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Fix concatenation of states in InFlightAutoBatcher #229

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@curtischong curtischong commented Aug 1, 2025

see #219

Summary

This is actually kinda a serious issue and I'll outline it here in a clear manner.

MD SimStates often track velocity. But on the first iteration, the states do NOT have velocity - so they are currently initialized as none.

But once the optimizer gets going, these states end up having a velocity attribute.

The problem is how we concatenate SimStates. Inside the autobatcher, when some SimStates finish before others, we swap those finished states with fresh states. This means inside the entire SimState, we have some systems with velocity set to none (since they were just swapped in and are fresh) and other systems with a set velocity.

When we concatenate these "mixed" SimStates (during the optimization process), we do torch.concatenate([torch.Tensor, none, none]). Where the first system's velocity exists (because it's a torch.Tensor, and the last 2 systems do NOT have a velocity - since they were just swapped in by the autobatcher.

PyTmorch cannot concatenate this because we're passing in none as an input which is invalid.

@t-reents 's solution works pretty well and is valid (which is why I'm touching it up in this PR). His solution is: "rather than initializing vector attributes as none, we initiliaze it as nan so we can do torch.concatentate between states that are old, and states that have just been swapped in.

  • Feature 1
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The `velocities` and `cell_velocities` are initialized to `None` in the `(FrechetCell)FIREState`. However, when using the `InFlightAutoBatcher` during an optimization, the current and new states are concatenated in `torch_sim.state.concatenate_states`. When trying to merge states that were already processed for a few iterations (i.e., velocities are not None anymore) and newly initialized ones, an error is raised because the code tries to merge a `Tensor` with a `None`.

Here, we initialize the `(cell_)velocities` as tensors full of `nan` instead, so that one can merge already processed and newly initialized states. During the first initialization, the `fire` methods look for `nan` rows and replace them with zeros.
@cla-bot cla-bot bot added the cla-signed Contributor license agreement signed label Aug 1, 2025
@curtischong curtischong marked this pull request as draft August 1, 2025 00:20
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maybe we should put this in tests and make separate unit and integration dirs. Whilst the example scripts are tested this doesn't particularly make much sense to me in this location

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I'll remove this file when this PR is out of draft. it's pure testing code rn

@@ -587,9 +587,11 @@ def fire_init(
masses=state.masses.clone(),
cell=state.cell.clone(),
atomic_numbers=state.atomic_numbers.clone(),
system_idx=state.system_idx.clone(),
system_index=state.system_idx.clone(),
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system_index vs system_idx still looks inconsistently applied? see line 863 below

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yes. this was a "hack" I was trying last night. but it will not work because of other parts of the code. I will have this renamed which makes it easier to see inconsistencies. Thanks for pointing out the inconsistency though

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for visibility I have thought of a good way to resolve this problem and will visit it hopefully later tonight. We'll definitely have a good PR by Saturday

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closing since this PR is superseded by #232

@curtischong curtischong closed this Aug 2, 2025
@curtischong curtischong deleted the fix-concatenation-tensors branch August 14, 2025 14:22
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