VaspInputSet.write_input: Improve error message

* src/pymatgen/io/vasp/inputs.py (PotcarSingle.from_symbol_and_functional):
Raise custom PMG_VASP_PSP_DIR_Error when PMG_VASP_PSP_DIR is not configured.

(PMG_VASP_PSP_DIR_Error): New error inheriting from ValueError.

* src/pymatgen/io/vasp/sets.py (VaspInputSet.write_input):
When PMG_VASP_PSP_DIR_Error provide a more helpful message advicing
that potcar_spec argument can be used.  The idea is to provide
instructions for users with no VASP license who are also not aware
about potcar_spec argument.

src/pymatgen/io/vasp/inputs.py

@@ -1733,6 +1733,11 @@ class OrbitalDescription(NamedTuple):
 POTCAR_STATS_PATH: str = os.path.join(module_dir, "potcar-summary-stats.json.bz2")
 
 
+class PMG_VASP_PSP_DIR_Error(ValueError):
+    """Error thrown when PMG_VASP_PSP_DIR is not configured, but POTCAR is requested."""
+    pass
+
+
 class PotcarSingle:
     """
     Object for a **single** POTCAR. The builder assumes the POTCAR contains
@@ -2278,7 +2283,7 @@ def from_symbol_and_functional(
         functional_subdir = SETTINGS.get("PMG_VASP_PSP_SUB_DIRS", {}).get(functional, cls.functional_dir[functional])
         PMG_VASP_PSP_DIR = SETTINGS.get("PMG_VASP_PSP_DIR")
         if PMG_VASP_PSP_DIR is None:
-            raise ValueError(
+            raise PMG_VASP_PSP_DIR_Error(
                 f"No POTCAR for {symbol} with {functional=} found. Please set the PMG_VASP_PSP_DIR in .pmgrc.yaml."
             )
         if not os.path.isdir(PMG_VASP_PSP_DIR):

src/pymatgen/io/vasp/sets.py

@@ -49,7 +49,7 @@
 from pymatgen.analysis.structure_matcher import StructureMatcher
 from pymatgen.core import Element, PeriodicSite, SiteCollection, Species, Structure
 from pymatgen.io.core import InputGenerator
-from pymatgen.io.vasp.inputs import Incar, Kpoints, Poscar, Potcar, VaspInput
+from pymatgen.io.vasp.inputs import Incar, Kpoints, PMG_VASP_PSP_DIR_Error, Poscar, Potcar, VaspInput
 from pymatgen.io.vasp.outputs import Outcar, Vasprun
 from pymatgen.symmetry.analyzer import SpacegroupAnalyzer
 from pymatgen.symmetry.bandstructure import HighSymmKpath
@@ -341,7 +341,15 @@ def write_input(
             zip_output (bool): If True, output will be zipped into a file with the
                 same name as the InputSet (e.g., MPStaticSet.zip).
         """
-        vasp_input = self.get_input_set(potcar_spec=potcar_spec)
+        try:
+            vasp_input = self.get_input_set(potcar_spec=potcar_spec)
+        except PMG_VASP_PSP_DIR_Error:
+            assert potcar_spec is False
+            raise ValueError(
+                "PMG_VASP_PSP_DIR is not set."
+                "  Please set the PMG_VASP_PSP_DIR in .pmgrc.yaml"
+                " or use potcar_spec=True argument."
+            )
 
         cif_name = None
         if include_cif: