[lammps-users] question about variable command

Dear developers,
I am not sure I understand all about variable command.
I ish to convert a temperature to a velocity
So I need to read the mass of the chosen atoms

So I define a
group SI type 1
group Al type 2
mass 1 28.00
mass 2 27.00
variable vz2 equal 0.909*sqrt(${Tvap}/mass[2]) # velocity of atom type 2 -> this line gives no error

print v_vz2 or print ${vz2} do not give the vz2 value . So it is not right written. What is the good syntax ?

I assumed that mass[2] return mass of type 2 atom. Is it correct ?

Thanks a lot for your help. It is certainly evident and I am probably not correctly reading the manual.

Hi all

Please forget my request. I found the solution.
In fact first define
variable mass2 index 26.98
variable Tvap index 500.00

variable vz2 equal 0.90897868*sqrt(\{Tvap\}/{mass2})
gives vz2 in A/ps

I was just blind.