Intra- and interpair electron correlation --Analysis of the G1 and G2 theories

The irrationality of the HLC correction in the G1 and G2 theories has been analyzed concretely and deeply. Meanwhile, how to improve the Post-HF calculation has been pointed out.

Rules on intrapair and interpair correlation energy for Cl, Cl^(-) and MCl(M=H,Li,Na,K)

According to the calculation results of the intrapair and interpair correlation energy for the title systems, it has been found that the intrapair correlation energy of K shell of Cl is almost a constant and both the intrashell and intershell correlation energy of K and L shell changes little. It has also been found that in MCI series compounds the value of Cl correlation energy contribution depends on the ionicity of MCI compounds, i.e., the Cl correlation energy contribution increases with the increase of the ionic bond strength of the compound and this value is always less than the correlation energy of Cl" anion but always larger than that of Cl atom. These rules are helpful for the estimation of the correlation energy of ionic compounds and the energy changes of chemical reactions.

HX体系电子对内、对间相关研究

在6-311+G~*基组水平上用CISD(configuration interaction with singly and doubly excited configurations)方法研究HX(X=Li-F,HBe^+,HBe)体系电子对内、对间的相关能.计算结果表明不同元素形成的HX(X=Li-F,HBe^+,HBe)体系,其价层电子对内、对间相关能的变化较大,它们之间存在着轨道差别,不宜将其相关贡献归为简单的常数.在使用相同理论方法和相同质量基组的前提下,电子数将直接影响到电子对间相关能的大小.对于多电子体系,电子对间相关在总相关中占有优势,若将其忽略会引起较大误差.

NHn（n=1—3）体系电子对内对间相关能的变化规律

众所周知, 电子相关能问题是量子化学的瓶颈问题[1]. 为了更深入地了解和认识电子相关能的轨道本质, 建立Post-HF理论校正模型, Ju等[2～6]应用电子对内对间电子相关模式, 指出国际著名量子化学家Pople等[7]提出的G1和G2理论中的高级校正项E(HLC)的逻辑缺陷. 在以G1和G2理论为基础的G3S理论[8]中, Pople等终于认识到该高级校正项的不合理性, 并舍弃了E(HLC)项而改用为标度能量项, 进一步证实了Ju指出的要建立正确的Post-HF理论校正模型必须有正确的科学思想作为指导[6]. 我们应用电子对内对间电子相关模式对离子化合物体系进行了研究并得出一些有意义的结论[9,10], 现对共价键体系, 运用电子对内对间相关模式和MELD程序[11]中MP2-OPT1[12,13]方法计算并分析了NH, NH2和NH3体系电子对内对间相关能的相对变化规律. 为了比较上述体系与其等电子原子体系电子相关能的差异性, 我们用同样的方法计算了O, F和Ne原子体系的电子对内对间相关能. 此外, 对所研究的氮氢体系及其对应的等电子原子体系还计算了包括三重激发和四重激发高激发项贡献的总电子相关能数值Ec(SDTQ), 并与只包括单重激发和两重激发贡献的总电子相关能数值Ec(SD)作了详细比较.

Simple estimation of electron correlation energy for multi-atomic strong ionic compounds KX and (KX)_2 (X=OH, NC)

On the basis of the calculations and analyses of the intrapair and interpair correlation energy of KX (X = OH, NC) molecules and the results of the transferability of both the innermost intrapair correlation energy and the inner core effect of K and X in KX molecules, we defined and calculated the Kδ- and Xδ-correlation contributions to the total correlation energy of KX molecules. With the comparison of the pair correlation energy of K+, X- and KX systems, we present a simple estimation method to estimate the electron correlation energy of strong ionic compound by summarizing the correlation energy of its constituent ion and ionic group. By using this simple method, the reasonable estimation results of the correlation energy of (KOH)2 and (KNC)2 have been obtained at mp2/6-311++G(d) level with Gaussian98 program, and the deviations are very small. Applying the scheme of 'Separate Large System into Smaller Ones' to the calculation of electron correlation energy of large ionic compounds, it can not only save lot of computation work but also reach the chemical accuracy.

计算基态X(X=Li-Ne,HC，H2C)电子相关能的研究

为深入系统研究电子相关能计算理论，本文报道在6-311＋G’基组水平用CISD（configuration interaction with singly and doubly excited configurations）方法计算基础体系X（X=Li-NeyHC，H2C）对内、对间电子相关能。结果表明不同元素所形成的基态X（X=Li-Ne，HC，H2C）体系，其价层电子对内、对间相关能的变化较大，它们之间存在着轨道差别，不宜将其相关贡献归为简单的常数。以使用相同理论方法和相同质量基组为前提，电子数直接影响电子对间相关能的大小。对于多电子体系，电子对间相关在总相关中占优势，若忽略则会产生较大误差。

Applications of Simple Estimation Scheme of Electron Correlation Energy to Strong Ionic Compounds

The calculation results of electron correlation energies of KF and （KF）2 were reported. The transferability of 1s^2 K , 1s^2 F and the inner core correlation effects of K and F in both K, K^＋, KF and F, F^-, KF systems were investigated respectively. The correlation energy contributions of K and F component to KF system were calculated. By applying the simple estimation scheme to the calculation of the correlation energy of the strong ionic compound KF and （KF）2, it was shown that such a powerful scheme could not only reach the chemical accuracy but also need little computational work.

Study on Pair Correlation Energies of Isoelectronic Systems F^-,HF and H_2F^+

The intrapair and interpair correlation energies of F -, HF and H 2F +systems are calculated and analyzed using MP2 OPT2 method of MELD program with cc PV5Z * basis set. From the analysis of pair correlation energies of these isoelectronic systems, it is found that the 1s F 2 pair correlation energy is transferable in these three isoelectronic systems. According to the definition of pair correlation contribution of one electron pair to a system, the pair correlation contribution values of these three systems are calculated. The correlation contribution values of inner electron pairs and H—F bonding electron pair in HF molecule with those in H 2F +system are compared. The results indicate that the bonding effect of a molecule is one of the important factors to influence electron correlation energy of the system. The comparison of correlation energy contributions including triple and quadruple excitations with those only including singles and doubles calculated with 6 311++G(d) basis set shows that the higher excitation correlation energy contribution gives more than 2% of the total correlation energy for these systems.