This popular method was proposed in J. Chem. Theory Comput., 5, 962 (2009), it focuses on deciphering orbital interaction between fragments. The key advantage of this analysis is that it is able to transform variation of electron density due to orbital interaction to a set of NOCV pairs, each pair has corresponding energy contribution to orbital interaction energy and has corresponding density which can be visualized to easily understand nature, therefore ETS-NOCV analysis provides very deep insight into orbital interaction. See Section 3.26 of Multiwfn manual for detailed introduction of this analysis and Section 4.23 for rich examples of applying ETS-NOCV on studying various kinds of interactions.

The ETS-NOCV module in Multiwfn has the following capabilities:
• Calculate NOCV orbital wavefunctions, eigenvalues and energies
• Visualize NOCV orbitals and export corresponding cube files
• Calculate energy contributions to ΔEorb of NOCV pairs
• Visualize NOCV pair densities and export corresponding cube files
• Visualize promolecular, frozen state and actual complex orbitals
• Visualize Δρ_Pauli, Δρ_orb, and Δρ isosurfaces