## on how to modify equations.f90 in CAMB (in general)

Use of Cobaya. camb, CLASS, cosmomc, compilers, etc.
Jiwon Park
Posts: 7
Joined: July 09 2019
Affiliation: Soongsil University

### on how to modify equations.f90 in CAMB (in general)

Dear all,

I'd like to ask how one should modify equations.f90 file in CAMB to add new features and species in general cases.

when I modify CAMB and add new species, I follow this recipe but it seems that there's a case that I can't get the result that I want.
But I'm not sure these steps are right as I'm using CAMB after a too long time.
So I'd like to check them just in case.
Could you please let me know if there is anything wrong with the steps?

- This is how I modify 'equation.f90' file

1. add new indexes for additional equations in module 'GaugeInterface,' line 43,44 like the following:
(note that all the line number here is based on the original file, not the modified one)

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    integer, parameter :: basic_num_eqns = 6 ! originally 4 ! line 43
integer, parameter :: ix_etak=1, ix_clxc=2, ix_clxb=3, ix_vb=4, ix_new_one=5, ix_new_two=6 !Scalar array indices for each quantity
! ix_new_one and ix_new_two are the new species

2. add initial conditions in 'subroutine initial(EV,y, tau),' like the following;

i) after the line 1769,

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    integer, parameter :: i_clxg=1,i_clxr=2,i_clxc=3, i_clxb=4, &
i_qg=5,i_qr=6,i_vb=7,i_pir=8, i_eta=9, i_aj3r=10,i_clxde=11,i_vde=12, & ! line 1769
i_new_one=13, i_new_two=14

ii) and for example, in the adiabatic initial condition, after the line 1856,

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    initv(1,i_eta)=-chi*2*EV%Kf(1)*(1 - x2/12*(-10._dl/Rp15 + EV%Kf(1))) ! line 1856
initv(1,i_new_one)=(###corresponding IC###)
initv(1,i_new_two)=(###corresponding IC###)

iii) and after the line 1927,

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    !  Baryons
y(ix_clxb)=InitVec(i_clxb)
y(ix_vb)=InitVec(i_vb) !line 1927

! new species
y(ix_new_one)=InitVec(i_new_one)
y(ix_new_two)=InitVec(i_new_two)

3. add corresponding new equations in 'subroutine derivs(EV,n,tau,ay,ayprime),' like the following:

i) first, after the line 2176, add the variables

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    real(dl) dgrho_de, dgq_de, cs2_de ! line 2176

real(dl) new_one, new_two ! new species
real(dl) new_var ! new variable

new_var=CP%new_var ! read input from inifile

ii) define the variables after the line 2196

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    !  Baryon variables
clxb=ay(ix_clxb)
vb=ay(ix_vb) ! line 2196

! new species
new_one=ay(ix_new_one)
new_two=ay(ix_new_two)
! first CDM, baryon, and then add new species following the order defined in line 44

iii) add equations after the line after the line 2311

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    !  Baryon equation of motion.
clxbdot=-k*(z+vb)
ayprime(ix_clxb)=clxbdot ! line 2311

! new species equation
new_one = (###new equation###)
new_two = (###new equation###)
! first CDM, baryon, and then add new species following the order defined in line 44

(I want to know this equation order is right or not, as different order gives sometimes different things and maybe errors too.
Plus, is this order mandatory in CAMB code?)

for example,

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    !  8*pi*a*a*SUM[rho_i*clx_i] - radiation terms
dgrho=dgrho + grhog_t*clxg+grhor_t*clxr + new_one/a ! line 2272


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    !  CDM equation of motion
clxcdot=-k*z
ayprime(ix_clxc)=clxcdot - k*new_one/new_two ! line 2307

4. For a final stage, modify other fortran files(like model.f90, camb.f90), python interface, etc. to read new inputs and to conduct other features you want.

(p.s. I note that this question is originally a subquestion of this question(viewtopic.php?f=11&t=3495).
I wrote my post too briefly and I thought I should explain more in detail to provide information to others so that someone may answer my post.
However, it seems to me that this question is worth treating another independent question so that others may refer to this post when modifying CAMB, and hence I write the question as a general one.)

Antony Lewis
Posts: 1723
Joined: September 23 2004
Affiliation: University of Sussex
Contact:

### Re: on how to modify equations.f90 in CAMB (in general)

This looks basically OK at a quick look.
If you are using fixed indices in derivs(), they do need to be part of the basic_num_eqns equations at the start of the arrays. Your output derivatives for the new variables are assigned as

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ayprime(ix_new_one) = [xxx]
etc.