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.     

Tunable raman excitation and vibrational relaxation in diatomic molecules

A generalization of the Raman vibrational excitation technique is presented. It is applied here to the room temperature study of V→T transfer in pure N₂ and O₂ or in mixtures with H₂ and He. The results on N₂ are the first obtained at room temperature, those on O₂ are in good agreement with existing data and provide a check of validity of the method.     

Finite electric field valence-shell calculations of molecular hyperpolarizabilities

Using the Finite Electric Field CNDO/II method discussed previously (Chem. Physics Letters 5, 507, (1970)), components of the first hyperpolarizability tensor _ijk are calculated for a series of linear molecules, for H₂O and NH₃ and for methane and its fluorinated derivatives. These are compared, where possible, with results of previous calculations and with experimental results.

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Zusammenfassung Mit Hilfe der zuvor diskutierten CNDO/II-Methode, bei der endliche Werte des störenden elektrischen Feldes Verwendung finden (Chem. Physics Letters 5, 507 (1970)), werden die Komponenten des ersten Hyperpolarisierbarkeitstensors _ijk für eine Reihe von linearen Molekülen, für H₂O, NH₃ und für Methan und seine fluorierten Derivate berechnet. Soweit möglich, werden die Ergebnisse mit früheren Rechnungen und experimentellen Werten verglichen.

     

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.     

Single-determinantal open- and closed-shell FSGO calculations for some diatomic molecules

Open-shell single-determinantal calculations are reported here for the molecular species H₂, Li₂⁺, N₂, O₂ (triplet), O₂^2?;, O₂^?, O₂²⁺, O₂⁺, and F₂; corresponding closed-shell calculations are reported for the species H₂, N₂, O₂ (singlet), O₂^2?, O₂²⁺, and F₂. The floating spherical Gaussian orbital (FSGO ) method was employed. The calculated trend in bond lengths of isonucleic diatomic molecules is in agreement with experiment. For heteronucleic diatomic molecules, however, the experimental trend in bond lengths is not obtained; in this connection, the effect of lone pairs on bond length is discussed. The dissociation energies of H₂ and Li₂⁺ are evaluated. The energy gap between the triplet and singlet states of the oxygen molecule is calculated to be 8.96 eV compared to the experimental value of 4.54 eV.     

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.     

Measuring polarizability anisotropies of rare gas diatomic molecules by laser-induced molecular alignment technique

The polarizability anisotropies of homonuclear rare gas diatomic molecules, Ar(2), Kr(2), and Xe(2), are investigated by utilizing the interaction of the induced electric dipole moment with a nonresonant, nanosecond laser pulse. The degree of alignment, which depends on the depth of the interaction potential created by the intense laser field, is measured, and is found to increase in order of Ar(2), Kr(2), and Xe(2) at the same peak intensity. Compared with a reference I(2) molecule, Ar(2), Kr(2), and Xe(2) are found to have the polarizability anisotropies of 0.45 ± 0.13, 0.72 ± 0.13, and 1.23 ± 0.21 A?(3), respectively, where the uncertainties (one standard deviation) in the polarizability anisotropies are carefully evaluated on the basis of the laser intensity dependence of the degree of alignment. The obtained values are compared with recent theoretical calculations and are found to agree well within the experimental uncertainties.     

Parent-molecule rotational depolarization of photofragment angular momentum distributions: diatomic and polyatomic molecules

We extend the A(q)(k) polarization-parameter model, which describes product angular momentum polarization from one photon photodissociation of polyatomic molecules in the molecular frame [J. Chem. Phys., 2010, 132, 224310], to the case of rotating parent molecules. The depolarization of the A(q)(k) is described by a set of rotational depolarization factors that depend on the angle of rotation of the molecular axis γ. We evaluate these rotational depolarization factors for the case of dissociating diatomic molecules and demonstrate that they are in complete agreement with the results of Kuznetsov and Vasyutinskii [J. Chem. Phys., 2005, 123, 034307] obtained from a fully quantum mechanical approach of the same problem, showing the effective equivalence of the two approaches. We further evaluate the set of rotational depolarization factors for the case of dissociating polyatomic molecules that have three (near) equal moments of inertia, thus extending these calculations to polyatomic systems. This ideal case yields insights for the dissociation of polyatomic molecules of various symmetries when we compare the long lifetime limit with the results obtained for the diatomic case. In particular, in the long lifetime limit the depolarization factors of the A(0)(k) (odd k), Re(A(1)(k)) (even k) and Im(A(1)(k)) (odd k) for diatomic molecules vanish; in contrast, for polyatomic molecules the depolarization factors for the A(0)(k) (odd k) reduce to a value of 1/3, whereas for the Re(A(1)(k)) (even k) and Im(A(1)(k)) (odd k) they reduce to 1/5.     

