ONIOM

ONIOM is a multilayer computational method used to perform quantum mechanics/molecular mechanics (QM/MM) calculations. The acronym stands for Our Own N-layered Integrated molecular Orbital and Molecular Mechanics (with occasional variants such as Our Own N-layered…). It was developed to enable accurate treatment of a small, chemically important region at a high level of theory while treating the surrounding environment with a lower level, thereby balancing accuracy and computational cost.

The method partitions a molecular system into hierarchical layers or zones. The inner region of interest is described with a high-level quantum mechanical method (for example DFT or ab initio). The outer region is modeled at a lower level (such as semi-empirical QM or molecular mechanics). The total ONIOM energy is formed by combining the high-level energy of the inner region with the low-level energy of the full system, minus the low-level energy of the inner region. This can be generalized to more than two layers, allowing multiple corrections to incorporate different levels across the system.

In practice, ONIOM calculations use boundary handling to cap bonds crossing layer boundaries, commonly with link atoms, and propagate gradients for geometry optimization across the layered scheme. The method is implemented in several quantum chemistry packages, notably Gaussian, and has been applied to study reaction mechanisms in enzymes, catalysis on surfaces, and large molecular systems where full high-level treatment would be computationally prohibitive.

Strengths of ONIOM include its flexibility and scalability, enabling targeted accuracy where it matters most. Limitations involve dependence on the choice of levels for each layer and potential boundary artifacts, which require careful selection and validation for reliable results.