Function compute_pm3 

pub fn compute_pm3(
    elements: &[u8],
    positions: &[[f64; 3]],
) -> Result<Pm3Result, String>

Run a PM3 semi-empirical calculation.

elements: atomic numbers. positions: Cartesian coordinates in Å, one [x,y,z] per atom.

Returns orbital energies, total energy, heat of formation, and Mulliken charges.