EVOLUTION: Ensure stable initial time step for embedded methods

Restricted initial time steps to values smaller than the CFL time step
to ensure numerical stability. This prevents instability issues that
may arise from excessively large initial steps.

Signed-off-by: Grzegorz Kowal <grzegorz@amuncode.org>

sources/evolution.F90

@@ -1283,9 +1283,9 @@ module evolution
       d(1:2) = sqrt(fnorm * d(1:2))
 
       if (minval(d(1:2)) >= 1.0d-05) then
-        h0 = 1.0d-02 * d(1) / d(2)
+        h0 = min(dt, 1.0d-02 * d(1) / d(2))
       else
-        h0 = 1.0d-06
+        h0 = dt
       end if
 
       m    = 10
@@ -1354,7 +1354,7 @@ module evolution
         if (h1 > 1.0d-15) then
           h1 = (1.0d-02 / h1)**(1.0d+00 / 3.0d+00)
         else
-          h1 = max(1.0d-06, 1.0d-03 * h0)
+          h1 = max(dt, 1.0d-03 * h0)
         end if
 
         dte = min(1.0d+02 * h0, h1)