Figure 1

(a) Instantaneous tetrahedral environment of a molecule. The O neighbors are labeled 1 to 4; the $x$ axis connects 1 and 2; the $y$ axis connects 3 and 4; the $z$ axis connects the center of 1–2 with the center of 3–4. This local frame is not orthogonal but the deviation is usually small. 1, 2, 3, and 4 identify a “cubic” cage shown by dashed lines, the center of which is near the O of the central molecule. The isodensity plots show MLWFs of the corner molecules participating in H bonds with the central molecule. The 1 and 2 MLWFs are lone pairs; 3 and 4 are bond pairs. The central molecule forms donor bonds with 1 and 2 and acceptor bonds with 3 and 4 (Pauling ice rule). In (b) and (c) the red dashed arrows show molecular dipole changes induced by a displacement of the central molecule along $z$ (b) and $-x$ (c); the full black arrows show the net effect of the coordinated dipolar changes.Reuse & Permissions

Figure 2

Calculated ${\mathrm{D}}_2\mathrm{O}$ IR spectrum. Arrows indicate the experimental peak positions from Ref. 2. The full spectrum of undeuterated ${\mathrm{H}}_2\mathrm{O}$ (solid line) from Ref. 4 and a spectrum of ${\mathrm{D}}_2\mathrm{O}$ (dashed line) in the range $1000{\mathrm{cm}}^{-1}$ to $2800{\mathrm{cm}}^{-1}$ from Ref. 34 are shown in the inset.Reuse & Permissions

Figure 3

Kinematical correlation function that blows up the translational modes (see text).Reuse & Permissions

Figure 4

IR spectrum calculated by retaining only the $i=j$ terms in the sum (solid line). Center-of-mass velocity autocorrelation function (dash-dotted line). The vertical dashed line is at $200{\mathrm{cm}}^{-1}$.Reuse & Permissions

Figure 5

Dynamical correlation functions, enhancing the IR activity due to intermolecular charge transfer (see text).Reuse & Permissions