Rowan Docs

Interaction Energy Decomposition

Python API · stjames models · API example

How it works

Rowan's interaction-energy-decomposition workflow uses symmetry-adapted perturbation theory (SAPT0) to calculate the interaction energy between two noncovalently interacting fragments at the supplied geometry. It separates the total into electrostatic, exchange, induction, and dispersion contributions, helping explain which effects favor or oppose the interaction.

Settings

  • Input: supply both fragments together in the geometry you want to analyze. Each fragment must be neutral and closed-shell (all electrons paired). Use complete fragments without cutting covalent bonds between them; optimize the dimer first when appropriate.
  • Energy decomposition settings: the web form uses SAPT0/jun-cc-pVDZ, implemented in Psi4. This field is fixed.
  • Fragment 1 indices: identify every atom in the first fragment; all remaining atoms form the second. Click into this field, then double-click a fragment in the viewer, select individual atoms, or enter atom numbers manually. Atom numbering starts at 1. Leave at least one atom in each fragment.

Notes

The results report the total interaction energy and four components in kcal/mol. Negative energies favor association; positive energies oppose it.

  • Electrostatic: attraction or repulsion between the fragments' charge distributions, including electrons and nuclei.
  • Exchange: repulsion associated with overlapping electron clouds and the Pauli exclusion principle.
  • Induction: polarization of each fragment by the other, including exchange-induction effects.
  • Dispersion: attraction from correlated fluctuations in the fragments' electron distributions, including exchange-dispersion effects.

These components combine to give the total interaction energy. A hydrogen bond or a π–π contact generally involves several components.

The calculation keeps the input geometry fixed and gives an electronic interaction energy. Solvent, entropy, and the energy needed to deform the isolated fragments are not included; these additional contributions matter when relating interaction energies to binding free energies or potency. Charged or open-shell fragments are not supported by this workflow.

The jun-cc-pVDZ basis is a truncated version of aug-cc-pVDZ. Basis choices and the performance of SAPT methods are discussed in Parker et al. (2014). A more thorough introduction to SAPT and SAPT0 is available in the first seven minutes of this video by Psi4 contributor Zach Glick.

Further reading