2. Molecular structure

MDT is built around python objects that try to mirror their physical counterparts. For instance, a Molecule‘s atoms are a list at molecule.atoms; its mass is stored at molecule.mass and its charge at molecule.charge. Atoms, similarly, have an atomic number (atom.atnum), a mass (atom.mass), a position (atom.position), and a momentum (atom.momentum).

These properties are interrelated - for instance, changing an atom’s mass will also affect its molecule’s total mass. As much as possible, MDT will automatically take care of this bookkeeping for you.

Below is a quick rundown of those objects and how they relate to one another.

For most of the examples here, we’ll look at a benzene molecule, which you can create as follows:
>>> import moldesign as mdt, moldesign.units as u
>>> benzene = mdt.from_name('benzene')

2.1. Atoms


An Atom object contains physical information,

>>> atom = mdt.Atom('C')
>>> atom.atomic_number
>>> atom.mass
12.0 * u.amu
>>> atom.position
[0.0, 0.0, 0.0] * u.angstrom

the bonds it’s involved in,

>>> atom = benzene.atoms[0]
>>> atom.bonds
[<Bond: C3 (#3) - C4 (#4) (order: 1)>,
 <Bond: C2 (#2) - C3 (#3) (order: 2)>,
 <Bond: C3 (#3) - H9 (#9) (order: 1)>]
>>> atom.bond_graph
 {<Atom C2 (elem C), index 2 in molecule Molecule: benzene>: 2,
  <Atom C4 (elem C), index 4 in molecule Molecule: benzene>: 1,
  <Atom H9 (elem H), index 9 in molecule Molecule: benzene>: 1}

and contains references to the larger structures it belongs to.

>>> mol = mdt.from_pdb('3fpp')
>>> atom = mol.atoms[0]
>>> atom
<Atom N (elem N), index 0 (res PRO43 chain A) in molecule Molecule: 3fpp>
>>> atom.molecule
<3fpp (Molecule), 4097 atoms>
>>> atom.residue
<Residue PRO43 (index 0, chain A) in Molecule: 3fpp>
>>> atom.chain
<Chain A in Molecule: 3fpp>

2.2. Bonds


An iterator over a molecule’s bonds is stored at molecule.bonds:

>>> list(benzene.bonds)
[<Bond: C4 (#4) - H10 (#10) (order: 1)>,
 <Bond: C4 (#4) - C5 (#5) (order: 2)>,

Each bond contains references to the two bonded atoms, and the Lewis structure bond order:

>>> bond = list(benzene.bonds)[0]
>>> bond
<Bond: C3 (#3) - C4 (#4) (order: 1)>
>>> bond.a1
<Atom C3 (elem C), index 3 in molecule Molecule: benzene>
>>> bond.a2
<Atom C4 (elem C), index 4 in molecule Molecule: benzene>
>>> bond.order

If you have one atom in a bond, and need to get the other atom, use the bond.partner method:

>>> atom = benzene.atoms[0]
>>> atom
<Atom C0 (elem C), index 0 in molecule Molecule: benzene>
>>> bond = atom.bonds[0]
>>> bond.partner(atom)
<Atom C1 (elem C), index 1 in molecule Molecule: benzene>

Bonding information can also be accessed as a bond graph:

>>> benzene.bond_graph
{<Atom C1 (elem C), index 1 in molecule Molecule: benzene>:
      {<Atom C0 (elem C), index 0 in molecule Molecule: benzene>: 2,
       <Atom C2 (elem C), index 2 in molecule Molecule: benzene>: 1,
       <Atom H7 (elem H), index 7 in molecule Molecule: benzene>: 1},
 <Atom C0 (elem C), index 0 in molecule Molecule: benzene>:
      {<Atom C1 (elem C), index 1 in molecule Molecule: benzene>: 2,

2.3. Molecules


You’ll spend most of your time in MDT working with Molecules - which, technically, represent completely molecular systems.

Each Molecule contains a list of Atoms.

>>> len(benzene.atoms)
>>> atom = benzene.atoms[3]
>>> atom.index

The 3D positions and momenta of all atoms in a molecule are stored as an Nx3 array:

>>> benzene.positions
<Quantity([[-0.76003939  1.16931777  0.02273989]
 [ 0.63314801  1.24365634 -0.01297972]
 ..., 'ang')>
>>> benzene.momenta
<Quantity([[ 0.  0.  0.]
 [ 0.  0.  0.]
 ..., 'amu * ang / fs')>

2.4. Molecular properties


Molecular properties are quantities that have been calculated by an energy model at the molecule’s current position.

For instance: if forces on the molecule have been calcualted, they are stored in an Nx3 array similarly to positions and momenta. However, they must first be calculated by an EnergyModel:

>>> benzene.forces
Traceback ...
NotCalculatedError: The 'forces' property hasn't been calculated yet. Calculate it with the molecule.calculate_forces() method
>>> benzene.set_energy_model(mdt.models.GAFF, charges='am1-bcc')
>>> benzene.calculate_forces()
>>> benzene.forces
<Quantity([[ -1.25171801e-01   1.93601314e-01   9.20898239e-04]
 ..., 'eV / ang')>

Any quantities calculated by the energy model are stored in molecule.properties:

>>> benzene.properties
{'am1-bcc': ...,
 'forces': ...,
 'potential_energy': ...,
 'positions': ...}

Properties almost always include potential_energy; other common properties include forces, wfn, and dipole. Calling mol.calculate() will only calculate the model’s default quantities (see model.DEFAULT_PROPERTIES for a given model). Additional properties, if available, can be requested via mol.calculate(requests=[prop1, prop2, ...])


These properties are only accessible if they correspond to the molecule’s current position - MDT will raise NotCalculatedError if you try to access properties at a new position without explicitly asking for a calculation.

>>> benzene.calculate()
>>> benzene.potential_energy
-6201.2054456 * u.eV
>>> mdt.set_angle(benzene.atoms[0], benzene.atoms[1], benzene.atoms[2], 5.0*u.degrees)
>>> benzene.potential_energy
Traceback (most recent call last):
NotCalculatedError: The 'potential_energy' property hasn't been calculated yet. ...
>>> benzene.calculate_potential_energy()
-6200.9382913 * u.eV

2.5. Electronic structure


If you’ve run an quantum chemistry calculation on a molecule, its electronic structure will also be available. This information is accessible through: mol.wfn (see its documentation for more details).

>>> mol.set_energy_model(mdt.models.RHF, basis='sto-3g')
>>> mol.calculate()
>>> wfn = benzene.wfn
>>> wfn.aobasis
<BasisSet (sto-3g) of Molecule: benzene>
>>> wfn.orbitals
{'atomic': <BasisSet (sto-3g) of Molecule: benzene>,
 'canonical': <36 canonical MolecularOrbitals in None/sto-3g wfn>}