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    "## Saving Systems to XML Files\n",
    "\n",
    "The XMLSerializer class can be used to save many kinds of objects to XML files: Systems, Integrators, States, and Forces.  Saving and loading State objects is most easily done by calling `saveState()` and `loadState()` on the Simulation.  For other objects, you need to invoke XMLSerializer directly.\n",
    "\n",
    "Here are some reasons you might want to save a System to a file.\n",
    "\n",
    "- Creating it with a class such as ForceField or CharmmPsfFile might be slow.  If you plan to run many simulations of the same System, writing it to a file and reloading it for future simulations will save time.\n",
    "- Saving it to a file helps reproducibility.  It preserves a record of exactly what you simulated.  Otherwise, you might forget what arguments you passed to `createSystem()` and be unsure how to reproduce your earlier work.\n",
    "- It can be used to communicate the exact system description.  You might have one script that builds the System and a different one that simulates it.  Or one person might be responsible for building it and a different person responsible for running the simulation.\n",
    "- You can use Python tools like ForceField to build the System, then load it into a simulation code written in C++.\n",
    "\n",
    "Let's start by creating a System."
   ]
  },
  {
   "cell_type": "code",
   "execution_count": 1,
   "id": "2c70bd0f",
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   "source": [
    "from openmm.app import *\n",
    "from openmm import *\n",
    "\n",
    "pdb = PDBFile('ala_ala_ala.pdb')\n",
    "forcefield = ForceField('amber14-all.xml')\n",
    "system = forcefield.createSystem(pdb.topology)"
   ]
  },
  {
   "cell_type": "markdown",
   "id": "5cc96bcb",
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   "source": [
    "The following lines save it to a file on disk."
   ]
  },
  {
   "cell_type": "code",
   "execution_count": 2,
   "id": "a93baea5",
   "metadata": {},
   "outputs": [],
   "source": [
    "with open('system.xml', 'w') as output:\n",
    "    output.write(XmlSerializer.serialize(system))"
   ]
  },
  {
   "cell_type": "markdown",
   "id": "f0546df7",
   "metadata": {},
   "source": [
    "Reloading the file and creating a new System is just as easy."
   ]
  },
  {
   "cell_type": "code",
   "execution_count": 3,
   "id": "4191151b",
   "metadata": {},
   "outputs": [],
   "source": [
    "with open('system.xml') as input:\n",
    "    system2 = XmlSerializer.deserialize(input.read())"
   ]
  },
  {
   "cell_type": "markdown",
   "id": "8df7c10f",
   "metadata": {},
   "source": [
    "Loading a serialized System in C++ is very similar:\n",
    "\n",
    "```c++\n",
    "ifstream input(\"system.xml\");\n",
    "System* system = XmlSerializer::deserialize<System>(input);\n",
    "```"
   ]
  }
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