## CAMB: calculating derivatives of Cl's wrt parameters

Use of Healpix, camb, CLASS, cosmomc, compilers, etc.
Posts: 18
Joined: July 17 2014
Affiliation: Johns Hopkins University

### CAMB: calculating derivatives of Cl''s wrt parameters

Is there any way to have CAMB calculate derivatives of CMB power spectra with respect to cosmological parameters? I.e. quantities like $\frac{\partial C_{\ell}}{\partial(\Omega_ch^2)}$

I can numerically estimate derivatives by perturbing parameters and re-running but wanted to check if there's some better / more efficient way.

Thanks

Antony Lewis
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Joined: September 23 2004
Affiliation: University of Sussex
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### Re: CAMB: calculating derivatives of Cl''''s wrt parameters

That's usually the best way. (unless you really want parameter errors/forecasts - in which case run CosmoMC).

Posts: 18
Joined: July 17 2014
Affiliation: Johns Hopkins University

### CAMB: calculating derivatives of Cl''s wrt parameters

OK. How about if I want to evaluate these derivatives at each step of a CosmoMC chain? Can I have CAMB called multiple times at each step with slightly different parameters?

Antony Lewis
Posts: 1423
Joined: September 23 2004
Affiliation: University of Sussex
Contact:

### Re: CAMB: calculating derivatives of Cl''''s wrt parameters

You probably could, but it might take some thought on how to code it.