Figure 1.

Illustration of the geometric quantities used in the water benchmarks. For the dimer tests and fits, we use the shorthand nomenclatures “QM/MM” and “MM/QM” to discern between the two possible subsystem configurations. Three angles provide insight into the hydrogen bonding network structure of liquid water, and are sampled in the molecular dynamics simulations: The donor angles $\alpha =\angle ({\mathrm{O}}_{\mathrm{D}},{\mathrm{H}}_{\mathrm{D}},{\mathrm{O}}_{\mathrm{A}})$ and $\beta =\angle ({\mathrm{H}}_{\mathrm{D}},{\mathrm{O}}_{\mathrm{D}},{\mathrm{O}}_{\mathrm{A}})$, and the acceptor angle $\theta =\angle ({\mathrm{H}}_{\mathrm{D}},{\mathrm{O}}_{\mathrm{A}},{\mathrm{H}}_{\mathrm{A}})$.