Add note about spin conventions in the orignal SEOBNRv4PHM model and …

…surrogate.
sxs-collaboration · Aug 25, 2024 · 9b6716f · 9b6716f
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diff --git a/gwsurrogate/surrogate.py b/gwsurrogate/surrogate.py
@@ -2528,6 +2528,23 @@ class SEOBNRv4PHMSur(SurrogateEvaluator):
 
 See the __call__ method on how to evaluate waveforms.
 In the __call__ method, x must have format x = [q, chi1, chi2].
+
+
+IMPORTANT NOTES:
+===============
+
+The original SEOBNRv4PHM model (arXiv:2004.09442) parameterizes the 
+direction of the BH spins relative to the Newtonian orbital angular momentum
+vector of the binary in the co-precessing frame. The SEOBNRv4PHMSur model, 
+however, parameterizes the spins relative to the direction where the radiation
+is always strongest along the z-axis, and the (ell,m)= (2, ±2) modes are dominant.
+This is the same convention used in the NRSur7dq4 surrogate model.
+
+These two frames are different for non-precessing systems. Hence, specifying
+the spins in the SEOBNRv4PHM-frame (e.g. when called through LALSimulation)
+will not give a physically equivalent waveform when these same spin directions
+are used in the SEOBNRv4PHMSur surrogate model. Ref. arXiv:2203.00381 provides
+further discussion on this point.
     """
 
     def __init__(self, h5filename):

diff --git a/tutorial/website/SEOBNRv4PHM.ipynb b/tutorial/website/SEOBNRv4PHM.ipynb
@@ -14,6 +14,15 @@
    "execution_count": 1,
    "metadata": {},
    "outputs": [
+    {
+     "name": "stdout",
+     "output_type": "stream",
+     "text": [
+      "lal.MSUN_SI != Msun\n",
+      "__name__ = gwsurrogate.new.spline_evaluation\n",
+      "__package__= gwsurrogate.new\n"
+     ]
+    },
     {
      "name": "stderr",
      "output_type": "stream",
@@ -42,15 +51,6 @@
       "\n",
       "  import lal\n"
      ]
-    },
-    {
-     "name": "stdout",
-     "output_type": "stream",
-     "text": [
-      "lal.MSUN_SI != Msun\n",
-      "__name__ = gwsurrogate.new.spline_evaluation\n",
-      "__package__= gwsurrogate.new\n"
-     ]
     }
    ],
    "source": [
@@ -151,7 +151,7 @@
   },
   {
    "cell_type": "code",
-   "execution_count": 4,
+   "execution_count": 3,
    "metadata": {},
    "outputs": [
     {
@@ -184,6 +184,23 @@
       " |  See the __call__ method on how to evaluate waveforms.\n",
       " |  In the __call__ method, x must have format x = [q, chi1, chi2].\n",
       " |  \n",
+      " |  \n",
+      " |  IMPORTANT NOTES:\n",
+      " |  ===============\n",
+      " |  \n",
+      " |  The original SEOBNRv4PHM model (arXiv:2004.09442) parameterizes the \n",
+      " |  direction of the BH spins relative to the Newtonian orbital angular momentum\n",
+      " |  vector of the binary in the co-precessing frame. The SEOBNRv4PHMSur model, \n",
+      " |  however, parameterizes the spins relative to the direction where the radiation\n",
+      " |  is always strongest along the z-axis, and the (ell,m)= (2, ±2) modes are dominant.\n",
+      " |  This is the same convention used in the NRSur7dq4 surrogate model.\n",
+      " |  \n",
+      " |  These two frames are different for non-precessing systems. Hence, specifying\n",
+      " |  the spins in the SEOBNRv4PHM-frame (e.g. when called through LALSimulation)\n",
+      " |  will not give a physically equivalent waveform when these same spin directions\n",
+      " |  are used in the SEOBNRv4PHMSur surrogate model. Ref. arXiv:2203.00381 provides\n",
+      " |  further discussion on this point.\n",
+      " |  \n",
       " |  Method resolution order:\n",
       " |      SEOBNRv4PHMSur\n",
       " |      SurrogateEvaluator\n",