# Langevin thermostat cumulative energy in/out

Dear Steve and Users,

I have two questions on Langevin thermostat cumulative energy in/out.

1. I am trying to understand the “unit” of the energy output from Langevin thermostat (fix commend) with tally yes. It looks like it gives the extensive scalar number and I am not sure what it should mean exactly. Does it give “total” cumulative energy to/from atoms (in group) in kcal/mol unit or just kcal (I am using the real unit) since it is “extensive”. Or “extensive” just meant by “total sum” of energy to/from all the atoms (in group).

2. Also, is this energy “net” energy in/out or total “external” energy in/out without considering the energy “loss” by the damping?

3. What does it mean by “potential” energy as an output (perhaps this question is also related to the Q#2)?

Hope my questions are clear enough.

If you can elaborate it, it would be great.

Thanks, Gisuk.

Dear Steve and Users,

I have two questions on Langevin thermostat cumulative energy in/out.

1. I am trying to understand the “unit” of the energy output from
Langevin thermostat (fix commend) with tally yes. It looks like it gives
the extensive scalar number and I am not sure what it should mean exactly.
Does it give “total” cumulative energy to/from atoms (in group) in kcal/mol
unit or just kcal (I am using the real unit) since it is “extensive”. Or
“extensive” just meant by “total sum” of energy to/from all the atoms (in
group).

​if you are using real units, the energy unit is kcal/mol (mol in this
context is the entire system). the langevin thermostate removes (damping)
and adds (random noise) energy to the system. ​the value returned is the
sum over all particles in the group over all time of these terms. if this
is output as a per particle fraction depends on your thermo settings. for
real units, that is by default the total sum.

in short, if you have no other manipulations of your system, then the sum
of the scalar returned by fix langevin and the etotal value be more or less
constant.

2. Also, is this energy “net” energy in/out or total “external”

energy in/out without considering the energy “loss” by the damping?

​"net".​

3. What does it mean by “potential” energy as an output (perhaps
this question is also related to the Q#2)?

​see above. ​the statement in the manual assumes that the kinetic energy is
constant (as it should with a balanced thermostat).

axel.