自然定域轨道方法研究

本文报导我们按自然定域轨道原理编制的NLMO程序以及对自然定域轨道计算方法的一些改进。用NLMO程序计算出七个具有不同成键特性分子(包括是否含有孤对或π键,是否含有多中心键等不同情况)的自然定域轨道,结果与用其它定域化方法得出的基本一致。由于自然定域轨道方法基本上具有内禀定域性质,而计算量明显小于其它定域化方法,可以认为它是有效和方便的。     

偶极积分与Boys法分子轨道定域化

定域分子轨道在分子体系的化学图象和物理图象之间充当重要的桥梁作用,它的产生依赖于定域化准则,其中最普遍使用的是Foster-Boys和Edmiston-Ruedenberg(E—R)提出的两种定域化准则。这两种定域化准则是等价的,因而结果也是一致的。但对于E—R定域化来说,由于涉及到大量的多中心积分的计算,计算极为费时,因而远不如Foster-Boys定     

链烃,环状烃和多面体烃的定域分子轨道研究

采用GAUSSAN 80程序对一系列直链烷烃、环状烃和多面体烷烃进行STO-3G基组下的曲initio计算,在所得非定域分子轨道自洽场结果的基础上,利用Boys方法进行定域化,进而研究了这些分子的定域分子轨道的键弯曲性质、轨道能量和集居数与几何结构之间的关系,用上述结果对一些典型分子的稳定性进行了讨论。     

定域分子轨道对称性

提出了利用定域分子轨道重心确定分子轨道的对称性。确定了线型的、非线型的、环状的分子定域分子轨道对称性。结果表明非线型分子和其中双中心键具有相同对称性,但含孤对或三中心键的分子则存在一些差别。还讨论了定域分子轨道的特征。     

有机链状分子的定域分子轨道的研究

利用Foster-Boys定域化程序和STO-3G ab initio方法,对含有C、H、O、N原子的100多个有机链状分子进行了研究,得到定域分子轨道能量及其相互作用参数。应用这些参数和定域分子轨道模型,对于众多的含有C、H、O、N原子的有机链状分子,可得到相应的正则分子轨道能量及其与定域分子轨道的关系。以此预测它们的电离能,结果与实验值符合较好。     

定域分子轨道中的σ—π分离

对含重键的分子体系,分子轨道定域化会涉及到两种完全不同的重键描述,即等价的重键或“香蕉”键和不等价的σ和π键,文献曾利用杂化轨道法对此进行过讨论。对于定域分子轨道,具体得出哪种描述取决于所采用的定域准则及计算中采用的近似方法。对于从头算法的定域化研究,Boys定域准则强烈地趋向于等价重键描述;Ruedenberg定域准则只是对未共轭的重键体系有较强的等价重键描述倾向,对共轭的重键体系,这种倾向性明显减     

定域分子轨道模型——含H、C、N、O有机小分子的研究

应用STO_(-3)G从头算方法和Fostre-Boys定域化分子轨道程序, 我们研究了20多个有机小分子(含H、C、N,O)的定域化分子轨道, 获得了它们的能量和相互作用参数。采用定域分子轨道模型和统一的参数, 对正醇类分子进行了计算, 与光电子能谱数据比较, 得到的电离能的计算结果是相当令人满意的。     

含双键有机小分子的定域分子轨道的研究

选择22种具有双键的有机小分子(含C、H、O和N原子)作模型分子,应用ab initioSTO-3G和Foster-Boys定域化分子轨道程序,对它们的定域分子轨道进行了研究,得到了定域分子轨道能量,确定了相应分子轨道间的相互作用。对烯类分子的光电子能谱进行了分析,结果较为满意。     

虚轨道定域化

通过引入虚轨道定域化函数，扩展了集居数的定域化方法，在从头算STO－3G水平上用护展后的方法计算无机小分子、烷烃、卤代烷、醇、胺及共轭烃等，结果表明，定域占据轨道与定域虚轨道的系数和能级分布相似的规律。     

