Song Lingchun, Mo Yirong, Gao Jiali
Department of Chemistry, Digital Technology Center and Supercomputing Institute, University of Minnesota, Minneapolis, MN 55455.
J Chem Theory Comput. 2009 Jan 1;5(1):174-185. doi: 10.1021/ct800421y.
An effective Hamiltonian mixed molecular orbital and valence bond (EH-MOVB) method is described to obtain an accurate potential energy surface for chemical reactions. Building upon previous results on the construction of diabatic and adiabatic potential surfaces using ab initio MOVB theory, we introduce a diabatic-coupling scaling factor to uniformly scale the ab initio off-diagonal matrix element H(12) such that the computed energy of reaction from the EH-MOVB method is in agreement with the target value. The scaling factor is very close to unity, resulting in minimal alteration of the potential energy surface of the original MOVB model. Furthermore, the relative energy between the reactant and product diabatic states in the EH-MOVB method can be improved to match the experimental energy of reaction. A key ingredient in the EH-MOVB theory is that the off-diagonal matrix elements are functions of all degrees of freedom of the system and the overlap matrix is explicitly evaluated. The EH-MOVB method has been applied to the nucleophilic substitution reaction between hydrosulfide and chloromethane to illustrate the methodology and the results were matched to reproduce the results from ab initio valence bond self-consistent valence bond (VBSCF) calculations. The diabatic coupling (the off-diagonal matrix element in the generalized secular equation) has small variations along the minimum energy reaction path in the EH-MOVB model, whereas it shows a maximum value at the transition state and has nearly zero values in the regions of the ion-dipole complexes from VBSCF calculations. The difference in the diabatic coupling stabilization is attributed to the large overlap integral in the computationally efficient MOVB method.
描述了一种有效的哈密顿混合分子轨道和价键（EH-MOVB）方法，用于获得化学反应的精确势能面。基于先前使用从头算MOVB理论构建非绝热和绝热势能面的结果，我们引入了一个非绝热耦合比例因子，以统一缩放从头算非对角矩阵元H(12)，使得从EH-MOVB方法计算得到的反应能量与目标值一致。该比例因子非常接近1，导致原始MOVB模型的势能面变化最小。此外，EH-MOVB方法中反应物和产物非绝热态之间的相对能量可以得到改善，以匹配实验反应能量。EH-MOVB理论的一个关键要素是，非对角矩阵元是系统所有自由度的函数，并且明确评估了重叠矩阵。EH-MOVB方法已应用于硫化氢与氯甲烷之间的亲核取代反应，以说明该方法，并且结果与从头算价键自洽价键（VBSCF）计算的结果相匹配。在EH-MOVB模型中，非绝热耦合（广义久期方程中的非对角矩阵元）沿最小能量反应路径的变化很小，而在VBSCF计算中，它在过渡态处显示出最大值，并且在离子-偶极复合物区域几乎为零。非绝热耦合稳定化的差异归因于计算效率高的MOVB方法中的大重叠积分。
