Fix gaussian input parser, and add test (#4127)

src/pymatgen/io/gaussian.py

@@ -306,9 +306,9 @@ def from_str(cls, contents: str) -> Self:
                 route += f" {line}"
                 route_index = idx
             # This condition allows for route cards spanning multiple lines
-            elif (line == "" or line.isspace()) and route_index:
+            elif (line == "" or line.isspace()) and route_index is not None:
                 break
-            if route_index:
+            if route_index is not None:
                 route += f" {line}"
                 route_index = idx
         functional, basis_set, route_paras, dieze_tag = read_route_line(route)

tests/files/io/gaussian/C60_gau.com

@@ -0,0 +1,69 @@
+#p M062X/cc-pVDZ GFPrint Pop=Full
+   Guess=Huckel Scfcyc=1500 SCF=Tight
+   6D Int(Grid=UltraFine)
+
+M06-2X Gaussian C60 Ih
+
+0 1
+C    -0.610401176921     0.810029720944    -0.764305079403
+C    -1.192040230402     2.057349251447    -0.954889339744
+C    -2.550768816579     2.301728679891    -0.503345793607
+C    -3.268831998991     1.288172163417     0.119165904258
+C    -2.660777640865    -0.015794741763     0.318405603513
+C    -1.361075404971    -0.249557755793    -0.113521834011
+C    -4.669739264514     1.085143064631    -0.206401705392
+C    -4.927493211517    -0.344302724279    -0.208373854526
+C    -3.685886645977    -1.024719708115     0.115974899928
+C    -5.291724656777     1.904490541619    -1.140337593858
+C    -6.199711641198     1.331603130650    -2.118660598022
+C    -6.446268153344    -0.035744187164    -2.120547072368
+C    -5.796035115872    -0.892302565114    -1.144196217325
+C    -4.541050428727     2.964078018356    -1.791120839251
+C    -3.201001854051     3.158287057841    -1.479696648649
+C    -5.462415534693    -2.145606959850    -1.798170466963
+C    -4.274747264674    -2.796465037103    -1.487912181554
+C    -3.366760280445    -2.223577626213    -0.509589177552
+C    -2.008031694237    -2.467957054545    -0.961132723659
+C    -1.027455824014    -1.502862150563    -0.767496083695
+C     0.187160591378     0.211586400804    -1.820485494071
+C    -0.070593355760    -1.217859388082    -1.822457643160
+C    -2.076278230991    -3.191879258295    -2.218524986693
+C    -3.477185496519    -3.394908357080    -2.544092596317
+C    -5.906460331781    -2.063633296247    -3.178699636034
+C    -6.514514689934    -0.759666390974    -3.377939335377
+C    -6.010204230835     2.037126715691    -3.374080711915
+C    -4.985095225753     3.046051682028    -3.171650008337
+C    -2.244139385630     3.443289820289    -2.534658208103
+C    -1.002532820230     2.762872836487    -2.210309453667
+C    -0.239619002072     2.190427295005    -3.220606918823
+C     0.368435356055     0.886460389826    -3.021367219568
+C    -0.135875103403    -1.910332716805    -3.025225842953
+C    -1.160984108672    -2.919257683102    -3.227656546400
+C    -3.901939948213    -3.316495821242    -3.864648346745
+C    -5.143546513762    -2.636078837446    -4.188997101249
+C    -6.333239925080    -0.084792401885    -4.578821060810
+C    -6.075485978123     1.344653386901    -4.576848911688
+C    -4.069801102713     3.318673257346    -4.180781568144
+C    -2.668893837362     3.521702356144    -3.855213958518
+C    -1.871332068979     2.923259035966    -4.911394373101
+C    -0.683663799358     2.272400958676    -4.601136087826
+C     0.300188819054     0.162538185986    -4.278759482538
+C     0.053632306775    -1.204809131727    -4.280645956853
+C    -1.605028905935    -2.837284019301    -4.608185715402
+C    -2.945077480671    -3.031493058655    -4.919609905823
+C    -4.954039103506    -1.930555252450    -5.444417215184
+C    -5.535678156438    -0.683235721865    -5.635001475308
+C    -5.118623509630     1.629656149468    -5.631810471044
+C    -4.138047639389     2.594751053411    -5.438173831076
+C    -2.779319053535     2.350371625022    -5.889717377153
+C    -0.350044218472     1.019096563840    -5.255110337422
+C    -0.854354677887    -1.777696542576    -5.258968960863
+C    -3.595310518635    -2.174934680895    -5.895960760705
+C    -4.785003928524     0.376351754708    -6.285784720673
+C    -3.485301692568     0.142588740749    -6.717712158047
+C    -2.460192688139     1.151513706802    -6.515281454472
+C    -1.218586123107     0.471096723020    -6.190932700028
+C    -1.476340070226    -0.958349065502    -6.192904849203
+C    -2.877247335830    -1.161378164125    -6.518472458604
+
+

tests/io/test_gaussian.py

@@ -153,6 +153,33 @@ def test_from_str(self):
         assert gau.molecule.composition.reduced_formula == "X3SiH4"
         assert set(gau.route_parameters) == {"opt", "freq", "scf"}
 
+    def test_from_str_no_link0(self):
+        gau_str = """#p mp2/6-31g* scf=direct
+        opt freq
+
+        SIH4+ H2---SIH2+ CS //MP2(full)/6-31G* MP2=-290.9225259
+
+        1,2
+        Si
+        X,1,1.
+        H,1,R1,2,HALF1
+        H,1,R1,2,HALF1,3,180.,0
+        X,1,1.,2,90.,3,90.,0
+        X,1,1.,5,THETA,2,180.,0
+        H,1,R3,6,HALF3,5,0.,0
+        H,1,R4,6,HALF3,7,180.,0
+
+        R1=1.47014
+        R3=1.890457
+        R4=1.83514
+        HALF1=60.633314
+        THETA=10.35464
+        HALF3=11.861807"""
+
+        gau = GaussianInput.from_str(gau_str)
+        assert gau.molecule.composition.reduced_formula == "X3SiH4"
+        assert set(gau.route_parameters) == {"opt", "freq", "scf"}
+
     def test_gen_basis(self):
         gau_str = """#N B3LYP/Gen Pseudo=Read