build_smirnoff_collection is really slow about openff-interchange HOT 4 CLOSED

Not great

In [1]: from openforcefield.typing.engines.smirnoff.forcefield import ForceField

In [2]: parsley = ForceField('openff-1.2.0.offxml')

In [3]: from system.typing.smirnoff import build_smirnoff_collection

In [4]: %timeit -n 10 -r 10 _ = build_smirnoff_collection(parsley)
42.9 s ± 2.1 s per loop (mean ± std. dev. of 10 runs, 10 loops each)

from openff-interchange.

Another option would be generate the SMIRKS map before converting the force field and only convert the parameters that are going to exist in the resulting system. This is probably pretty dangerous, though, as it would significantly limit modification later on.

from openff-interchange.

The above is implemented in 1196a74 and works ok, but is only a temporary solution.

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Resolved #30

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