Energy Decomposition and Local Correlation

ORCA

Define fragments and analyze interaction energies with EDA-NOCV and local energy decomposition while keeping method and fragment conventions explicit.

Role: Electronic structure and EDA
Typical input: Geometry, fragments, method, basis
Typical output: Interaction and deformation terms, NOCVs

1. Interaction energy

At a fixed complex geometry, a basic interaction energy is the energy of the complex minus the energies of isolated fragments evaluated under a defined geometry and basis convention.

{ΔE}_{int} = E_{AB} - E_{A} - E_{B}

Distinguish interaction energy from binding energy: the latter may additionally include deformation, zero-point, thermal, solvation, and standard-state terms.

2. Fragments and preparation

Fragment charge, multiplicity, geometry, orbital reference, and atom assignment are part of the EDA definition. Verify fragment labels in the output before interpreting components.

Fragment choice defines the result

Changing fragment charge, spin, orbital occupations, or the bond that is conceptually cleaved changes the EDA path and therefore the numerical components.

3. EDA-NOCV

EDA separates preparation and interaction terms; EDA-NOCV further resolves orbital interaction through natural orbitals for chemical valence and deformation densities. The exact partition is method-dependent.

{ΔE}_{int} = {ΔE}_{elstat} + {ΔE}_{Pauli} + {ΔE}_{orb} + {ΔE}_{disp}

Electrostatics describes interaction between unrelaxed fragment densities, Pauli repulsion arises from antisymmetrization, orbital interaction includes polarization and charge transfer, and dispersion follows the selected dispersion treatment.

4. EDA-NOCV input

! BP86 TZVP EDA

%EDA
  FRAG1 "BP86 TZVP"
  FRAG2 "BP86 TZVP"
  FRAG1_C 1
  FRAG1_M 1
  FRAG2_C 0
  FRAG2_M 1
END

* xyz 1 1
Li (1)  1.986554  0.245933  0.000797
N  (2) -0.424027 -0.022414 -0.010889
H  (2) -0.914047 -0.381589 -0.841562
H  (2) -0.958117 -0.388240  0.789150
H  (2)  0.468043 -0.535155  0.011083
*

This follows the ORCA 6.1 Li+-NH3 example. Syntax and available EDA variants depend on the version; confirm fragment assignments in the output and use the manual for the installed release.

5. NOCV pairs

Inspect paired eigenvalues and deformation-density plots together. A visual pattern can support a donation or polarization assignment, but the label should be consistent with fragment orbitals and geometry.

Report both the deformation-density isovalue and the paired energy contribution. The overall sign of a plotted deformation-density pair can be reversed, so interpret density flow with the program convention and fragment definition.

6. Local Energy Decomposition

LED is associated with local-correlation calculations and partitions interaction contributions in a framework different from EDA-NOCV or SAPT. Do not compare similarly named terms as if definitions were identical.

LED is most naturally paired with DLPNO-CCSD(T)-type calculations. Its preparation, electrostatic, exchange, and correlation-related terms are not numerically interchangeable with ETS-NOCV or SAPT components.

! DLPNO-CCSD(T) def2-TZVP def2-TZVP/C def2/JK   RIJCOSX VeryTightSCF TightPNO LED

* xyz 0 1
# Assign atoms to fragments according to the installed-version manual.
*

7. Choosing SAPT, EDA-NOCV, or LED

Method	Main question
SAPT	How do intermolecular electrostatics, exchange, induction, and dispersion contribute?
EDA-NOCV	How do chosen fragments reorganize and exchange density when a complex forms?
LED	How is a local-correlation interaction energy partitioned?

8. Validation

Check ORCA version and manual syntax.
Verify fragment charges, spins, and atom membership.
State geometry and basis conventions.
Inspect SCF and correlation convergence.
Compare total interaction energies before individual terms.

9. References

Last reviewed: August 4, 2026. Check the linked official documentation for syntax specific to the installed software version.