How to ADD a new parameter in cosmomc(NOT how to re-interpret an existing one)

[Patrice Okouma]

I guess that one can add N more parameters to cosmomc.
My question is : what's the upper bound for N ?
Up to now I've been essentially converting 1 non used parameters into another one.

Basically what I want to do in the mcmc part is :
1 / Generate a large set of new parameters , say {S_i}, along the "standard" ones;
2 / save them for use in the camb module (this part I know how to do).

Could you give me some suggestions please ?
At least how to add a totally new parameter will help. I do not want to re-interpret an existing one. I want to add.

Thanks in advance,
Patrice

[Antony Lewis]

There's no fixed upper bound, but the more you add the less likely it is to work well. Certainly should be no problem adding around 10 new well-behaved parameters. See

http://cosmologist.info/cosmomc/readme.html#Programming

[Patrice Okouma]

Antony,
thanks for the hint on the upper bound. I've also found how to add 1 or more parameters.
Your link to how to do it is not pedagogical enough. I will post a tentative 5 steps brief guide in a few.
Cheers,
Patrice

[Patrice Okouma]

Some tips for adding parameters to cosmomc (PLEASE IMPROVE):
-- Thanks to suggestions from a network of Actors in cosmology ---

If you want to add a slow parameter (a parameter whose the change of
value requires that the transfer functions - the slowest part of camb
- are recalculated), increase num_hard in settings.f90,

If you want to add a spectral index type of parameter (fast parameter) increase
num_initpower in settings.f90

If you want to add a new amplitude kind of parameter (fast parameter) increase
num_norm in settings.f90

If you want to add any other type of FAST parameter increase either of
the above, and interpret correspondingly. If you wnat to add a nuisance paramer increase
num_nuisance_params in setting.f90

Example:
let's add a new slow parameter, say w_a, without
reinterpreting any of the existing parameters.
(Assuming that the required equations have been amended)

Step 1 :
Increase num_hard from 7 to 8 in setting.f90.

Step 2 :
Then in cmbtypes.f90,
add a new entry to the record.
e.g Type CMBParams. Originally it was

Type CMBParams
real nuisance(1:num_nuisance_params)
!unit Gaussians for experimental parameters
real norm(1:num_norm)
!These are fast parameters controling amplitudes, calibrations, etc.
real InitPower(1:num_initpower)
!These are fast paramters for the initial power spectrum
!Now remaining (non-independent) parameters
real omb, omc, omv, omnu, omk, omdm
real ombh2, omch2, omnuh2, omdmh2
real zre, nufrac
real h, H0
real w
real reserved(5)

end Type CMBParams

now add into this a line
real cool_wa ! time-varying eqn of state
probably just after w.

Step 3 :

Now in params_CMB.f90 you must put your new hard parameter (recall it is
parameter 8) from params(8) to CMB%cool_wa. So in subroutine
SetForH(Params,CMB,H0) we need to add the line
CMB%cool_wa = Params(8).

There is also an opposite routine CMBParamsToParams(CMB, Params).
Here we add Params(8) = CMB%cool_wa.

Step 4 :

One final thing is now to pass the new CMB%cool_wa to camb. That is done
in CMB_Cls_simple.f90 in subroutine CMBToCAMB(CMB,P). So there we put
P%new_cool_wa = CMB%cool_wa.

Note that here P% is the camb parameter record defined in
../camb/modules.f90. So above It is assumed that we have edited camb so
that it has one new parameter (C)P%new_cool_wa which is then used
throughout camb to calculate (e.g) w from w_0 and w_a at each time.

Another thing to note is that if you increase num_initpower from 3 to 4
and keep all the other things in cosmomc unchanged then your new
parameter is param # 11 etc.

WE PROBABLY MADE A MISTAKE OR OMITTED AN IMPORTANT POINT SOMEWHERE, PLEASE HELP BY radically IMPROVING THIS.