Song Lingchun, Gao Jiali
Department of Chemistry and the Minnesota Supercomputing Institute, Digital Technology Center, University of Minnesota, Minneapolis, Minnesota 55455, USA.
J Phys Chem A. 2008 Dec 18;112(50):12925-35. doi: 10.1021/jp803050e.
A theoretical model is presented for deriving effective diabatic states based on ab initio valence bond self-consistent field (VBSCF) theory by reducing the multiconfigurational VB Hamiltonian into an effective two-state model. We describe two computational approaches for the optimization of the effective diabatic configurations, resulting in two ways of interpreting such effective diabatic states. In the variational diabatic configuration (VDC) method, the energies of the diabatic states are variationally minimized. In the consistent diabatic configuration (CDC) method, both the configuration coefficients and orbital coefficients are simultaneously optimized to minimize the adiabatic ground-state energy in VBSCF calculations. In addition, we describe a mixed molecular orbital and valence bond (MOVB) approach to construct the CDC diabatic and adiabatic states for a chemical reaction. Note that the VDC-MOVB method has been described previously. Employing the symmetric S(N)2 reaction between NH(3) and CH(3)NH(3)(+) as a test system, we found that the results from ab initio VBSCF and from ab initio MOVB calculations using the same basis set are in good agreement, suggesting that the computationally efficient MOVB method is a reasonable model for VB simulations of condensed phase reactions. The results indicate that CDC and VDC diabatic states converge, respectively, to covalent and ionic states as the molecular geometries are distorted from the minimum of the respective diabatic state along the reaction coordinate. Furthermore, the resonance energy that stabilizes the energy of crossing between the two diabatic states, resulting in the transition state of the adiabatic ground-state reaction, has a strong dependence on the overlap integral between the two diabatic states and is a function of both the exchange integral and the total diabatic ground-state energy.
本文提出了一种理论模型，通过将多组态价键哈密顿量简化为有效的双态模型，基于从头算价键自洽场（VBSCF）理论推导有效非绝热态。我们描述了两种用于优化有效非绝热构型的计算方法，从而产生了两种解释此类有效非绝热态的方式。在变分非绝热构型（VDC）方法中，非绝热态的能量通过变分法最小化。在一致非绝热构型（CDC）方法中，构型系数和轨道系数同时进行优化，以在VBSCF计算中最小化绝热基态能量。此外，我们描述了一种混合分子轨道和价键（MOVB）方法，用于构建化学反应的CDC非绝热态和绝热态。请注意，VDC - MOVB方法先前已有描述。以NH(3)和CH(3)NH(3)(+)之间的对称S(N)2反应作为测试系统，我们发现使用相同基组的从头算VBSCF和从头算MOVB计算结果吻合良好，这表明计算效率高的MOVB方法是凝聚相反应VB模拟的合理模型。结果表明，随着分子几何结构沿反应坐标从各自非绝热态的最小值发生畸变，CDC和VDC非绝热态分别收敛到共价态和离子态。此外，稳定两个非绝热态之间能量交叉从而导致绝热基态反应过渡态的共振能量，强烈依赖于两个非绝热态之间的重叠积分，并且是交换积分和总非绝热基态能量的函数。
