# May compute temperture have little problem?

Hello Axel,

In manual, compute temp command say it calculate the temperture by tensor. Tensor should be consider as matrix(in fact ,it is not strict, but it could be in this problem).
Then the trace of matrix is invariant, so we often use the trace to denfine physics.

Therefore, the mean of trace of temperture tensor should be temperture vaule, so I output c_TTTT[1],[2],[3], and their mean is about 0.92

This problem is like Rg tensor, I also calculate the Rg tensor, and output c_Rg[1],c_Rg[2],c_Rg[3], and the value is what i need

So, I’m confused! May be I’m stupid to make a mistake, but I try my best.

Thanks,

Yongjin Ruan

Hello Axel,

In manual, compute temp command say it calculate the temperture by
tensor.

no, it doesn't say so and it doesn't do it. you get the *temperature*
for the compute group by accessing the *scalar* c_TTTT. that is what
the docs say and that is what gives you the temperature. the tensor
c_TTTT is a *kinetic energy* tensor. you can easily confirm this
from reading the source code. it looks like "almost 1.0" only because
you are using reduced units (conversion from m*v^2 to energy is 1.0
and k_B is 1.0) and also all computed _extensive_ properties (i.e.
that depend on the number of atoms like kinetic energy) get normalized
(i.e. divided by the total(!) number of atoms). temperature (the
scalar) is an _intensive_ property (does not depend on the number of
atoms) does not get normalized (the number of degrees of freedom for
the group of atoms is already factored in by the compute).

your group as 12144 out of 13200 atoms with a kinetic energy of ~1
epsilon per atom. => 12144/13200 = 0.92

thermo_modify norm no

and you should see a drastic difference in the thermo output for all
extensive properties.

Tensor should be consider as matrix(in fact ,it is not strict, but
it could be in this problem).
Then the trace of matrix is invariant, so we often use the trace to
denfine physics.

Therefore, the mean of trace of temperture tensor should be temperture
vaule, so I output c_TTTT[1],[2],[3], and their mean is about 0.92

This problem is like Rg tensor, I also calculate the Rg tensor, and
output c_Rg[1],c_Rg[2],c_Rg[3], and the value is what i need

So, I'm confused! May be I'm stupid to make a mistake, but I try my
best.

there is a simple way to avoid confusion: read the documentation *all
the way* and also check out the sections that it refers to for further
information.