组态相互作用计算中耦合系数的快速确定

提出了组态相互作用 (CI)计算中确定耦合系数和分子积分新的算法 .这些算法明显地提高了CI计算程序的效率 .比较了改进的CI程序与MELD和Gaus sian 94程序包中CI计算的效率 .     

双收缩CI方法及其应用

提出了一种近似的基于空穴-粒子对称的双收缩CI方法. 该方法一般能使组态数减少3个数量级, 计算时间减少一个数量级. 通过一系列比较计算, 表明该方法能够重现非收缩CI计算结果, 并达到了理想的计算精度.     

H_2和I_2反应的赝势法量子化学研究

用赝势价轨道从头算方法并采用双组态行列式波函数,找出了 H_2 与两个碘原子反应形成的直线型过渡态的几何构型。根据这个构型,利用 CIPSI 程序进行了微扰 CI计算,得到了反应活化能的近似值。根据所得到的结果对 H_2 与 I_2 反应的机理进行了探讨,支持了三分子反应的观点。     

H2和I2反应的赝势法量子化学研究

用赝势价轨道从头算方法并采用双组态行列式波函数, 找出了H_2与两个碘原子反应形成的直线型过渡态的几何构型。根据这个构型, 利用CIPSI程序进行了微扰CI计算, 得到了反应活化能的近似值。根据所得到的结果对H_2与I_2反应的机理进行了探讨, 支持了三分子反应的观点。     

建立在群对称轨道(SMO)及特征组态(CD)基础上的线性分子SMO-CDCI方法

定义和讨论了线性分子的群对称轨道(SMO)及特征组态函数(CD),它能够对线性分子的分子轨道与组态函数进行简单而完全的对称分类.SMO－CDCI方法是一种高效的计算方法,大大节省了线性分子CI计算的机时与内存.     

H~3^+的精确量子化学计算

本文采用组态相互作用方法(CI), 对H~3^+的基态和三个低激发态3^A~1,1^A~1和3^Σ~u^+进行了计算研究。结果表明, 键函数方法不失为一个可供选择的精确量子化学计算方法。     

MRCI程序的并行化

设计了基于空穴-粒子对称的并行化组态相互作用程序,并在一个有12个结点的计算机集群上测试了程序的运行.测试表明,程序的并行化基本上达到了预期的结果,8个结点的加速比达到6.33,在12个结点上成功地完成了5.8亿自旋匹配组态的超大规模MRCI计算.     

可调节的收缩CI方法

提出了一个新的收缩CI计算方案———可调节收缩CI方案 ,并完成了相应的程序 .通过实例计算考察了该方案的效率 .一系列计算表明 ,新方案的应用比较灵活 ,相关能损失比较小 .     

运用微机求等价电子j-j偶合的J值

推求等价电子组态 j-j 偶合谱项的难点在于推求组合组态(j)_J~x 的 J 值。我们使用 PASCAL 语言编制了计算程序,在 Apple-Ⅱ、IBM-PC/XT 微机上调试、运行。整个程序由三个子程序完成。为准确求出组合组态(j)_J~x 的 J 值,使用了递归调用技术,使求取 J 值的繁杂工作变得异常简便。该程序的特点是快     

求解弱相互作用分子A-BC的振转能级和波函数的SCF-CI方法

给出了一种计A－BC型弱相互作用分子体系的分子间振转激发态的自洽场-组态相互作用(SCF－CI)方法. 首先使用SCF方法代化径向伸缩振动和弯曲振动的基函数,再用CI方法确定精确的振转激发态能级. 在求解-维的伸缩振动SCF方程时,采用Numerov－Johnson算法在给定区间上求解获得振动波函数的数值解. 具体计算了Ar－HCI和Ar－N₂体系的振转激发态的能级以验证该方法. 结果表明,本方法可用较少的组态就能获得可与其它大计算量的方法具有相比拟的精度的结果.     

Theoretical investigation of the cyclic conformer of ozone

Ab initio SCF and CI calculations are reported which find the cyclic conformer of ozone to lie 16 kcal/mole above the preferred (open-chain) form of this substance. Polarization functions in the AO basis set are found to be quite important in this determination, strongly favoring the cyclic species. The possible experimental significance of such a relatively stable ring conformer of ozone is assessed.     

Cluster expansion of the wave function. Electron correlations in singlet and triplet excited states,ionized states,and electron attached states by SAC and SAC–CI theories

The symmetry-adapted-cluster (SAC ) and SAC –CI theories reported previously have been applied to the study of electron correlations in ground state, singlet and triplet excited states, ionized state, and electron attached state. Formulas for calculations of one-electron properties and transition properties from the SAC and SAC –CI wave functions are given. Calculations were carried out for the ground and Rydberg excited states of water and its positive and negative ions, with the use of the simpler computational scheme than the previous one. The results compare well with experiments.     

