**5. Conclusions**

In the present work, we have employed standard density functional methods to computationally characterize a broad variety of unusual "windowpane" clusters that may play a role in the high-density fluid phase(s) of water. Despite their diverse topological forms and unusual angular features, we have demonstrated that these clusters are fully compliant with water's known facility in forming doubly (*d*-type), triply (*t*-type), and quadruply (*q*-type) coordinative linkages to other water molecules, leading to multiply connected ("water-wired") networks of increasing energetic stability when proper Grotthuss-type proton ordering is maintained. The *Aufbau* construction approach also suggests the mechanistic sequence by which such Grotthuss-ordered clusters can readily form from successive aggregation with water dimers.

We have also employed natural bond orbital (NBO) and natural resonance theory (NRT) analysis tools to demonstrate the consistency and accuracy with which H-bonding in these clusters conforms to the general conceptual picture of *resonance–covalency* ("charge transfer") as the authentic origin of intermolecular O−H···O attractions. The charge flows and adaptive bond order and structural shifts in these clusters are shown to obey familiar bond order–bond length (BOBL) correlations with high accuracy (|χ| > 0.9). Moreover, the BOBL correlations also exhibit the expected *deviations* from linearity in the asymptotic limit of vanishing bond order where *R*O···<sup>O</sup> distance becomes divergent. Although connections can be shown between NBO and Bader-type descriptors [65], we believe that the NRT bond orders of the present work provide broader predictive utility and more nuanced inclusion of resonance effects than the topological descriptors as employed in previous studies of water clustering (e.g., [66]).

The reader is reminded that "correlation is not causation." Nevertheless, the *continuity* of robust BOBL correlations that stretch across the broad extremes of supramolecular (subinteger) vs. molecular (multi-integer) bond orders strongly implies their *shared* origin in unified "covalency" concepts, contrary to the dichotomous viewpoint that still dominates freshman-level teaching of chemical principles and many facets of force-field methodology.

**Supplementary Materials:** The following supporting information can be downloaded at: https: //www.mdpi.com/article/10.3390/molecules27134218/s1. The Supporting Information (SI) file contains optimized geometrical coordinates, NBO/NRT keyword input, and other computational details in ready-to-run Gaussian input files for all equilibrium water clusters described in the paper. The file also contains tables of computed natural atomic charges and natural bond orders for all water clusters of the work.

**Funding:** Support for computational facilities was provided in part by the National Science Foundation Grant CHE-0840494.

**Institutional Review Board Statement:** Not applicable.

**Informed Consent Statement:** Not applicable.

**Data Availability Statement:** Not applicable.

**Acknowledgments:** Thanks are due to Eric Glendening (Indiana State University) for assistance in wrangling consistent NRT bond orders for the many challenging H-bonding interactions in this study. **Conflicts of Interest:** The author declares no conflict of interest.

**Sample Availability:** Samples of the clusters are not available from the author.
