Department of Chemistry, University of Cambridge, Cambridge CB2 1EW, United Kingdom.
J Chem Phys. 2009 Oct 21;131(15):154504. doi: 10.1063/1.3250438.
The density functional theory based molecular dynamics (DFTMD) method for the computation of redox free energies presented in previous publications and the more recent modification for computation of acidity constants are reviewed. The method uses a half reaction scheme based on reversible insertion/removal of electrons and protons. The proton insertion is assisted by restraining potentials acting as chaperones. The procedure for relating the calculated deprotonation free energies to Brønsted acidities (pK(a)) and the oxidation free energies to electrode potentials with respect to the normal hydrogen electrode is discussed in some detail. The method is validated in an application to the reduction of aqueous 1,4-benzoquinone. The conversion of hydroquinone to quinone can take place via a number of alternative pathways consisting of combinations of acid dissociations, oxidations, or dehydrogenations. The free energy changes of all elementary steps (ten in total) are computed. The accuracy of the calculations is assessed by comparing the energies of different pathways for the same reaction (Hess's law) and by comparison to experiment. This two-sided test enables us to separate the errors related with the restrictions on length and time scales accessible to DFTMD from the errors introduced by the DFT approximation. It is found that the DFT approximation is the main source of error for oxidation free energies.
基于密度泛函理论的分子动力学（DFTMD）方法用于计算氧化还原自由能已在先前的出版物中介绍过，最近又对其进行了修改以计算酸度常数。该方法使用基于可逆电子和质子插入/去除的半反应方案。质子的插入由作为伴侣的约束势辅助。详细讨论了将计算得到的去质子化自由能与布伦斯特酸度（pK(a)）以及氧化自由能与相对于标准氢电极的电极电势相关联的过程。该方法在应用于水相 1,4-苯醌的还原中得到了验证。对苯二酚转化为苯醌可以通过多种替代途径发生，这些途径由酸离解、氧化或脱氢的组合组成。计算了所有基本步骤（总共十个）的自由能变化。通过比较同一反应的不同途径的能量（赫斯定律）以及与实验的比较来评估计算的准确性。这种双向测试使我们能够将与 DFTMD 可及的长度和时间尺度限制相关的误差与 DFT 逼近引入的误差分开。结果发现，DFT 逼近是氧化自由能误差的主要来源。
