## Builder


Build a molecule atom by atom on a tetrahedral grid.


Usage

``` python
Builder(
    up=(0.0, 0.0, 1.0),
    left=(-1.0, 0.0, 0.0),
    *,
    xh_length=XH_LENGTH,
    xy_length=XY_LENGTH
)
```


Every atom has four bond slots, one per direction of the [tetrahedral_basis](tetrahedral_basis.md#tetramol.tetrahedral_basis). Bonded atoms have opposite parities: an atom of parity +1 bonds along the basis directions, and one of parity -1 along their negatives. A child added through slot `i` therefore bonds back to its parent through its own slot `i`. Filling in carbons gives the diamond lattice, and neighboring centers are always staggered.


## Example

A C-N bond pointing up, with a methyl group to the left of the carbon:

> > > b = Builder(up=(0, 0, 1), left=(-1, 0, 0)) c = b.add("C") n = b.add("N", c, toward=(0, 0, 1)) c2 = b.add("C", c, toward=(-1, 0, 0)) b.add_hydrogens() \[3, 4, 5, 6, 7, 8, 9\] b.geometry().symbols \['C', 'N', 'C', 'H', 'H', 'H', 'H', 'H', 'H', 'H'\]