# Run a simulation from Python

**You will learn:** how to drive ACEMD from a Python script using the {py:func}`~acemd.acemd.acemd` function, including overriding input-file options at call time.

**Prerequisites:**

- ACEMD installed (see [Installation](../installation.md)).
- A directory with an `input.yaml` and the referenced structure/coordinates files.

## Setup

```python
from acemd import acemd
```

## Run a simulation from a directory

{py:func}`~acemd.acemd.acemd` takes a directory containing `input.yaml` and runs the simulation there — equivalent to `cd`-ing into that directory and calling `acemd` on the command line:

```python
acemd(".")                          # current directory
acemd("/tmp/my_simulation")
```

## Override input-file options

Any input-file option (lowercased) is accepted as a keyword argument. The keyword overrides the value from `input.yaml`:

```python
acemd("/tmp/my_simulation", run="2ns")
acemd("/tmp/my_simulation", run="2ns", barostat=True)
```

This is convenient for sweeps — e.g. running the same setup at different temperatures or for different lengths without editing the input file each time.

## Override CLI flags

Command-line flags also pass through as keywords (lowercased name, same value):

```python
acemd("/tmp/my_simulation", device=[0, 1])
acemd("/tmp/my_simulation", precision="single")
```

## Use a non-default input file name

By default ACEMD looks for `input.yaml`. Use the `inputfile` keyword to load a different file from inside the directory:

```python
acemd("/tmp/my_simulation", inputfile="equilibration.yaml")
```

## Gotchas

- {py:func}`~acemd.acemd.acemd` blocks until the simulation completes — use a job scheduler or subprocess if you need it to run in the background.

## See also

- {py:func}`~acemd.acemd.acemd`
- [Set up equilibration and production](equilibrate-and-produce.md) — Python helpers that generate the `input.yaml` for you.
- [Input options](../reference/input-options.md)
- [Command-line arguments](../reference/cli.md)