# Re: [MPB-discuss] Unit problem with Mpb software

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Steven G. Johnson <stevenj.mit <at> gmail.com> writes:```
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>
>
>
> On Jul 14, 2009, at 11:08 AM, Alexandre Besnier wrote:
>   Good morning, I'm a student and I work currently on your software
Mpb, I try to make a rectangular lattice of 0.204µm x 0.561µm size, and
I would like to understand units give by band diagram. On your wiki
pages you say that frequency is given in c/a unit, but I don't
understand several things :     - In band diagram, if I obtain frequency
= 1, that say a=c, ie a=3.10^8 meters ?
>
>
> No, it means that frequency = 1 (c/a).  Since frequency = c / lambda,
this means that
>
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>       c/lambda = 1 (c/a)
>
> and thus
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>       lambda = a
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>
> Put another way, the frequency in MPB's units is exactly the same as
a/lambda  (where lambda = vacuum wavelength).
>
>      - Normally, "a" is the lattice period, but with rectangular
lattice, I have two periods, so what is "a" compared with my two periods
?
>
> "a" is whatever unit of distance you want; it doesn't have to have
anything to do with the lattice constant(s).
>
> You are perfectly free to use a=1um, for example, and then specify the
size of your computation cell as 0.204 x 0.561, and give all distance
units in microns.  In this case the frequencies returned by MPB are
equivalent to 1/(lambda in microns).
>
>      - In the page "Mpb User Tutorial/A few words on units", you say
at the last paragraph : "Thus, the corresponding vacuum wavelength is a
over the frequency eigenvalue." It is not "Thus, the corresponding
vacuum wavelength is c over the frequency eigenvalue." ? Because
frequency * lambda = c ?
>
> See above.  This is a consequence of using c/a units of frequency.
>
> | Finally,  if I would like to have frequency in Hz, what I have to do
to convert it ?
>
> Multiply Meep's units by c/a in SI units.
>
>
> Steven
>
>
>
>
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Hi Professor Steven G. Johnson

I am trying to simulate the rectangular rods in rectangular lattice and
facing difficulty in the process. I am not sure how to set the
rectangular rods length in program as in book it is mentioned for square
rods in sqare lattice that square side has a relation as side/a=0.4(as
an example) but how can we apply it for the case rectangular rods in
rectangular lattice. For example my dimensions for rectangular rod and
rectangular lattice are  x1 x y1 and x2 x y2 then what will be my inputs
in program. Thanks
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