MEHMO的理论分析及其在碱金属双原子分子上的应用

THEORETICAL ANALYSIS AND APPLICATION OF MEHMO TO AKALINE METAL DIATOMICMOLECULES

EHMO[2]是一种半经验的分子轨道理论,由于其计算量较小,近年来在固体和复相催化等领域的研究中得到了广泛的应用,这种方法常被用来预示分子的几何构形.然而当它被用于CH?F时却发现构型误差很大;对于H2、碱金属等的双原子分子,位能曲线无极小值.A.B.Anderson等认为,这可能是由于EHMO方法未能很好地描述分子中两个原子间的静电相互作用,因而提出了一种校正方法。

Starting from Roothaan equation, this paper has analysed the theoretical background and physical interpretation of the factor (I-Y)e^(-(1/2)R), proposed by Jiang Fenglin et al. for improving EHMO method by adding this factor into the diagonal element of H matrix. It has been shown that for hydrogenlike molecules M_2(M=H), Li, Na, K, Rb, Cs) under the core approximation, we have (I-Y)e^((1/2)R)≈<11|12>, that is, (I-Y)e^((1/2)R) may be considered as the Coulomb repulsive energy between an electron in φ_1 and the electronic charge in the overlop part of φ_1 and φ_2, so far as single Slater determinant issued to reprsent approximately the wave function of the system being discussed. Thus MEHMO (Modified EHMO) method has had a clear and firm theoretical foundation for Problems about hydrogenlike molecules, it is expected that this method will reach satisfied result when it is applied to alkali metal diatomic molecules.This paper has calculated the equilibrium nuclear distanes and dissociative energies of a series of alkali metal homonuclear and heteronuclear diatomic molecules, the results are fairly good agreement with the experimental results.The calculation results about the fundamental vibrational frequencies of molecules in ground state are also of some useful value.