still a draft, nbody0 is almost done now.

teuben · Jan 20, 2025 · 590b0d2 · 590b0d2
1 parent 4f59089
commit 590b0d2
Showing 1 changed file with 132 additions and 5 deletions.
diff --git a/scripts/notebooks/aarseth.ipynb b/scripts/notebooks/aarseth.ipynb
@@ -5,13 +5,13 @@
    "id": "0b0acc08-6ce4-4c7a-abec-966e5abdf370",
    "metadata": {},
    "source": [
-    "# The Aarseth NBODY family of codes\n",
+    "# The Aarseth NBODY family of codes (*draft*)\n",
     "\n",
     "With the \n",
     "[passing of Sverre Aarseth on Dec 28, 2024](https://ascl.net/wordpress/2025/01/07/sverre-aarseth-father-of-open-source-stellar-dynamics-has-passed-on-to-a-higher-orbit/),\n",
-    "we wanted to remember him by showcasing some examples of some of his legacy in NEMO, in the process cleaning up some of NEMO's code.\n",
+    "we remember him by showcasing some examples of some of his legacy codes in NEMO.\n",
     "\n",
-    "Sverre has always made his code available for anybody to use. His current body of work can be found at https://people.ast.cam.ac.uk/~sverre/web/pages/nbody.htm\n",
+    "Sverre has always made his code available for anybody to use. His current body of work can still be found at https://people.ast.cam.ac.uk/~sverre/web/pages/nbody.htm  as well as an [entry in ASCL](https://ascl.net/1102.006).\n",
     "\n",
     "\n",
     "The following programs are avaiilable in NEMO:\n",
@@ -27,6 +27,10 @@
     "* u4tos, stou4 - conversion programs of Sverre's \"UNIT4\" files to and from NEMO snapshot's\n",
     "* u3tos - conversion program of Sverre's \"UNIT3\" file to a NEMO snapshot\n",
     "\n",
+    "and Sverre's 1999 paper [\"From NBODY1 to NBODY6: The Growth of an Industry\"](https://ui.adsabs.harvard.edu/abs/1999PASP..111.1333A) \n",
+    "outlines the history behind this series.\n",
+    "\n",
+    "\n",
     "\n",
     "# Loading NEMO\n",
     "\n",
@@ -55,11 +59,11 @@
    "id": "4f6c80e3-688b-4b95-be5b-d8cbf8fb0d98",
    "metadata": {},
    "source": [
-    "# nbody0, nbody00\n",
+    "# nbody0, nbody00, nbody0_ff, nbody0h4\n",
     "\n",
     "This code was published in the Appendix of the 1987 (first) edition of Binney & Tremaine's *Galactic Dynamics*. The code\n",
     "can be found in \n",
-    "**$NEMO/src/nbody/evolve/aarseth/nbody0**.  The FORTRAN version reads input from \n"
+    "**$NEMO/src/nbody/evolve/aarseth/nbody0**, where several derivatives of this *Micky Mouse* version are available."
    ]
   },
   {
@@ -72,6 +76,110 @@
     "man nbody0"
    ]
   },
+  {
+   "cell_type": "markdown",
+   "id": "76d7e27d-c05b-47ae-8ad1-3be1597c6fc4",
+   "metadata": {},
+   "source": [
+    "### Creating initial conditions\n",
+    "\n",
+    "By default the FORTRAN code is compiled with space for a maximum of 2048 particles. We thus create a Plummer (1911) sphere with 2048 particles. We fix the seed to have reproducable results, and integrate a few crossing times to keep the CPU loaded for a few seconds."
+   ]
+  },
+  {
+   "cell_type": "code",
+   "execution_count": null,
+   "id": "47a0a006-b7ac-4c9e-88c3-64e9f99a5f62",
+   "metadata": {},
+   "outputs": [],
+   "source": [
+    "rm -f p2048\n",
+    "mkplummer p2048 2048 seed=123\n",
+    "tsf p2048"
+   ]
+  },
+  {
+   "cell_type": "markdown",
+   "id": "06ff1c6d-5a36-4182-9624-95e7ba744193",
+   "metadata": {},
+   "source": [
+    "### Comparing FORTRAN and C\n",
+    "\n",
+    "We now compare the performance of the FORTRAN and C versions. We use **out=.** to not have to write an output file, saving some overhead.\n",
+    "The default integration time **tcrit=2** is used."
