Function compute_xtb 

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

Run an xTB tight-binding calculation.

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

Returns orbital energies, total energy, gap, and Mulliken charges.