Lacerda Evanildo G, Sauer Stephan P A, Mikkelsen Kurt V, Coutinho Kaline, Canuto Sylvio
Department of Chemistry, University of Copenhagen, Universitetsparken 5, DK-2100, Copenhagen Ø, Denmark.
Instituto de Física da Universidade de São Paulo, Rua do Matão 1371, 05508-090, São Paulo, SP, Brazil.
J Mol Model. 2018 Feb 20;24(3):62. doi: 10.1007/s00894-018-3600-4.
In this work we investigate the level of theory necessary for reproducing the non-linear variation of the Xe nuclear magnetic resonance (NMR) chemical shift with the density of Xe in supercritical conditions. In detail we study how the Xe chemical shift depends under supercritical conditions on electron correlation, relativistic and many-body effects. The latter are included using a sequential-QM/MM methodology, in which a classical MD simulation is performed first and the chemical shift is then obtained as an average of quantum calculations of 250 MD snapshots conformations carried out for Xe clusters (n = 2 - 8 depending on the density). The analysis of the relativistic effects is made at the level of 4-component Hartree-Fock calculations (4c-HF) and electron correlation effects are considered using second order Møller-Plesset perturbation theory (MP2). To simplify the calculations of the relativistic and electron correlation effects we adopted an additive scheme, where the calculations on the Xe clusters are carried out at the non-relativistic Hartree-Fock (HF) level, while electron correlation and relativistic corrections are added for all the pairs of Xe atoms in the clusters. Using this approach we obtain very good agreement with the experimental data, showing that the chemical shift of Xe in supercritical conditions is very well described by cluster calculations at the HF level, with small contributions from relativistic and electron correlation effects.
在这项工作中，我们研究了在超临界条件下重现氙核磁共振（NMR）化学位移随氙密度的非线性变化所需的理论水平。详细地说，我们研究了在超临界条件下氙化学位移如何依赖于电子关联、相对论效应和多体效应。后者通过顺序量子力学/分子力学（QM/MM）方法包含在内，其中首先进行经典分子动力学（MD）模拟，然后将化学位移作为对氙簇（根据密度，n = 2 - 8）的250个MD快照构象进行量子计算的平均值获得。相对论效应的分析在四分量哈特里 - 福克（4c - HF）计算水平上进行，电子关联效应使用二阶莫勒 - 普莱塞特微扰理论（MP2）考虑。为了简化相对论效应和电子关联效应的计算，我们采用了一种加和方案，其中对氙簇的计算在非相对论哈特里 - 福克（HF）水平上进行，而对簇中所有氙原子对添加电子关联和相对论修正。使用这种方法，我们与实验数据取得了很好的一致性，表明在超临界条件下氙的化学位移可以通过HF水平的簇计算很好地描述，相对论效应和电子关联效应的贡献很小。
