Small modifications in code and comments to clarify (#11) (#539)

Co-authored-by: Ignacio Pickering <ign.pickering@gmail.com>

torchani/aev.py

@@ -36,17 +36,17 @@ def radial_terms(Rcr: float, EtaR: Tensor, ShfR: Tensor, distances: Tensor) -> T
     .. _ANI paper:
         http://pubs.rsc.org/en/Content/ArticleLanding/2017/SC/C6SC05720A#!divAbstract
     """
-    distances = distances.unsqueeze(-1).unsqueeze(-1)
+    distances = distances.view(-1, 1, 1)
     fc = cutoff_cosine(distances, Rcr)
     # Note that in the equation in the paper there is no 0.25
     # coefficient, but in NeuroChem there is such a coefficient.
     # We choose to be consistent with NeuroChem instead of the paper here.
     ret = 0.25 * torch.exp(-EtaR * (distances - ShfR)**2) * fc
-    # At this point, ret now have shape
-    # (conformations, atoms, N, ?, ?) where ? depend on constants.
-    # We then should flat the last 2 dimensions to view the subAEV as one
-    # dimension vector
-    return ret.flatten(start_dim=-2)
+    # At this point, ret now has shape
+    # (conformations x atoms, ?, ?) where ? depend on constants.
+    # We then should flat the last 2 dimensions to view the subAEV as a two
+    # dimensional tensor (onnx doesn't support negative indices in flatten)
+    return ret.flatten(start_dim=1)
 
 
 def angular_terms(Rca: float, ShfZ: Tensor, EtaA: Tensor, Zeta: Tensor,
@@ -63,7 +63,7 @@ def angular_terms(Rca: float, ShfZ: Tensor, EtaA: Tensor, Zeta: Tensor,
     .. _ANI paper:
         http://pubs.rsc.org/en/Content/ArticleLanding/2017/SC/C6SC05720A#!divAbstract
     """
-    vectors12 = vectors12.unsqueeze(-1).unsqueeze(-1).unsqueeze(-1).unsqueeze(-1)
+    vectors12 = vectors12.view(2, -1, 3, 1, 1, 1, 1)
     distances12 = vectors12.norm(2, dim=-5)
 
     cos_angles = vectors12.prod(0).sum(1) / distances12.prod(0)
@@ -74,11 +74,11 @@ def angular_terms(Rca: float, ShfZ: Tensor, EtaA: Tensor, Zeta: Tensor,
     factor1 = ((1 + torch.cos(angles - ShfZ)) / 2) ** Zeta
     factor2 = torch.exp(-EtaA * (distances12.sum(0) / 2 - ShfA) ** 2)
     ret = 2 * factor1 * factor2 * fcj12.prod(0)
-    # At this point, ret now have shape
-    # (conformations, atoms, N, ?, ?, ?, ?) where ? depend on constants.
-    # We then should flat the last 4 dimensions to view the subAEV as one
-    # dimension vector
-    return ret.flatten(start_dim=-4)
+    # At this point, ret now has shape
+    # (conformations x atoms, ?, ?, ?, ?) where ? depend on constants.
+    # We then should flat the last 4 dimensions to view the subAEV as a two
+    # dimensional tensor (onnx doesn't support negative indices in flatten)
+    return ret.flatten(start_dim=1)
 
 
 def compute_shifts(cell: Tensor, pbc: Tensor, cutoff: float) -> Tensor: