投稿时间：2022-01-25
修订日期：2022-03-10
网络发布日期：2022-05-06
中文摘要:地质流体中不同络合物之间的稳定同位素平衡分馏系数是利用稳定同位素示踪地质流体演化过程的必要前提。但由于受实验条件的限制，流体中部分元素不同络合物之间的同位素分馏系数难以通过实验精确测定。基于第一性原理的理论计算是获得溶液中不同络合物之间稳定同位素平衡分馏系数的一种重要手段，能够获取各种极端条件下同位素分馏系数。在应用第一性原理计算溶液中不同络合物间同位素平衡分馏系数时，需要对溶液的动态特性和水分子之间的相互作用进行模拟。目前，显式溶剂模型和隐式溶剂模型是模拟地质流体中水溶液环境的2种主要模型，均已成功应用于同位素分馏系数的计算。文章系统介绍了这2种模型的计算方法，并且对比研究了这2种模型计算约化配分函数比(β)的优势及不足，结果显示，2种模型的主要差异表现在3个方面：①计算耗时的不同，隐式溶剂模型计算的原子数目少，结构单一，因此耗时少；显式溶剂模型计算的原子数目多，结构复杂多样，因此在同一计算和理论水平条件下，耗时比隐式溶剂模型多；②水分子数和构型的不同，隐式溶剂模型只有一个构型，因而不能评估络合物构型对β值的影响；而显式溶剂模型能够通过添加不同数量的水分子及建立不同的初始构型有效评估水分子数量及构型对计算结果的影响；③隐式溶剂模型将溶质周围的水环境定义为具有均匀介电常数的连续介质，没有充分考虑水的动态特性和水分子之间的相互作用，导致计算结果误差较大；而显式溶剂模型能更好的处理水分子的动态特征及分子间形成的氢键，计算结果更为准确。通过2种模型的对比研究表明，显式溶剂模型能够更好的模拟水分子和溶质间的相互作用，计算获得的各络合物之间的同位素平衡分馏系数更为准确。本研究结果可以为今后开展相关同位素分馏计算时选择何种模型提供重要依据。
Abstract:The equilibrium isotope fractionation factors between complexes in nature fluids play a key role in using stable isotope to trace the evolution process of nature fluids. However, due to the limitations of experiments, it is difficult to accurately determine the isotope fractionation factors between complexes. Based on the first principle, quantum calculation is an important method to obtain the stable isotope equilibrium fractionation factor between different complexes, which cannot be obtained by experimental method for various extreme conditions. It is necessary to simulate the dynamic characteristics of solution and the interaction between water molecules when applying the first principle to calculate the equilibrium isotope fractionation factors between complexes. At present, there are two methods to simulate the environment of aqueous solution in geological fluid, explicit solvent model and implicit solvent model, which have been successfully applied to the calculation of equilibrium isotope fractionation factors of many elements. In this study, we systematically introduce the two models, and compare the advantages and disadvantages of the two models in calculating reduced isotope partition function ratio (RPFR or β) value. The results show that the differences between the two methods are mainly reflected in three aspects: ① for computing time, due to the small number of atoms and a single structure of implicit solvent model, the calculation time is less; however, due to the large number of atoms and complex structure of the explicit solvent model, the calculation takes a long time under the same calculation and theoretical level with the implicit solvent model; ② For different configurations, the implicit solvent model has only one configuration, which cannot evaluate the effect of configuration on β values, while the explicit solvent model can effectively evaluate the influence of configuration on the results by establishing different initial configurations; ③ The implicit solvent model defines the aqueous solution environment around the solute as a continuum of uniform dielectric constant, which does not fully consider the dynamic characteristics of water and the interaction between water molecules, which may lead to large errors in the calculation results. The explicit solvent model can well express the dynamic characteristics of water and the hydrogen bond formed between water molecules, which lead to more accurate calculation results. The comparative study of the two models shows that the explicit solvent model can better simulate the interaction between water molecules and solutes, and the calculated equilibrium isotope fractionation factors between complexes is more accurate, which can provide a basis and reference for choosing which model to use in the calculation of isotopic fractionation in the future.
keywords:geochemistry first principles isotope fractionation complexes ore-bearing fluids explicit solvent model and implicit solvent model
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基金项目:国家自然科学基金重大研究计划重点支持项目(编号:92062218)资助
引用文本：
孙明光,王佳新,袁顺达.2022.地质流体中络合物同位素平衡分馏系数的第一性原理计算[J].矿床地质,41(2):372~382SUN MingGuang,WANG JiaXin,YUAN ShunDa.2022.First principles calculations of equilibrium isotope fractionations for complexes in geological fluid[J].Mineral Deposits41(2):372~382
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