# Tweaking and Inspecting Parameters

Interchange stores force field parameters in potential handlers, which link the entry in the force field to the parameter as applied to a topology. This makes it easy to inspect and even modify how a parameter is applied to a system.

## Handlers

Potential handlers describe how force field parameters are applied to a chemical system. This attribute is a dictionary whose keys are string identifiers and whose values are PotentialHandler objects. Instead of linking parameters to abstract chemical environments like SMIRNOFF force fields, or copying parameters into place like a traditional force field format, a PotentialHandler links an input parameter in the force field to every place in the topology where it is used.

Like SMIRNOFF force fields, each parameter in an Interchange knows where it came from, but like traditional force fields, the parameterized system is readily defined and values can be retrieved quickly. This allows changes to an input parameter to be reflected instantly in the parameterized system. Unlike a SMIRNOFF force field, the chemistry of system itself cannot be changed; a new Interchange must be defined and parameterized.

There are three central components in each handler: topology keys, potentials, and potential keys.

TopologyKey objects are unique identifiers of locations in a topology. These objects do not include physics parameters. The basic information is a tuple of atom indices, which can be of any non-zero length. For example, a topology key describing a torsion will have a 4-length tuple, and a topology key describing a vdW parameter will have a 1-length tuple.

Potential objects store the physics parameters that result from parameterizing a chemical topology with a force field. These do not know anything about where in the topology they are applied. The parameters are stored in a dictionary attribute .parameters in which keys are string identifiers and values are the parameters themselves, tagged with units.

PotentialKey objects uniquely identify physics parameters so that many topology keys can refer to the same potential. Potential keys do not know anything about the topology they are associated with. In SMIRNOFF force fields, SMIRKS patterns uniquely identify a parameter within a parameter handler, so (with some exceptions) the SMIRKS pattern is all that is needed to construct a potential key. For classically atom-typed force fields, a key can be constructed using atom types or combinations thereof.

These objects are strung together with two mappings, each stored as dictionary attributes of a PotentialHandler. The .slot_map attribute maps segments of a topology to the potential keys (TopologyKey to PotentialKey mapping). The .potentials attribute maps the potential keys to the potentials (PotentialKey to Potential). This allows many topology keys to map to the same Potential by sharing a PotentialKey. If the Potential is updated, all the places in the topology where it is used are updated immediately. Despite this, getting the Potential for a place in the topology is a constant time operation. For example, parametrizing a thousand water molecules each with two identical bonds will produce only one Potential, rather than two thousand.

Each potential handler inherits from the base PotentialHandler class and describes a single type of parameter from a single source. Potential handlers for SMIRNOFF force fields are found in the openff.interchange.components.smirnoff module, while those for Foyer are found in the openff.interchange.components.foyer module.

## Inspecting an assigned parameter

Construct a simple Interchange

>>> from openff.interchange import Interchange
>>> from openff.toolkit import Molecule, ForceField
>>>
>>> ethane = Molecule.from_smiles("CC")
>>> sage = ForceField("openff-2.0.0.offxml")
>>> box = box = [[1, 0, 0], [0, 1, 0], [0, 0, 1]]
>>> interchange = Interchange.from_smirnoff(sage, [ethane], box=box)



The Interchange.handlers attribute maps names to the corresponding handler:

>>> interchange.handlers.keys()
dict_keys(['Bonds', 'Constraints', 'Angles', 'ProperTorsions',
'ImproperTorsions', 'vdW', 'Electrostatics'])
>>> # Ethane has no improper torsions, so both maps will be empty
>>> interchange.handlers['ImproperTorsions']
SMIRNOFFImproperTorsionHandler(type='ImproperTorsions',
expression='k*(1+cos(periodicity*theta-phase))',
slot_map={},
potentials={})



In the bond handler for example, each pair of bonded atoms maps to one of two potential keys, one for the carbon-carbon bond, and the other for the carbon-hydrogen bonds. It’s clear from the SMIRKS codes that atoms 0 and 1 are the carbon atoms, and atoms 2 through 7 are the hydrogens:

>>> interchange.handlers['Bonds'].slot_map
{TopologyKey(atom_indices=(0, 1), ...): PotentialKey(id='[#6X4:1]-[#6X4:2]', ...),
TopologyKey(atom_indices=(0, 2), ...): PotentialKey(id='[#6X4:1]-[#1:2]', ...),
TopologyKey(atom_indices=(0, 3), ...): PotentialKey(id='[#6X4:1]-[#1:2]', ...),
TopologyKey(atom_indices=(0, 4), ...): PotentialKey(id='[#6X4:1]-[#1:2]', ...),
TopologyKey(atom_indices=(1, 5), ...): PotentialKey(id='[#6X4:1]-[#1:2]', ...),
TopologyKey(atom_indices=(1, 6), ...): PotentialKey(id='[#6X4:1]-[#1:2]', ...),
TopologyKey(atom_indices=(1, 7), ...): PotentialKey(id='[#6X4:1]-[#1:2]', ...)}



Question

Which atom indices represent hydrogens bonded to carbon atom 0, and which are bonded to carbon atom 1?

The bond handler also maps the two potential keys to the appropriate Potential. Here we can read off the force constant and length:

>>> interchange.handlers['Bonds'].potentials
{PotentialKey(id='[#6X4:1]-[#6X4:2]', ...):
Potential(parameters={'k': <Quantity(529.242972, 'kilocalorie / angstrom ** 2 / mole')>,
'length': <Quantity(1.52190126, 'angstrom')>}, ...),
PotentialKey(id='[#6X4:1]-[#1:2]', ...):
Potential(parameters={'k': <Quantity(740.093414, 'kilocalorie / angstrom ** 2 / mole')>,
'length': <Quantity(1.09389949, 'angstrom')>}, ...)}



We can even modify a value here, export the new interchange, and see that all of the bonds have been updated:

>>> from openff.interchange.models import TopologyKey
>>> from openff.units import unit
>>> # Get the potential from the first C-H bond
>>> top_key = TopologyKey(atom_indices=(0, 2))
>>> pot_key = interchange.handlers['Bonds'].slot_map[top_key]
>>> potential = interchange.handlers['Bonds'].potentials[pot_key]
>>> # Modify the potential
>>> potential.parameters['length'] = 3.1415926 * unit.nanometer
>>> # Write out the modified interchange to a GROMACS .top file
>>> interchange.to_top("out.top")
>>> with open("out.top") as f:
...     print(f.read())
; Generated by Interchange
...
[ bonds ]
;     ai      aj  func  r              k
1       2   1     0.152190126495 221435.2592902858
1       3   1     3.1415926      309655.084322414
1       4   1     3.1415926      309655.084322414
1       5   1     3.1415926      309655.084322414
2       6   1     3.1415926      309655.084322414
2       7   1     3.1415926      309655.084322414
2       8   1     3.1415926      309655.084322414
...