Function compute_simplified_neb_path_configurable 

pub fn compute_simplified_neb_path_configurable(
    smiles: &str,
    start_coords: &[f64],
    end_coords: &[f64],
    n_images: usize,
    n_iter: usize,
    spring_k: f64,
    step_size: f64,
    method: &str,
) -> Result<NebPathResult, String>

Build a simplified NEB path using a configurable energy backend.

method: one of "uff", "mmff94", "pm3", "xtb", "gfn1", "gfn2", "hf3c".