块定域波函数方法及其应用

提出了块定域波函数方法以定量分析分子内的电子定域现象或分子间的电荷传递效应。对于一个假想的严格定域的分子,我们通过将全部的电子和基轨道配分成几个子空间来构造其相应的波函数。其中每一个分子轨道只对某一个子空间展开,各子空间内的分子轨道相互正交,但不同子空间内的分子轨道间是非正交的。Hartree-Fock波函数和块定域波函数之间的能量之差即为分子内的电子定域能或分子间的电荷传递能。我们应用块定域波函数方法讨论了丁二烯分子中的旋转势垒。     

Determination of extremely localized molecular orbitals in the framework of density functional theory

Extremely localized molecular orbitals are rigorously localized on only a preselected set of atoms and do not have any tails outside the localization region. The importance of these orbitals lies in their ability to be transferred from one molecule to another one. A new algorithm to determine extremely localized molecular orbitals in the framework of the density functional theory method is presented. This could also be a valuable tool in the quantum mechanics/molecular mechanics methodology where localized molecular orbitals are used to describe covalent bonds across the frontier region. The present approach is used to build up the electron density of thymopentin, a polypeptide constituted by five residues, starting from extremely localized molecular orbitals determined on a set of model molecules. The results obtained confirm good transferability properties for these orbitals.Proceedings of the 11th International Congress of Quantum Chemistry satellite meeting in honor of Jean-Louis Rivail     

猪胰岛素的活力位置的电子结构分析

本文采用负因子计数方法对猪胰岛素分子作了完整分子的量子化学计算,其中对矩阵元的计算采用EHMO方法。我们将前线轨道的概念推广为靠近Fermi能级的能量区域中的分子轨道,将局域于少数氨基酸残基上的前线轨道简称为活性轨道。计算结果表明,猪胰岛素70％的活力敏感位置上局域有活性轨道。对全部结果的分析说明猪胰岛素的活性轨道和生物活力之间存在某些内在联系,但不是一一对应关系;猪胰岛素的三级构象的变化可以引起电子结构的变化;相同种类的氨基酸残基在分子的不同位置上可以有不同的活性轨道能级和轨道分布。     

Localization scheme for relativistic spinors

A new method to determine localized complex-valued one-electron functions in the occupied space is presented. The approach allows the calculation of localized orbitals regardless of their structure and of the entries in the spinor coefficient matrix, i.e., one-, two-, and four-component Kramers-restricted or unrestricted one-electron functions with real or complex expansion coefficients. The method is applicable to localization schemes that maximize (or minimize) a functional of the occupied spinors and that use a localization operator for which a matrix representation is available. The approach relies on the approximate joint diagonalization (AJD) of several Hermitian (symmetric) matrices which is utilized in electronic signal processing. The use of AJD in this approach has the advantage that it allows a reformulation of the localization criterion on an iterative 2 × 2 pair rotating basis in an analytical closed form which has not yet been described in the literature for multi-component (complex-valued) spinors. For the one-component case, the approach delivers the same Foster-Boys or Pipek-Mezey localized orbitals that one obtains from standard quantum chemical software, whereas in the multi-component case complex-valued spinors satisfying the selected localization criterion are obtained. These localized spinors allow the formulation of local correlation methods in a multi-component relativistic framework, which was not yet available. As an example, several heavy and super-heavy element systems are calculated using a Kramers-restricted self-consistent field and relativistic two-component pseudopotentials in order to investigate the effect of spin-orbit coupling on localization.     

群分解EHMO计算机程序的编制及GaN晶体能谱的计算

本文把变换算符对原子轨道的作用概括成两个法则,还给出了一个求属于某个高阶不可约表示的基分子轨道的简便方法,即用投影算符求出其中一个分子轨道,然后用这个轨道在变换算符作用下的变换性质来求其余的分子轨道。用这种算法编制EHMO计算机程序在文中也作了扼要的说明。作为程序应用的示例,计算了GaN“分子簇”(包括不同原子数目、掺杂和空位等各种情况)的单电子能级,为实验工作者对GaN(Zn)中什么是施主和什么是受主的推测提供了量子化学的解释。     

硼烷结构规则的量子化学计算

利用EHMO方法,对n=5～12的完整多面体硼烷骨架、缺一个和两个顶点以及戴一个和两个帽原子的各种异构体进行了量子化学计算。直接对硼烷的价成键轨道数公式BMO=4n-[f+3(s+1)]进行了验证。在对计算结果讨论的基础上,进一步探讨了骨架几何结构与成键轨道数之间的关系。     