+   ]
+  },
+  {
+   "cell_type": "code",
+   "execution_count": null,
+   "id": "c9609d60-5891-4cb0-a239-0c6972cbbae2",
+   "metadata": {
+    "scrolled": true
+   },
+   "outputs": [],
+   "source": [
+    "/usr/bin/time nbody0 p2048 . tcrit=2\n",
+    "/usr/bin/time nbody00 p2048 . tcrit=2"
+   ]
+  },
+  {
+   "cell_type": "markdown",
+   "id": "86ef0f50-6344-491d-8578-a4e13c14eecc",
+   "metadata": {},
+   "source": [
+    "### Reproducability\n",
+    "\n",
+    "If NEMO's random number generator is working correctly, the number of steps and energy at time=2 should be identical to\n",
+    "```\n",
+    "   time = 2   steps = 161238   energy = -0.244397\n",
+    "```\n",
+    "and although the CPU time varies per machine, my 2023 \"Ultra 7 155H\" laptop CPU took about 1.7sec for **nbody0** and 2.0sec for **nbody00**. Also notable is that the C version does use a small amount (4%) of system time, whereas FORTRAN took 0. "
+   ]
+  },
+  {
+   "cell_type": "markdown",
+   "id": "14849a05-f74a-49f3-b915-9cc55f9069bb",
+   "metadata": {},
+   "source": [
+    "### nbody00_ff\n",
+    "\n",
+    "The pure FORTRAN version does not have a NEMO CLI. It reads a one line header from stdin, followed by N (the number of bodies) lines containing the mass, position and velocity. The header contains **n,eta,deltat,tcrit,eps2,reset**.   Such an input file can be easily created using basic NEMO tools."
+   ]
+  },
+  {
+   "cell_type": "code",
+   "execution_count": null,
+   "id": "30e65f69-09c2-4ece-8e23-eef25839859f",
+   "metadata": {},
+   "outputs": [],
+   "source": [
+    "rm -f p5\n",
+    "mkplummer p5 5 seed=123\n",
+    "\n",
+    "# convert the snapsshot to the input file that nbody0_ff needs\n",
+    "echo \"5,0.02,1.0,10,0.0001,1\" > input5\n",
+    "snapprint p5 m,x,y,z,vx,vy,vz format=%.15g >> input5\n",
+    "\n",
+    "# run nbody0_ff\n",
+    "nbody0_ff < input5 \n",
+    "\n",
+    "# run nbody0, and compare the phase space coordinated at times=10\n",
+    "nbody0 p5 - deltat=1 eps=0.01 tcrit=10 | snaptrim - - times=10 | snapprint -"
+   ]
+  },
+  {
+   "cell_type": "markdown",
+   "id": "82c27183-5e94-43d2-b4c2-2b44a93c610e",
+   "metadata": {},
+   "source": [
+    "### Comparing nbody0 and nbody0_ff\n",
+    "\n",
+    "Apart from the limited accuracy that nbody0_ff shows, the comparison is excellent, as well as number of steps taken and the energy in the final snapshot:\n",
+    "```\n",
+    "     time = 10   steps = 2351   energy = -0.181269\n",
+    "```"
+   ]
+  },
   {
    "cell_type": "markdown",
    "id": "6fc2c8ee-6e47-4d5e-9988-195a1307fb1c",
@@ -84,6 +192,14 @@
     "This version also introduces the more formal *run* interface in NEMO to allow one to run legacy codes with a NEMO command line interface. It also includes conversion between NEMO's snapshots and NBODYx files (unit3, unit4)."
    ]
   },
+  {
+   "cell_type": "code",
+   "execution_count": null,
+   "id": "340163de-36bd-4a5c-9f47-0bebf26b4047",
+   "metadata": {},
+   "outputs": [],
+   "source": []
+  },
   {
    "cell_type": "code",
    "execution_count": null,
@@ -120,6 +236,7 @@
    "metadata": {},
    "outputs": [],
    "source": [
+    "rm -f p128 \n",
     "mkplummer p128 128"
    ]
   },
@@ -253,6 +370,16 @@
    "id": "63d9eff9-c644-4c6d-8df7-8efcaf74ca31",
    "metadata": {},
    "outputs": [],
+   "source": [
+    "man runbody6"
+   ]
+  },
+  {
+   "cell_type": "code",
+   "execution_count": null,
+   "id": "18d6be8d-8a5c-4591-8600-a77b919b91cc",
+   "metadata": {},
+   "outputs": [],
    "source": []
   }
  ],