using NVE for a group means the group energy will not fluctuate significantly during simulation process.

no. you know wrong. please discuss this with your adviser. your
question indicates that you need to get a better understanding of
statistical mechanics.
fix nve does time integration only and no manipulation of the system
otherwise. if there is *NOTHING ELSE GOING ON*, it will result in an
NVE ensemble.

please also note that you are talking about energy *transfer* and not
fluctuations. the latter are linked to the system size and its
compressibility, but not the time integration.

axel.