## Builder.add()


Add an atom.


Usage

``` python
Builder.add(
    symbol,
    parent=None,
    slot=None,
    *,
    toward=None,
    length=None,
)
```


## Parameters


`symbol: str`  
Atomic symbol.

`parent: int | None = None`  
Atom to bond to. Must be `None` for the first atom, which goes at the origin.

`slot: int | None = None`  
Slot of `parent` to bond through. If neither this nor `toward` is given, use the first empty slot.

`toward: ArrayLike | None = None`  
Direction to bond in. The empty slot of `parent` closest to this direction is used.

`length: float | None = None`  
Bond length. Defaults to `xh_length` for bonds to hydrogen and `xy_length` otherwise.


## Returns


`int`  
Index of the new atom.


## Raises


`ValueError`  
If the parent, slot, or position is invalid.