Potential-Energy Landscape Study of the Amorphous-Amorphous Transformation in ${\mathrm{H}}_2\mathrm{O}$

We study the potential energy landscape explored during a compression-decompression cycle for the simple point charge extended model of water. During the cycle, the system changes from low density amorphous (LDA) ice to high density amorphous ice. After the cycle, the system does not return to the same region of the landscape, supporting the interesting possibility that more than one significantly different configuration corresponds to LDA. We find that the regions of the landscape explored during this transition have properties remarkably different from those explored in thermal equilibrium in the liquid phase.

Figure 1

(a) $P_{\mathrm{I}\mathrm{S}}$ and (b) $e_{\mathrm{I}\mathrm{S}}$ for $T=0\mathrm{K}$ as functions of $\rho$, where a starting glass configuration (black dot) is decompressed to $\rho =0.8$, and also compressed to $\rho =1.4\mathrm{g}/{\mathrm{c}\mathrm{m}}^3$ and then decompressed to $\rho =0.8\mathrm{g}/{\mathrm{c}\mathrm{m}}^3$. The density in each step of the compression/decompression cycle is changed by $\Delta \rho =5\times {10}^{-5}\mathrm{g}/{\mathrm{c}\mathrm{m}}^3$. (c) Squared distance between a sequence of IS configurations differing by ${10}^{-2}\mathrm{g}/{\mathrm{c}\mathrm{m}}^3$ along the compression/decompression path. Note the correlation between the large molecular rearrangements in part (c) and the corresponding discontinuities in $P_{\mathrm{I}\mathrm{S}}$ and $e_{\mathrm{I}\mathrm{S}}$ in parts (a) and (b), such as one finds at $\rho =1.20\mathrm{g}/{\mathrm{c}\mathrm{m}}^3$.

Figure 2

$P_{\mathrm{I}\mathrm{S}}$, $e_{\mathrm{I}\mathrm{S}}$, and ${\mathcal{S}}_{\mathrm{I}\mathrm{S}}$, averaged over 16 different realizations, as functions of $\rho$ during the compression and decompression at $T=0\mathrm{K}$ (left panels) and $T=77\mathrm{K}$ (right panels) for the fast (solid line) and slow (dashed line) compression rates. Error bars are shown in the left lower corner.

Figure 3

(a) $P_{\mathrm{I}\mathrm{S}}$ and (b) ${\mathcal{S}}_{\mathrm{I}\mathrm{S}}$ as functions of $e_{\mathrm{I}\mathrm{S}}$ at different densities for the equilibrium liquid (filled symbols) and for the compression/decompression of glass (thin line) at both temperatures studied. Open symbols indicate the density during the compression/decompression. The relative location of the open symbol with respect to the location of the same filled symbol is a measure of the difference between the landscape properties in equilibrium and in the glass. The significant difference in the location confirms that the compressed/decompressed glass explores PEL regions not explored under equilibrium conditions at the same density. Densities are 0.9 (○), 1.0 (◇), 1.1 (⊲), 1.2 (▽), 1.3 (⊳), and 1.4 (△). The error bars for these densities are indicated in the figures. Data for equilibrium liquid are averaged over 2000 realizations and error bars are smaller than the corresponding symbol size.