# Visualize forces

**You will learn:** how to render per-atom force vectors in VMD on top of a trajectory frame — useful when [debugging a crash](debug-a-simulation-crash.md) and the cause isn't obvious from coordinates alone.

**Prerequisites:**

- A trajectory (`output.xtc`) and a matching forces file (`forces.xtc`) produced by setting `trajforcefile: "forces.xtc"` in the ACEMD input.
- VMD installed and on `PATH`.

## Setup

Forces are written into an XTC by ACEMD when `trajforcefile` is set. Format-wise, an XTC normally holds coordinates — ACEMD reuses it to store force vectors instead.

```python
from acemd.utils import view_forces
from moleculekit.molecule import Molecule

mol = Molecule("structure.prmtop")
mol.read("output.xtc")
view_forces(mol, "forces.xtc", step=0)
```

`step=0` picks the first frame; bump it to look at later frames.

VMD opens with the molecule plus an arrow per atom — direction and length follow the force vector.

## Parameters that matter

{py:func}`~acemd.utils.view_forces` accepts:

- `step` — trajectory frame index to visualise.
- `threshold` (default `500`) — atoms with force magnitude (in kcal/mol/Å) below this are not drawn. Raise to declutter; lower to see weaker forces.
- `normalize` (default `True`) — when true, every drawn arrow is rendered at unit length so a few huge forces don't dominate the view. The default is what you want for finding the source of a crash.
- `scalef` (default `100`) — divisor applied to each force vector before drawing. Larger `scalef` → shorter arrows. **Has no effect when `normalize=True`** because the subsequent unit-vector step cancels it; only set it when you have switched `normalize=False` to draw arrows whose lengths actually encode force magnitude.

## Reading the picture

Wait a moment for VMD to finish drawing — the per-atom arrows are added one at a time and the scene is slow to converge. The bigger the arrow, the larger the force. Atoms surrounded by abnormally long arrows are where the simulation is breaking down — look at their immediate neighbours for the clash, mis-parameterised residue, or extrapolating NNP atom.

## See also

- [Debug a simulation crash](debug-a-simulation-crash.md)
- {py:func}`~acemd.utils.view_forces`
- `trajforcefile` in [Input options](../reference/input-options.md)