It is well-established that strong electric fields (EFs) can align water dipoles, partially order the H-bond network of liquid water, and induce water splitting and proton transfers. To illuminate the fundamental behavior of water under external EFs, we present the first benchmark, to the best of our knowledge, of DFT calculations of the water dimer exposed to intense EFs against coupled cluster calculations. The analyses of the vibrational Stark effect and electron density provide a consistent picture of the intermolecular charge transfer effects driven along the H-bond by the increasing applied field at all theory levels. However, our findings prove that at extreme field regimes (∼1-2 V/Å) DFT calculations significantly exaggerate by ∼10-30% the field-induced strengthening of the H-bond, both within the GGA, hybrid GGA, and hybrid meta-GGA approximations. Notably, a linear correlation emerges between the vibrational Stark effect on OH stretching and H-bond strengthening: a 1 kcal mol-1 increase corresponds to an 80 cm-1 red-shift in OH stretching frequency.

Department of Chemical Biological Pharmaceutical and Environmental Science University of Messina 98166 Messina Italy

Dipartimento di Chimica Biologia e Biotecnologie Università degli Studi di Perugia 06123 Perugia Italy

Dipartimento di Chimica Materiali e Ing Chimica G Natta Politecnico di Milano Piazza Leonardo da Vinci 32 20133 Milano Italy

Institute for Chemical Physical Processes National Research Council of Italy 98158 Messina Italy

J Heyrovský Institute of Physical Chemistry Czech Academy of Sciences Dolejškova 3 18223 Prague Czechia

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Hydrogen Bonds under Electric Fields with Quantum Accuracy