👌 allow early-time Δa limiter to be 0 (#446)

doc/parameters/simulation.rst

@@ -194,7 +194,22 @@ random numbers used for the primordial noise.
 
                          Δa_max_early = 0.0008
 -- --------------- -- -
-\  **Example 1**   \  Effectively disregard this limiter completely by setting
+\  **Example 1**   \  Ignore this limiter by setting it equal to :math:`0`:
+
+                      .. code-block:: python3
+
+                         Δa_max_early = 0
+
+                      .. note::
+                         As the instantaneous Hubble time :math:`H^{-1}` is
+                         part of the same limiter as the early-time
+                         :math:`\Delta a` (see the paper on
+                         ':ref:`The cosmological simulation code CO𝘕CEPT 1.0<the_cosmological_simulation_code_concept_10>`'),
+                         doing this means that :math:`H^{-1}` will be used to
+                         set the global time step size at early times,
+                         leading to finer time-stepping at such times.
+-- --------------- -- -
+\  **Example 2**   \  Effectively disregard this limiter completely by setting
                       it equal to its
                       :ref:`late-time version <Deltaa_max_late>`:
 
@@ -210,10 +225,11 @@ random numbers used for the primordial noise.
                       .. note::
                          As the instantaneous Hubble time :math:`H^{-1}` is
                          part of the same limiter as the early-time
-                         :math:`\Delta a` (again, see the paper on
+                         :math:`\Delta a` (see the paper on
                          ':ref:`The cosmological simulation code CO𝘕CEPT 1.0<the_cosmological_simulation_code_concept_10>`'),
-                         this also removes any explicit limitation of the
-                         global time step size due to :math:`H^{-1}`.
+                         doing this effectively also removes any explicit
+                         limitation of the global time step size due
+                         to :math:`H^{-1}`.
 == =============== == =
 
 

src/commons.py

@@ -5891,8 +5891,8 @@ def open_hdf5(filename, raise_exception=False, **kwargs):
 # The maximum allowed Δa at late times is supposed to be bigger than the
 # maximum allowed value of Δa at early times,
 # and they must both be positive.
-if Δa_max_early <= 0:
-    abort('You must have Δa_max_early > 0')
+if Δa_max_early < 0:
+    abort('You must have Δa_max_early ≥ 0')
 if Δa_max_late <= 0:
     abort('You must have Δa_max_late > 0')
 if Δa_max_early > Δa_max_late:

src/main.py

@@ -773,12 +773,13 @@ def get_base_timestep_size(components, static_timestepping_func=None):
         Δt_hubble = fac_hubble/H
         bottleneck_hubble = 'the Hubble time'
         # Constant Δa overrule Hubble at early times
-        a_next = a + Δa_max_early
-        if a_next < 1:
-            Δt_Δa_early = Δt_base_background_factor*(cosmic_time(a_next) - t)
-            if Δt_Δa_early > Δt_hubble:
-                Δt_hubble = Δt_Δa_early
-                bottleneck_hubble = 'the maximum allowed Δa (early)'
+        if Δa_max_early > 0:
+            a_next = a + Δa_max_early
+            if a_next < 1:
+                Δt_Δa_early = Δt_base_background_factor*(cosmic_time(a_next) - t)
+                if Δt_Δa_early > Δt_hubble:
+                    Δt_hubble = Δt_Δa_early
+                    bottleneck_hubble = 'the maximum allowed Δa (early)'
         if Δt_hubble < Δt_max:
             Δt_max = Δt_hubble
             bottleneck = bottleneck_hubble