make a new excpetion for dropping gaff support (#333)

* make a new excpetion for dropping gaff support

* [pre-commit.ci] auto fixes from pre-commit.com hooks

for more information, see https://pre-commit.ci

* make sure we catch GAFFNotSupportedError if needed

* raise the error if the user wants gaff

* lol whats syntax??

* Update openmmforcefields/generators/system_generators.py

Co-authored-by: Matt Thompson <mattwthompson@protonmail.com>

---------

Co-authored-by: pre-commit-ci[bot] <66853113+pre-commit-ci[bot]@users.noreply.github.com>
Co-authored-by: Matt Thompson <mattwthompson@protonmail.com>

openmmforcefields/generators/system_generators.py

@@ -247,6 +247,7 @@ def __init__(
         # Create and cache a residue template generator
         from openmmforcefields.generators.template_generators import (
             SmallMoleculeTemplateGenerator,
+            GAFFNotSupportedError,
         )
 
         self.template_generator = None
@@ -260,10 +261,17 @@ def __init__(
                         template_generator_kwargs=self.template_generator_kwargs,
                     )
                     break
-                except (ValueError, NotImplementedError) as e:
+                except (ValueError, GAFFNotSupportedError) as e:
                     _logger.debug(f"  {template_generator_cls.__name__} cannot load {small_molecule_forcefield}")
                     _logger.debug(e)
             if self.template_generator is None:
+                if small_molecule_forcefield.startswith("gaff"):
+                    raise GAFFNotSupportedError(
+                        "This release (0.13.x) of openmmforcefields temporarily drops GAFF support and "
+                        "thereby the GAFFTemplateGenerator class. Support will be re-introduced in "
+                        "future releases (0.14.x). To use this class, install version 0.12.0 or older."
+                    )
+
                 msg = (
                     "No registered small molecule template generators could load force field "
                     f"'{small_molecule_forcefield}'\n"

openmmforcefields/generators/template_generators.py

@@ -27,6 +27,10 @@ class ForceException(Exception):
     """Exception for forces"""
 
 
+class GAFFNotSupportedError(Exception):
+    """Exception for missing GAFF support"""
+
+
 class SmallMoleculeTemplateGenerator:
     """
     Abstract base class for small molecule template generation for OpenMM ForceField.
@@ -503,7 +507,7 @@ def __init__(self, molecules=None, forcefield=None, cache=None, **kwargs):
 
         Newly parameterized molecules will be written to the cache, saving time next time!
         """
-        raise NotImplementedError(
+        raise GAFFNotSupportedError(
             "This release (0.13.x) of openmmforcefields temporarily drops GAFF support and "
             "thereby the GAFFTemplateGenerator class. Support will be re-introduced in "
             "future releases (0.14.x). To use this class, install version 0.12.0 or older."