Bonding in negative ions: the role of d orbitals in the heavy analogues of pyridine and furan radical anions

We have used a potential wall method to investigate the role of d orbitals in the a(2) singly-occupied molecular orbitals of (2)A(2) negative ion states of two molecular series: pyridine, phosphabenzene, arsabenzene, stibabenzene (C(5)H(5)X, X = {N, P, As, Sb}), and furan, thiophene, selenophene, tellurophene (C(4)H(4)X, X = {O, S, Se, Te}). Unlike for the lower lying doubly occupied orbitals, heteroatom d-carbon p in-phase (bonding) interactions in these a(2) orbitals are clearly identified and explain the 0.5 eV stabilization of the (2)A(2) radical anion state in those compounds where the heteroatoms have d orbitals in the valence shell, compared to compounds where d orbitals are missing in the valence shell of the heteroatoms. The performance of both the potential wall approach and the approximate expression of Tozer and De Proft for calculating negative electron affinities has been also investigated, through a comparison with results obtained using electron-transmission spectroscopy experiments.     

Computational developments in generalized valence bond calculations

In this article a procedure for generating starting orbitals for generalized valence bond (GVB) calculations is presented. This is achieved by selecting orbitals which correspond to specific bonds or electron pairs. These orbitals can be identified from the localized molecular orbitals, for both occupied and virtual orbitals, which are obtained through a unitary transformation of the Hartree-Fock canonical molecular orbitals using the Boys's localization method. A scheme has also been implemented which achieves optimum convergence of the pairwise orbital optimization. An object-oriented GVB program is developed which automatically generates reliable initial GVB orbitals, leading to proper and fast convergence. © 1996 by John Wiley & Sons, Inc.     

Tight-binding density functional theory: an approximate Kohn-Sham DFT scheme

The DFTB method is an approximate KS-DFT scheme with an LCAO representation of the KS orbitals, which can be derived within a variational treatment of an approximate KS energy functional. But it may also be related to cellular Wigner-Seitz methods and to the Harris functional. It is an approximate method, but it avoids any empirical parametrization by calculating the Hamiltonian and overlap matrices out of DFT-derived local orbitals (atomic orbitals, AO's). The method includes ab initio concepts in relating the Kohn-Sham orbitals of the atomic configuration to a minimal basis of the localized atomic valence orbitals of the atoms. Consistent with this approximation, the Hamiltonian matrix elements can strictly be restricted to a two-center representation. Taking advantage of the compensation of the so-called "double counting terms" and the nuclear repulsion energy in the DFT total energy expression, the energy may be approximated as a sum of the occupied KS single-particle energies and a repulsive energy, which can be obtained from DFT calculations in properly chosen reference systems. This relates the method to common standard "tight-binding" (TB) schemes, as they are well-known in solid-state physics. This approach defines the density-functional tight-binding (DFTB) method in its original (non-self-consistent) version.     

Topological localized molecular orbitals

Intrinsic and external π-orbital localization procedures which rely only on molecular topology are proposed and discussed. Localized molecular orbitals obtained by application of these procedures are referred to as ‘topological localized molecular orbitals”.     

Performance of the density matrix functional theory in the quantum theory of atoms in molecules

The generalization to arbitrary molecular geometries of the energetic partitioning provided by the atomic virial theorem of the quantum theory of atoms in molecules (QTAIM) leads to an exact and chemically intuitive energy partitioning scheme, the interacting quantum atoms (IQA) approach, that depends on the availability of second-order reduced density matrices (2-RDMs). This work explores the performance of this approach in particular and of the QTAIM in general with approximate 2-RDMs obtained from the density matrix functional theory (DMFT), which rests on the natural expansion (natural orbitals and their corresponding occupation numbers) of the first-order reduced density matrix (1-RDM). A number of these functionals have been implemented in the promolden code and used to perform QTAIM and IQA analyses on several representative molecules and model chemical reactions. Total energies, covalent intra- and interbasin exchange-correlation interactions, as well as localization and delocalization indices have been determined with these functionals from 1-RDMs obtained at different levels of theory. Results are compared to the values computed from the exact 2-RDMs, whenever possible.