# Command-line arguments

```text
acemd [--input <path>]
      [--platform <platform>]
      [--ncpus <n> | --ngpus <n> | --device <d1> [<d2> ...]]
      [--precision <precision>]
      [--license]
      [--version]
      [--help]
```

## `--input`

- **Type:** path to a YAML/JSON input file, or a directory containing one.
- **Default:** `.` (current directory).

The input file describes the system and the run — see [Input options](input-options.md) for every key. When `--input` points at a directory, ACEMD looks inside for the first of: `input.yaml`, `input.yml`, `input.json`, `input`.

## `--platform`

- **Values:** `CUDA`, `OpenCL`, `CPU`.
- **Default:** `CUDA`.

OpenMM platform used for the integration. `CUDA` targets NVIDIA GPUs; `OpenCL` covers other vendors; `CPU` exists for debugging — it is orders of magnitude slower and should not be used for production.

## `--ncpus`

- **Values:** positive integer.
- **Default:** unset — OpenMM picks its own default (typically a subset of available threads).

Number of CPU threads to use. Only relevant for `--platform CPU`.

## `--ngpus`

- **Values:** positive integer.
- **Default:** unset.

Number of GPUs to use. ACEMD picks devices `0..ngpus-1`. **If `--ngpus` and `--device` are both set, `--ngpus` wins** — it rewrites `--device` to `[0, ..., ngpus-1]`.

## `--device`

- **Values:** **space-separated** list of GPU device indices.
- **Default:** unset (OpenMM picks the fastest visible device).

Specific GPU device indices to use. Get device indices from `nvidia-smi`.

```bash
acemd --device 0 2
```

See [Select GPU devices](../how-to/select-gpu-devices.md) for a fuller walk-through.

## `--precision`

- **Values:** `single`, `mixed`, `double`.
- **Default:** `mixed`.

Numerical precision used by the GPU integrator. `mixed` is the recommended balance of speed and stability. `single` is faster but can lose energy on long runs; `double` is for validating numerical behaviour.

## `--license`

Print the ACEMD license status and exit.

## `--version`

Print the ACEMD version and exit.

## `--help`

Print usage and exit.