ENH: expose TPR box dimensions through timestep

package/AUTHORS

@@ -280,6 +280,7 @@ Chronological list of authors
   - Amarendra Mohan
   - Shubham Mittal  
   - Charity Grey
+  - Tanmay Baranwal
 
 External code
 -------------

package/CHANGELOG

@@ -18,10 +18,13 @@ The rules for this file:
          spyke7, talagayev, tanii1125, BradyAJohnston, hejamu, jeremyleung521,
          harshitgajjela-droid, kunjsinha, aygarwal, jauy123, Dreamstick9,
          ollyfutur, Amarendra22, charity-g, ParthUppal523
-
+ 
  * 2.11.0
 
 Fixes
+ * TPRReader now exposes box dimensions when loading TPR files
+   as coordinates-only Universes, fixing missing `Universe.dimensions`
+   values. (Issue #5375, PR #27)
  * `MDAnalysis.analysis.nucleicacids.WatsonCrickDist`, `MinorPairDist`,
    and `MajorPairDist` now match residue names against the full resname
    instead of only the first character, fixing incorrect behaviour with

package/MDAnalysis/coordinates/TPR.py

@@ -51,6 +51,7 @@
 
 """
 
+from ..lib.mdamath import triclinic_box
 from . import base
 from ..lib import util
 from .timestep import Timestep
@@ -110,7 +111,15 @@ def _read_first_frame(self):
 
         state_ngtc = th.ngtc  # done init_state() in src/gmxlib/tpxio.c
         if th.bBox:
-            tpr_utils.extract_box_info(data, th.fver)
+            box_info = tpr_utils.extract_box_info(data, th.fver)
+            # box vectors are stored in nm
+            box_angstrom = np.array(box_info.size) * 10.0
+
+            ts.dimensions = triclinic_box(
+                box_angstrom[0],
+                box_angstrom[1],
+                box_angstrom[2],
+            )
 
         if state_ngtc > 0:
             if th.fver < 69:  # redundancy due to  different versions

testsuite/MDAnalysisTests/topology/test_tprparser.py

@@ -438,3 +438,23 @@ def test_resids(resid_from_one, resid_addition):
         resids,
         err_msg="tpr_resid_from_one kwarg not switching resids",
     )
+
+
+class TestTPRBoxVectors:
+    """Tests for TPRParser box vector support."""
+
+    def test_tpr_only_dimensions_not_none(self):
+        import MDAnalysis as mda
+        from MDAnalysisTests.datafiles import TPR
+
+        u = mda.Universe(TPR)
+
+        assert u.dimensions is not None
+
+    def test_tpr_only_dimensions_shape(self):
+        import MDAnalysis as mda
+        from MDAnalysisTests.datafiles import TPR
+
+        u = mda.Universe(TPR)
+
+        assert u.dimensions.shape == (6,)