Promolecule: add default multiplicities for non-int charge (#82) (#84)

* Promolecule: add default multiplicities for non-int charge

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

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* Add fix to make_promolecule

Added support for making promolecules with fractional charges and
no default multiplicities.

---------

Co-authored-by: pre-commit-ci[bot] <66853113+pre-commit-ci[bot]@users.noreply.github.com>
Co-authored-by: Marco Martínez González <[email protected]>

atomdb/promolecule.py

@@ -584,8 +584,12 @@ def make_promolecule(
 
     # Handle default multiplicity parameters
     if mults is None:
-        # Force non-int charge to be integer here; will be overwritten below.
-        mults = [MULTIPLICITIES[(atnum, charge)] for (atnum, charge) in zip(atnums, charges)]
+        # if all charges are integers, get corresponding multiplicities
+        if all(isinstance(charge, Integral) for charge in charges):
+            mults = [MULTIPLICITIES[(atnum, charge)] for (atnum, charge) in zip(atnums, charges)]
+        else:
+            # set each multiplicity to None
+            mults = [None for _ in atnums]
 
     # Construct linear combination of species
     promol = Promolecule()
@@ -615,6 +619,41 @@ def make_promolecule(
                     "of other species",
                     stacklevel=1,
                 )
+        # Non-integer charge, default multiplicity
+        if mult is None:
+            # get floor and ceiling charges
+            f_charge, c_charge = int(np.floor(charge)), int(np.ceil(charge))
+            fmult, cmult = MULTIPLICITIES[(atnum, f_charge)], MULTIPLICITIES[(atnum, c_charge)]
+            # get coefficients for linear combination
+            f_coeff = np.ceil(charge) - charge
+            c_coeff = charge - np.floor(charge)
+            # get corresponding species
+            try:
+                specie_f = load(
+                    atom,
+                    f_charge,
+                    fmult,
+                    dataset=dataset,
+                    datapath=datapath,
+                    remotepath=remotepath,
+                )
+                specie_c = load(
+                    atom,
+                    c_charge,
+                    cmult,
+                    dataset=dataset,
+                    datapath=datapath,
+                    remotepath=remotepath,
+                )
+                promol._extend((specie_f, specie_c), (coord, coord), (f_coeff, c_coeff))
+                continue
+            except FileNotFoundError:
+                warn(
+                    "Unable to load species corresponding to `charge, mult`; "
+                    "generating species via linear combination "
+                    "of other species",
+                    stacklevel=1,
+                )
 
         # Non-integer charge and multiplicity
         #