I'm attempting to reproduce figure 1 of Scott et al. 2016 (https://arxiv.org/abs/1603.03550), but my results don't match the paper at all at low multipoles. Please see my attempt at reproducing figure 1 for [math]. At low multipoles, my TT curve has a dip at [math] where the Scott et al. curve does not. This is only the most glaring difference; none of the curves match at low multipoles.
To try to debug the problem, I plotted [math] as a function of [math].
Because the Fisher matrix (especially in this case) depends essentially on the square of the derivative, I also plotted the absolute value, and the dip at [math] appears.
I spoke with Dr. Scott about this, and he points out that this dip is unphysical. He suggested I check the accuracy at low multipoles, because he had similar difficulties when he was developing the paper, but couldn't recall exactly what he had to do to correct it.
Here is my CAMBpars instance that I'm using to get the [math].
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Using CAMB version 1.0.8 class: <CAMBparams> WantCls = True WantTransfer = False WantScalars = True WantTensors = False WantVectors = False WantDerivedParameters = True Want_cl_2D_array = True Want_CMB = True Want_CMB_lensing = True DoLensing = True NonLinear = NonLinear_lens Transfer: <TransferParams> high_precision = False accurate_massive_neutrinos = False kmax = 0.9 k_per_logint = 0 PK_num_redshifts = 1 PK_redshifts = [0.0] want_zstar = False want_zdrag = False min_l = 2 max_l = 3149 max_l_tensor = 600 max_eta_k = 18000.0 max_eta_k_tensor = 1200.0 ombh2 = 0.022277 omch2 = 0.11843 omk = 0.0 omnuh2 = 0.000645138398938 H0 = 67.8708311184 TCMB = 2.7255 YHe = 0.245361301871 num_nu_massless = 2.03066666667 num_nu_massive = 1 nu_mass_eigenstates = 1 share_delta_neff = False nu_mass_degeneracies = [1.0153333333333332] nu_mass_fractions = [1.0] nu_mass_numbers =  InitPower: <InitialPowerLaw> tensor_parameterization = tensor_param_rpivot ns = 0.9683 nrun = 0.0 nrunrun = 0.0 nt = -0.0 ntrun = -0.0 r = 0.0 pivot_scalar = 0.05 pivot_tensor = 0.05 As = 2.14044747364e-09 At = 1.0 Recomb: <Recfast> min_a_evolve_Tm = 0.00110987795051 RECFAST_fudge = 1.125 RECFAST_fudge_He = 0.86 RECFAST_Heswitch = 6 RECFAST_Hswitch = True AGauss1 = -0.14 AGauss2 = 0.079 zGauss1 = 7.28 zGauss2 = 6.73 wGauss1 = 0.18 wGauss2 = 0.33 Reion: <TanhReionization> Reionization = True use_optical_depth = True redshift = 10.0 optical_depth = 0.0666 delta_redshift = 0.5 fraction = -1.0 include_helium_fullreion = True helium_redshift = 3.5 helium_delta_redshift = 0.4 helium_redshiftstart = 5.5 tau_solve_accuracy_boost = 1.0 timestep_boost = 1.0 max_redshift = 50.0 DarkEnergy: <DarkEnergyFluid> w = -1.0 wa = 0.0 cs2 = 1.0 use_tabulated_w = False NonLinearModel: <Halofit> Min_kh_nonlinear = 0.005 halofit_version = mead HMCode_A_baryon = 3.13 HMCode_eta_baryon = 0.603 Accuracy: <AccuracyParams> AccuracyBoost = 1.0 lSampleBoost = 1.0 lAccuracyBoost = 1.0 AccuratePolarization = True AccurateBB = False AccurateReionization = True TimeStepBoost = 1.0 BackgroundTimeStepBoost = 1.0 IntTolBoost = 1.0 SourcekAccuracyBoost = 1.0 IntkAccuracyBoost = 1.0 TransferkBoost = 1.0 NonFlatIntAccuracyBoost = 1.0 BessIntBoost = 1.0 LensingBoost = 1.0 NonlinSourceBoost = 1.0 BesselBoost = 1.0 LimberBoost = 1.0 SourceLimberBoost = 1.0 KmaxBoost = 1.0 neutrino_q_boost = 1.0 SourceTerms: <SourceTermParams> limber_windows = True limber_phi_lmin = 100 counts_density = True counts_redshift = True counts_lensing = False counts_velocity = True counts_radial = False counts_timedelay = True counts_ISW = True counts_potential = True counts_evolve = False line_phot_dipole = False line_phot_quadrupole = False line_basic = True line_distortions = True line_extra = False line_reionization = False use_21cm_mK = True z_outputs =  scalar_initial_condition = initial_adiabatic InitialConditionVector =  OutputNormalization = 1 Alens = 1.0 MassiveNuMethod = Nu_best DoLateRadTruncation = True Evolve_baryon_cs = False Evolve_delta_xe = False Evolve_delta_Ts = False Do21cm = False transfer_21cm_cl = False Log_lvalues = False use_cl_spline_template = True SourceWindows =  CustomSources: <CustomSources> num_custom_sources = 0 c_source_func = None custom_source_ell_scales =