# Can lammps calculate rotational spectra( rotational density of state) ?

Dear Lammps Users,

I have a question about the calculation of rotational density of state.

I am trying to compute the absorption spectrum of water via the Fourier-transform of the time auto-correlation function of the total dipole moment M of system.

The formula like this : alph(omega)=F(omega)integral(dt exp(iomega*t)<M(0)*M(t)>). I use SPCE -rigid water model.

I would like to know which COMMAND I should use if I want to dump the out put dipole moment M in the dump file.

Thank you very much in advance!

Best Regards

Liyi

Dear Lammps Users,

I have a question about the calculation of rotational density of state.

​no, you don't.

I am trying to compute the absorption spectrum of water via the
Fourier-transform of the time auto-correlation function of the total dipole
moment M of system.

​that is a very different thing from the rotational density of state.​

The formula like this : alph(omega)=F(omega)*integral(dt
exp(i*omega*t)<M(0)*M(t)>). I use SPCE -rigid water model.

I would like to know which* COMMAND* I should use if I want to dump the
out put dipole moment M in the dump file.

​there is no such command, since ​you cannot output global properties in a
dump file. dump files can only store per-atom properties. ​

​axel.​

Dear Axel,

Actually,I want to output the quantity of angular momentum.

I was trying to use fix momentum command and compute angmom/chunk command to calculate and output by fix ave/time.But it does not work.

Could you please recommend which command should I use to calculate angular momentum and output command?

I am grateful all the suggestions and help.

Best Regards

“But it does not work.” is not very descriptive so it is difficult for us to be of any help. How does it not work? What error(s) do you get? What output? What does your input script look like? Etc…

Dear Axel,

​???

Actually,I want to output the quantity of angular momentum.

​angular momentum of what?​

I was trying to use* fix momentum *command and *compute angmom/chunk*
command to calculate and output by fix ave/time.But it does not work.

​have you actually read the documentation of these commands? i mean
properly and not just the summary on top of the pages.
and how exactly have you used them?​ and how exactly have they failed to
work?

Could you please recommend which command should I use to calculate angular
momentum and output command?

​nobody can help you unless you describe in a comprehensible way what it is
exactly, that you want to ​do. so far, you are giving the impression that
you don't really know yourself what that is.

axel.