Molecule Polymer Linear (discrete)
====================================
Arguments:

  * `nmolecule` (int) - Number of molecules in simulation
  * `moleculeX` (json object) - Information about `moleculeX` where `X` ranges from 0 to `nmolecule-1`. In each `moleculeX` object the following must be specified:

    * `type` (string) - Type of molecule. Here `PolymerLinear`
    * `label` (string) - label for this molecule, defaults to `molX` 
    * `chain_type` (string) - Type of chain statistics. Either `discrete` (as documented here) or `continuous`.
    * `nbeads` (int) - Number of beads in discrete chain
    * `volume_frac` (float) - Volume fraction of `moleculeX` in the simulation. Must be 0-1.
    * `ncopies` (float) - number of copies of `moleculeX` in the simulation. Note that either `volume_frac` or `ncopies` can be specified, not both. 
    * There are two options to initialize the polymer sequence

      1. Initialize by block

        * `nblocks` (int) - Number of blocks in polymer
        * `block_species` (list string) - the species of each block. Length is `nblocks`. Must corresponds to a `label` entry in :ref:`Species`.
        * `block_fractions` (list float) - fractional length of each block. Length is `nblocks`, must sum to 1.

      2. Initialize by monomer sequence
    
        * `monomer_sequence` (string) - String of length `nbeads` where each letter corresponds to a :ref:`Species` label. To use this option, each :ref:`Species` label must be a single character.


Example (python)::

  # a block polymer
  fts.add_molecule(nbeads=100,nblocks=2,block_fractions=[0.5, 0.5],block_species=['A', 'B'],type='PolymerLinear',chain_type='discrete',volume_frac=1.0)

  # sequence defined polymer
  seq = "AABAAAAAABAAABAABBAAABABBABBABAABBABBBBBBBBBBBBAAA"
  fts.add_molecule(nbeads=50,type='PolymerLinear',chain_type='discrete',volume_frac=1.0,monomer_sequence=sv20)

Example (json)::

  # block polmer
  "molecules": {
    "molecule0": {
      "nbeads": 100,
      "nblocks": 2,
      "block_fractions": [0.5, 0.5],
      "block_species": ["A","B"],
      "type": "PolymerLinear",
      "chain_type": "discrete",
      "volume_frac": 1.0
    },
    "nmolecule": 1
  },

  # sequence defined polymer
  "molecules": {
    "molecule0": {
    "chain_type": "discrete",
    "monomer_sequence": "AABAAAAAABAAABAABBAAABABBABBABAABBABBBBBBBBBBBBAAA",
    "nbeads": 50,
    "type": "PolymerLinear",
    "volume_frac": 1.0
    },
    "nmolecule": 1
  },
                                               

Molecule Polymer Discrete Formalism
--------------------------------------

TODO should consolidate. For now, see :ref:`Continuous vs Discrete Chains`.