I'm finding significant differences between the matter power spectrum obtained through a direct CAMB call, and that available through the PK%PowerAt function in CosmoMC.
Here's the minimal way I was able to reproduce the issue:
1) I downloaded the latest version of CAMB (as of the time of this post), changed "get_transfer = T" in params.ini and plotted the contents of the resulting "test_matterpower.dat"
2) I downloaded the latest version of CosmoMC and added a print statement that prints a k array and the output of PK%PowerAt at those values of k and z=0. I compiled and ran CosmoMC with a minimal .ini file that sets the cosmological parameters to those printed out by CAMB in step 1). I copied the output and overplotted. I find that the resulting matter power spectra P(k,z=0) agree to roughly 1%.
3) I now changed only w=−1 to w=−1.5 in both the CAMB params.ini file and the CosmoMC .ini file and reran both. This results in a disagreement of nearly a factor of two around k~0.01, smoothly changing to percent level agreement at k~1.
A colleague of mine independently verified this problem. Thanks in advance for any help or insights you can offer.
CAMB/CosmoMC: Matter Power Spectrum for w!=−1

 Posts: 7
 Joined: May 30 2013
 Affiliation: UC Berkeley

 Posts: 1353
 Joined: September 23 2004
 Affiliation: University of Sussex
 Contact:
Re: CAMB/CosmoMC: Matter Power Spectrum for w!=1
Are you sure the parameters being used by CAMB are the same?
in Case (1) for example, CosmoMC will be setting transfer_high_precision =T (and the kmax is probably also different)
In Case(2), If you just changed the w input parameter in Cosmomc, that will change H0 at fixed theta (since theta is also another input parameter, which I assume you left unchanged?). In the CAMB .ini file, H0 is specified directly instead of theta, so you will be changing w at fixed H0 (and hence varying theta).
in Case (1) for example, CosmoMC will be setting transfer_high_precision =T (and the kmax is probably also different)
In Case(2), If you just changed the w input parameter in Cosmomc, that will change H0 at fixed theta (since theta is also another input parameter, which I assume you left unchanged?). In the CAMB .ini file, H0 is specified directly instead of theta, so you will be changing w at fixed H0 (and hence varying theta).

 Posts: 7
 Joined: May 30 2013
 Affiliation: UC Berkeley
CAMB/CosmoMC: Matter Power Spectrum for w!=1
After updating theta along with w in CosmoMC so as to get a consistent H0, I find the two agree at the percent level. Thanks for the help!