Calcium oxalate minerals are broadly present in nature. They form through biogenic, geogenic, and pathogenic processes that lead to different pseudopolymorphs. Being the most common solid phase in human nephrolithiasis, calcium oxalate monohydrate (COM) in particular has been the focus of much investigation. It exists in several crystalline forms, two of which appear to be of biological and medical relevance: the low- and high-temperature forms (COM-LT and COM-HT, respectively). While there is broad consensus on the ordered structure of COM-LT, which possesses the P2₁/n space group symmetry, for COM-HT controversy remains. Experimental results suggest that there is a certain degree of structural disorder in the high-temperature form. However, the exact character of disorder in COM-HT is yet an open question. Here, we examine the effect of the disorder of water molecules on the structure of COM using first-principles calculations based on dispersion-augmented density functional theory. Such calculations allow for controlled examination of specific disorder features and their effect on crystal structure and stability. On the basis of our first-principles analysis, we suggest that in COM-HT each water dimer site is randomly occupied by any of four water dimer arrangements present in COM-LT, leading to statistical 2/m point symmetry at each site and a statistical I2/m space group symmetry.

中文翻译：

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一水草酸钙的有序和无序：从第一性原理计算中得出的见解

草酸钙矿物质广泛存在于自然界中。它们通过导致不同假多晶型物的生物，地理和致病过程形成。草酸钙一水合物（COM）是人类肾结石病中最常见的固相，已成为许多研究的重点。它以几种晶体形式存在，其中两种似乎具有生物学和医学意义：低温和高温形式（分别为COM-LT和COM-HT）。尽管对COM-LT的有序结构拥有广泛共识，该结构具有P 2 ₁ / n空间群的对称性，对于COM-HT的争议仍然存在。实验结果表明，高温形式存在一定程度的结构紊乱。但是，COM-HT中疾病的确切特征仍是一个悬而未决的问题。在这里，我们使用基于弥散增强的密度泛函理论的第一原理计算来检验水分子紊乱对COM结构的影响。这样的计算允许对特定的无序特征及其对晶体结构和稳定性的影响进行受控检查。根据我们的第一性原理分析，我们建议在COM-HT中，每个水二聚体位点被COM-LT中存在的四个水二聚体排列中的任意一个随机占据，从而导致每个位点的统计2 / m点对称性和统计I2 / m空间群对称。