键解离焓的理论方法研究

Assessment of Contemporary Theoretical Methods for Bond Dissociation Enthalpies

计算键解离焓的方法除了对很小的体系采用高精度量子化学方法外，最常使用的便是密度泛函方法（DFT）．但是碍于其计算精度有限，因此希望能寻找到适合处理较大体系且精度更高的理论方法．本文考察的方法有DFT、双杂化密度泛函方法、以及高精度方法，所研究的对象包括了单环及多环芳香化合物、支链烷烃、以及小的无机化合物分子等．结果表明，所有方法中，mPW2PLYP方法和G4MP2方法的综合表现最好．根据计算结果，最后建议在计算小尺寸的体系（原子数〈20）时，选择G4MP2方法；计算大尺寸f20≤原子数〈50）的芳香化合物时，双杂化密度泛函方法最为适宜，计算在此尺寸范围内的长链或支链烷烃时，适宜选择有经验色散校正的双杂化密度泛函方法；处理更大尺寸（原子数≥50）的体系时，建议使用DFT方法，其中M06—2X方法和B3P86方法均有不错表现．此外，不同方法的最优结构的差别以及基组的影响均在考察内容中．

The density functional theory （DFT） is the most popular method for evaluating bond dis- sociation enthalpies （BDEs） of most molecules. Thus, we are committed to looking for alternative methods that can balance the computational cost and higher precision to the best for large systems. The performance of DFT, double-hybrid DFT, and high-level com- posite methods are examined. The tested sets contain monocyclic and polycyclic aromatic molecules, branched hydrocarbons, small inorganic molecules, etc. The results show that the mPW2PLYP and G4MP2 methods achieve reasonable agreement with the benchmark val- ues for most tested molecules, and the mean absolute deviations are 2.43 and 1.96 kcal/mol after excluding the BDEs of branched hydrocarbons. We recommend the G4MP2 is the most appropriate method for small systems （atoms number≤20）; the double-hybrid DFT methods are advised for large aromatic molecules in medium size （20≤atoms number≤50）, and the double-hybrid DFT methods with empirical dispersion correction are recommended for long-chain and branched hydrocarbons in the same size scope; the DFT methods are ad- vised to apply for large systems （atoms number〉50）, and the M06-2X and B3P86 methods are also favorable. Moreover, the differences of optimized geometry of different methods are discussed and the effects of basis sets for various methods are investigated.