Relativistic Dirac–Fock calculations for closed-shell molecules

A new code has been written to perform relativistic Dirac–Fock self-consistent field (SCF) calculations on closed-shell molecules of any symmetry. The choice of the basis set allows us to work at different levels of approximation depending on the precision required. Calculations on the H₂Po molecule show that accurate results on specific problems like geometry optimization can be obtained by evaluating the two-electron integrals on half the basis spinors. © 1994 by John Wiley & Sons, Inc.     

A.M.O. calculations for some first row diatomic molecules

AMO wavefunctions for LiH, Li₂, HF, F₂ are presented. An explicit formula for computing the energy of a closed shell system composed by doubly occupied MO's and singly filled AMO's is given.     

Resonance Raman scattering amplitude calculations for diatomic molecules

A direct procedure for calculating resonance Raman scattering amplitudes for diatomic molecules is presented and compared with other calculational procedures. An extension of the present procedure is outlined for cases where there is more than one excited intermediate state and these states interact with one another.     

Extended basis NDDO calculations on diatomic molecules

The ab initio NDDO method as described by Roby has been investigated for a range of diatomic species, using symmetric orthonormalization and various basis set sizes. No limiting behaviour is observed with basis set extension.     

Simultaneous ab initio calculations of isoelectronic diatomic molecules

Large gaussian basis sets are employed in simultaneous configuration interaction calculations for the ground states of isoelectronic diatomic molecules. The resulting potential energy curves for three members respectively of four different isoelectronic molecule sequences show the applicability of the method. Comparisons with available results of standard configuration interaction calculations for selected molecules are given. Using our method we often get lower upper bounds for the electronic energy, save computer time and treat physically totally different molecules simultaneously.     

Spectroscopic calculations of electron diffraction parameters of diatomic molecules

Using spectroscopic data, the electron diffraction parameters r_g and l_e for the iodine and oxygen molecules are calculated at various temperatures by numerical diagonalization of the vibrational hamiltonian matrix and by the density matrix approach. The results are discussed and compared with available experimental data. A comparison is also made with the results of second-order perturbation calculations for iodine reported in the literature.     

Interactions in diatomic dimers involving closed-shell metals

Interaction energies of dimers containing alkaline earth (Be, Mg, and Ca) metals have been investigated using symmetry-adapted perturbation theory (SAPT) and supermolecular (SM) methods. Also, to enable broader comparisons, some calculations have been performed on the Zn dimer and on the He-Mg dimer. Although all of the investigated metallic atoms have closed electronic shells, the quasidegeneracy of the ground states of these atoms with the lowest-lying excited states leads to convergence problems in theories based on a single-determinant reference state. The main goal of the present work was to establish how the quality of the interaction energies computed using various electronic-structure methods changes across the range of atoms. We show that although the convergence problems become somewhat less severe with the increase of the atomic number, single-determinant-based methods do not provide reliable interaction energies for any of the investigated metallic dimers even at the level of the coupled-cluster method with single, double, and noniterative triple excitations [CCSD(T)]. However, interaction energies accurate to within a few percent can be obtained if CCSD(T) calculations in large basis sets are extrapolated to the complete basis set limit and followed by full configuration interaction (FCI) calculations with a frozen-core (FC) approximation. Since the systems considered contain only two valence electrons, FCI/FC calculations have been feasible for all of them except for Zn2, providing the best theoretical estimates of the binding energies to date. We found that a large part of the error of the SAPT results originates from limiting some exchange components to terms proportional to the squares of the intermonomer orbital overlap integrals. When the neglected terms were approximately accounted for, the accuracy improved significantly and became comparable to that of CCSD(T), allowing us to obtain for the first time a physical interpretation of the interaction energies in metallic dimers.     

