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

Posted: **August 08 2017**

by **Graeme Addison**

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

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

Thanks

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

Posted: **August 09 2017**

by **Antony Lewis**

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

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

Posted: **August 10 2017**

by **Graeme Addison**

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?

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

Posted: **August 10 2017**

by **Antony Lewis**

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

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

Posted: **August 10 2017**

by **Graeme Addison**

Hmm OK. I think I'll start with figuring out how much the derivatives are really varying in case I don't need to do the recalculation each step. Thanks.