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采用一系列开壳层分子来检验我们最近提出的开壳层微扰理论(J Chem Theory Comput,2009,5:931―936,简称OSPT)的有效性.我们首先采用双重态分子NH2来讨论OSPT的收敛性.微扰能量计算至400阶.基函数为6-13G.选择这个基函数是为了易于进行完全组态相互作用计算(简称FCI),以及和FCI结果比较.几何结构有3种:(1)N-H键长为平衡键长,(2)N-H键长为平衡键长的1.5倍,(3)N-H键长为平衡键长的2.0倍.在这3种情况下,键角∠HNH均为103.2°.在第一种情况,NH2完全为单参考态分子.随着N-H键长逐渐偏离平衡键长,NH2波函数开始显示出多参考态性质.从微扰量能的角度来看,在第一种情况,微扰能量很快就单调收敛至完全组态相互作用(简称FCI)的计算结果.第二种情况,微扰能量稍微有一点振荡,但也很快收敛至FCI的结果.第三种情况对单参考态微扰理论是一种考验,微扰能量振荡比较明显,收敛较慢.在这3种情况下,我们提出的OSPT微扰方法均比文献中报道的ZAPT(the Z-averaged perturbation theory)方法和RMP方法(restricted Mφller-Plesset theory)收敛快.
A series of open-shell molecules was used to test the validity of our recently proposed open shell perturbation theory (J Chem Theory Comput, 2009, 5: 931-936, or simply OSPT). We first discuss the OSPT The perturbation energy is calculated to 400. The basis function is 6-13 G. This base function is chosen for ease of complete configuration interaction calculations (FCI) and for comparison with FCI results.There are 3 geometries: (1) The NH bond length is the equilibrium bond length, (2) the NH bond length is 1.5 times the equilibrium bond length, and (3) the NH bond length is 2.0 times the equilibrium bond length. In these three cases, the bond angle ∠HNH Are both 103.2 °. In the first case, NH2 is completely a single reference state molecule. As the NH bond length gradually shifts away from the equilibrium bond length, the NH2 wave function begins to show a multi-reference state property. In the first case, the perturbation energy quickly monotonically converges to the result of complete configuration interaction (FCI). In the second case, the perturbation energy oscillates a little but quickly converges to FCI Results The third case is a test for the single reference state perturbation theory, the perturbation energy oscillation is obvious and the convergence is slow, Under conditions, OSPT perturbation method we propose ZAPT than reported in the literature (the Z-averaged perturbation theory) method and the RMP method (restricted Mφller-Plesset theory) and fast convergence.