Interactions of 2P atoms with closed-shell diatomic molecules: alternative diabatic representations for the electronic anisotropy

The matrices of electrostatic and spin-orbit Hamiltonians for the system of a 2P atom interacting with a closed shell diatomic molecule in uncoupled, coupled, and complex-valued representations for electronic diabatic basis functions are rederived, and the unitary transformations connecting them are given explicitly. The links to previous derivations are established and existing inconsistencies are identified and eliminated. It is proven that the block-diagonalization of a 6 x 6 matrix of the electronic Hamiltonian is a result of using the basis functions with well-defined properties with respect to time reversal. Consideration of time-reversal symmetry also enforces phase consistency relevant for applications to multisurface reactive scattering and photodetachment spectroscopy calculations, as well as for perspective studies of inelastic effects in cold and ultracold environments. These and further developments are briefly sketched.     

FSGO calculations of octahydrotriborate anion and tetraborane

The FSGO quantum mechanical model is used to makeab initio calculations of the geometrical structures and energies of the ground state configurations of the octahydrotriborate anion, B₃H ₈ ⁻ , and tetraborane, B₄H₁₀. Both molecules are assumed to belong to theC _2v symmetry point group during these computations. Comparisons with available experimental data show good agreement. B₄H₁₀ calculations are also compared with results of SCF calculations.     

Hartree-Fock-Roothaan calculations of dipolar polarizabilities for closed-shell atoms

Methods are suggested for optimization of Slater type atomic orbitals in polarizability calculations of closed-shell atoms using “bound” perturbation theory (algebraic version of the Hartree-Fock method). The size and composition of the basis set of atomic orbitals providing the Hartree-Fock limit for perturbation parameters are considered. Dipolar polarizabilities are calculated for He, Be, Ne, and Mg atoms and their isoelectronic series. Translated fromZhurnal Strukturnoi Khimii, Vol. 41, No. 3, pp. 439-448, May–June, 2000.     

Variable-screening method for diatomic molecules

A variable-screening method is proposed for the calculation of electronic energies of diatomic molecules. This new method is applied to the ground state of HeH⁺ in order to investigate its utility.     

Variable-screening model for diatomic molecules

An effective hamiltonian method based on a one-electron potential is proposed. The potential is represented by a sum of two coulombic interactions with effective nuclear charges depending upon the internuclear distance. This potential preserves the separability of Schrödinger equation. The method can be usefully applied to various atom (ion)-atom collision problems. Calculations are carried out for some states of HeH⁺ and HeH using one configuration built up from a minimal basis set chosen to ensure correct dissociation.     

X-ray spectra and core hole energy curves of some diatomic molecules

Soft X-ray emission spectra of the molecules CO, N₂, NO and O₂ are examined for the purpose of deriving information on their core hole energy curves. Molecular force constant and equilibrium bond lengths are determined for the core hole species C*O and N₂*, and a qualitative analysis is made for CO*, N*O, NO* and O₂*. The results show that differences of equilibrium geometries between the core hole states and the ground states are very well reproduced (better than 1 pm) by SCF calculations within the Hartree-Fock formalism. Inclusion of anharmonicity in the Franck-Condon analysis gives a small but significant effect on the best fitted value for the core hole state bond lengths (about 0.5 pm). Oxygen is binding energies determined from the X-ray spectra are shown to agree with ESCA data, in most cases within a few tenths of an eV. Calculated ΔSCF transition energies reproduce the experimental data within a few eV.     

On the calculation of Rydberg states of some diatomic molecules

The virtual orbital method and the variational method for the calculation of the Rydberg states are compared. The potential curves for the first ⁴Σ^? and ²Σ^? Rydberg states of OH are calculated to be unstable.     

Selected valence electron split-shell molecular orbital calculations on the diatomic interhalogen molecules

Selected valence electron split-shell molecular orbital calculations have been performed on the diatomic interhalogen molecules in order to obtain their binding energies, equilibrium internuclear distances, vibrational force constants, dipole moments and nuclear quadrupole coupling constants. The results are compared with the corresponding closedshell values and with those of some previous semiempirical and nonempirical all valence electron calculations. It is observed that the selected valence electron split-shell molecular orbital method which involves the least amount of computations yields results in better agreement with experiment than other methods.     

Finite electric field valence shell calculations of polarizability gradients and raman depolarization ratios for diatomic molecules

Calculation of polarizability gradients have been made for a number of diatomic molecules using the Finite Field CNDO/II approximate SCF method. Comparison with experimental results suggests that the method will be generally useful for the prediction and interpretation of Raman intensities.