Configuration interaction calculations for the N2 molecule and its three lowest dissociation limits

A series of multi-reference double-excitation CI calculations is reported for the N₂ molecule in its equilibrium conformation as well as for its three lowest dissociation limits ⁴S + ⁴S, ⁴S + ²D and ⁴S + ²P respectively. Special emphasis is placed upon satisfying the requirement that the molecular CI energies at large internuclear separations must converge to the same values as are obtained in an analogous treatment for the corresponding atomic limits. In the most extensive CI undertaken the lowest three dissociation limits are calculated to occur at 9.33, 12.08 and 13.39 eV respectively compared to the experimental values of 9.90, 12.29 and 13.48 eV. The advantages of employing bond-centered functions in obtaining the proper balance between the bound molecule and its dissociated atoms are underscored in the calculations.     

Correlated ab initio calculation of electronic g-tensors using a sum over states formulation

An implementation of a sum-over-states (SOS) formulation for the electronic g-tensor is reported and evaluated through test calculations. The calculations include contributions from the relativistic mass correction, the gauge correction and the second-order orbital Zeeman/spin–orbit coupling part. Five different configuration interaction (CI) based methods ranging from single excitation CI to accurate multireference methods were evaluated. The results were compared to experimental data and other high-level calculations.     

The valence orbitals of NH3 by electron momentum spectroscopy: Quantitative comparisons using Hartree-Fock limit and correlated wavefunctions

The complete valence shell binding energy spectra and valence orbital electron momentum distributions for NH₃ have been measured by high-momentum-resolution electron momentum spectroscopy (EMS). The results are quantitatively compared with theoretical calculations using SCF wavefunctions ranging from DZ quality to a newly developed 126-GTO wavefunction essentially at the Hartree-Fock limit. The 3a₁ and to a lesser extent the 2a₁ valence orbital are not adequately described even at the Hartree-Fock limit with basis set saturation including diffuse functions. The differences between theory and experiment are largely resolved by ion-neutral overlap calculations using CI wavefunctions to incorporate the effects of electron correlation. The 126-G (CI) wavefunctions provide accurate calculation of a wide range of electronic properties of NH₃ and also give good quantitative prediction of the three valence orbital momentum distributions as well as a reasonable prediction of the many-body pole strength distribution observed in the (2a₁)⁻¹ inner valence binding energy spectrum. The present EMS results are compared with recent investigations of wavefunction tails by exterior electron distribution calculations and Penning ionization electron spectroscopy measurements reported by Ohno et al.     

Selection of terms for a CI wavefunction to preserve potential surface features

A cumulative selection procedure for choosing configuration functions for inclusion in CI calculations is described. The objective of the method is to obtain equal energy loss, relative to unselected calculations, for different states and different regions of the potential surface. Results obtained from calculations on the BH molecule indicate an overall advantage in comparison to the threshold selection procedure, particularly with regard to molecular geometry changes.     

Accuracy of energy extrapolation in multireference configuration interaction calculations

The accuracy of extrapolation procedures in conjunction with energy-based configuration selection in CI calculations is examined. The normally high accuracy of such extrapolation can deteriorate in multireference CI calculations when configuration functions of low weight are included in the root (reference) set. This is due to the inadequacy of second-order energy contribution estimates for the very large number of discarded low-contribution functions generated as single and double excitations from the minor members of the root set. The problem may be overcome by increasing the number of configurations included in the zero-order function used for the energy contribution estimation process. Illustrative results are presented for excited states of the H₂O molecule and the H₂O⁺ ion.     

Analytic second derivatives for general coupled-cluster and configuration-interaction models

Analytic second derivatives of energy for general coupled-cluster (CC) and configuration-interaction (CI) methods have been implemented using string-based many-body algorithms. Wave functions truncated at an arbitrary excitation level are considered. The presented method is applied to the calculation of CC and CI harmonic frequencies and nuclear magnetic resonance chemical shifts up to the full CI level for some selected systems. The present benchmarks underline the importance of higher excitations in high-accuracy calculations.     

Usefulness of modified virtual orbitals in multireference CI procedure illustrated by calculations on lithium clusters

The maximization of the exchange interaction between the canonical Hartree–Fock virtual and occupied orbitals leads to a transformed set of virtual orbitals which are well suited as one-electron functions for CI calculations. The procedure, generally known for a long time is seldom applied, despite its simplicity and very low computational demand. However, it is found to be particularly useful in the case of multireference CI, since an improved energy is obtained with a considerable shortening of the CI expansion. Moreover, in the final CI wave function, several configurations appear with considerable weight, thus allowing an easy choice of additional configurations to be inserted in the definition of a new zero-order wave function. The efficiency of the computational procedure is discussed for the case of a Li₆ cluster of D_3h symmetry and for the NaCO and PdCO complexes. Results are reported for the relative stability of four different geometrical arrangements of the Li₆ cluster.     

Ground-state correlation energy of Ne

A series of basis sets and configuration interaction (CI ) wave functions, both of which were constructed so as to systematically approach to the complete set limit and the full CI limit, were used for the ground state of Ne. These calculations yielded an estimated correlation energy of ?0.3891 au, which is 99.6% of a recent theoretical estimate of ?0.3905 au. The CI value, ?0.3821 au, was obtained by SDCI calculation with seven reference configurations by using Slater-type orbitals (STO s) from s to h functions. © 1994 John Wiley & Sons, Inc.