Halogen bonds are not purely electrostatic

Relativistic DFT shows halogen and hydrogen bonds share HOMO–LUMO covalency; halogen bonds have weaker electrostatics but stronger orbital mixing.

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Are halogen and hydrogen bonds purely electrostatic?

No—neither is predominantly, let alone purely, electrostatic.

Where does the covalent piece come from?

HOMO–LUMO mixing of the acceptor np lone pair with D–X or D–H σ*.

How do halogen-bond electrostatics compare with hydrogen bonds?

Weaker, because dihalogens DX are less polar than hydrogen halides DH.

Why can halogen-bond orbital interactions be stronger?

The halogen–halogen σ* sits lower than the stronger H–X σ*.

How does FH···A− strength change from F− to I−?

ΔE weakens from −53 to −18 kcal mol−1 as r(H···A) grows from 1.159 to 2.319 Å.

Which DFT setup was used?

ZORA-BP86/TZ2P relativistic DFT.

What ΔE values describe FF···A− along F− to I−?

−52, −43, −44, and −48 kcal mol−1.

In general, how do DX···A− and DH···A− change from F− to I−?

They become weaker as halide radius grows and np energies drop.

What electronic-structure method underpins the bonding analysis?

Relativistic density functional theory.

What is unusual about DF···A− fluorine-bond strength?

It is partially inverted: ΔE becomes more stabilizing along Cl−, Br−, I−.

Can halogen bonds be stronger than analogous hydrogen bonds?

Yes—they can be stronger or weaker depending on the pair.