In that case this might be a bug in how we set comoving units for artio data. The one artio test dataset we have has DeltaDC equal to 0.0, so we're not testing the case where this is nonzero at all. Is there any chance you can create an issue on our bug tracker that demonstrates the problem (at https://github.com/yt-project/yt/issues/new)? If you can, please include a link to a sample dataset that triggers the issue as well as a script demonstrating the problem so one of us can reproduce locally and hopefully fix it. -Nathan On Fri, Oct 13, 2017 at 2:57 PM, Nick Gnedin <gnedin@fnal.gov> wrote:

Britton,

In the simulations with the non-zero DC mode there are two scale factors to consider, a_uni (global scale factor of the universe) and a_box (the scale factor of the simulated region). YT recognizes this when reading ART data, but appears to use a wrong scale factor in computing comoving units.

n

On 10/13/2017 2:44 PM, Britton Smith wrote:

...

Hi Huanqing,

I am a bit unclear on what the problem is. I think it is correct that the conversion between proper and comoving uses the expansion factor. I'm not sure what abox is. Can you elaborate on that?

Britton

On Thu, Oct 12, 2017 at 4:27 PM, Huanqing <hqchen@oddjob.uchicago.edu <mailto:hqchen@oddjob.uchicago.edu>> wrote:

Hi all,

I’m using yt to generate halo catalog. However, when I check the halos created by HaloCatalog function, I found the corresponding comoving unit uses the expansion factor of the universe rather than a_box.

For example, My boxsize is 20cMpc/h, but if I use .in_units(‘Mpccm/h’), the halos are offset by some amount (see attached). I wonder if this is a bug in converting physical unit to comoving unit?

Thanks, Huanqing

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