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CAMB: halofit matter power spectrum derivative
 
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Matthew Digman



Joined: 03 Feb 2017
Posts: 7
Affiliation: The Ohio State University

PostPosted: September 29 2017  Reply with quote

Hey,
I was running a test involving python camb 0.1.6.1's Takahashi halofit matter power spectrum, and I notice that there is a large spike around kh=0.005 h Mpc−1 in the second derivative of the matter power spectrum with respect to k, shown here:

I assume the spike itself is because the implementation of halofit switches to using the linear power spectrum for kh below Min_kh_nonlinear = 0.005, which creates a discontinuity, but I would not expect that to produce an extended feature like this, unless something is smoothing it out. Is camb applying a smoothing to the halofit output, or is this feature something else?
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Antony Lewis



Joined: 23 Sep 2004
Posts: 1293
Affiliation: University of Sussex

PostPosted: September 29 2017  Reply with quote

It depends on what function you are using. You can either get the raw k sampling, or a spline object which does bicubic spline interpolation on P(k,z) over the sampled points. The raw k sampling may not be very fine by default.
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Matthew Digman



Joined: 03 Feb 2017
Posts: 7
Affiliation: The Ohio State University

PostPosted: September 29 2017  Reply with quote

Ok, I see that increasing set_accuracy does decrease the width of the spike, as expected. Thanks!
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Antony Lewis



Joined: 23 Sep 2004
Posts: 1293
Affiliation: University of Sussex

PostPosted: September 29 2017  Reply with quote

You can change just the k sampling by setting k_per_logint using set_matter_power.
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Matthew Digman



Joined: 03 Feb 2017
Posts: 7
Affiliation: The Ohio State University

PostPosted: September 30 2017  Reply with quote

Oh, awesome. I hadn't realized that was different from setting npoints.
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