SCF MO CI perturbation calculations of inner shell and valence shell vertical ionization potentials

A CI method for calculating inner and valence shell vertical ionization potentials is presented. It is based on ab initio SCF MO calculations for the neutral closedshell ground state followed by CI perturbation calculations for the ground and ion states including all spin and symmetry adapted singly and doubly excited configurations with respect to the main configurations of the state of interest. The state energy is computed by performing a CI calculation for a set of selected configurations, and then adding the contributions of the remaining configurations as estimated by second order Brillouin-Wigner perturbation theory. The use of the same set of MO's for all states together with the CI perturbation method makes the method rather rapid. The numerical results are, in spite of the limited Gaussian basis sets used, in good agreement with experiment.     

Performance of dynamically weighted multiconfiguration self-consistent field and spin-orbit coupling calculations of diatomic molecules of Group 14 elements

The efficacy of several multiconfiguration self-consistent field (MCSCF) methods in the subsequent spin-orbit coupling calculations was studied. Three MCSCF schemes to generate molecular orbitals were analyzed: state-specific, state-averaged, and dynamically weighted MCSCF. With Sn(2)(+) as the representative case, we show that the state-specific MCSCF orbitals lead to discontinuities in potential energy curves when avoided crossings of electronic states occur; this problem can be solved using the state-averaged or dynamically weighted MCSCF orbitals. The latter two schemes are found to give similar results when dynamic electron correlation is considered, which we calculated at the level of multiconfigurational quasidegenerate perturbation theory (MCQDPT). We employed the recently developed Douglas-Kroll spin-orbit adapted model core potential, ZFK3-DK3, and the dynamically weighted MCSCF scheme to calculate the spectroscopic constants of the mono-hydrides and compared them to the results obtained using the older set of potentials, MCP-TZP. We also showed that the MCQDPT tends to underestimate the dissociation energies of the hydrides and discussed to what extent coupled-cluster theory can be used to improve results.     

Observed adiabatic corrections to the born-oppenheimer approximation for diatomic molecules with ten valence electrons

Using millimeter-wave (mw) spectroscopy pure rotational transitions were measured with very high precision in several vibrational states for many compounds of the group III/VII and IV/VI diatomic molecules. The spectra were fitted to the usual Dunham expansion adopting the normal mass relations for the Y_lk except for Y₀₁ in order to combine all data of different isotopes for the same compound. For Y₀₁ the atomic mass relation given by Watson is used which introduces phenomenological parameters Δ₀₁^A, Δ₀₁^B for molecule AB taking the adiabatic and nonadiabatic corrections to the Born-Oppenheimer approximation into account. All observed spectra are well described by such a procedure. From these calculations the correction parameters Δ₀₁^A, Δ₀₁^B were obtained with an accuracy of ≈ 10% or better. Using known values of the rotational gJ factor and of the electric dipole moment the nonadiabatic part was calculated and with this result the adiabatic part was evaluated from Δ₀₁ for each atom. The adiabatic correction does not change very much for one specific atom by varying the chemical counterpart and in general it is less than 30% of the total correction for this class of molecules. The only exceptions are InI and the Tl and Pb compounds for which the adiabatic corrections are obtained ten to hundred times larger than those of the other compounds. No explanation is known for this behavior in the published literature.     

Advances in structural analysis of fluoroaluminates using DFT calculations of 27Al electric field gradients

Based on the analysis of 23 aluminum sites from 16 fluoroaluminates, the present work demonstrates the strong potential of combining accurate NMR quadrupolar parameter measurements, density functional theory (DFT)-based calculations of electric field gradients (EFG), and structure optimizations as implemented in the WIEN2k package for the structural and electronic characterizations of crystalline inorganic materials. Structure optimizations are essential for compounds whose structure was refined from usually less accurate powder diffraction data and provide a reliable assignment of the 27Al quadrupolar parameters to the aluminum sites in the studied compounds. The correlation between experimental and calculated EFG tensor elements leads to the proposition of a new value of the 27Al nuclear quadrupole moment Q(27Al) = 1.616 (+/-0.024) x 10(-29) m2. The DFT calculations provide the orientation of the 27Al EFG tensors in the crystal frame. Electron density maps support that the magnitude and orientation of the 27Al EFG tensors in fluoroaluminates mainly result from the asymmetric distribution of the Al 3p orbital valence electrons. In most cases, the definition of relevant radial and angular distortion indices, relying on EFG orientation, allows correlations between these distortions and magnitude and sign of the Vii.     

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.     

The role of Hartree-Fock instability in calculations on electron transitions and polarizability for simple molecules

The stability problem is discussed for the Hartree-Fock solutions for simple molecules; the nonempirical GAUSSIAN-76 program has been supplemented with modules that enable one to monitor the course of the self-consistent calculation by means of stability matrices. The process is illustrated by a calculation on the nitrogen molecule.Translated from Teoreticheskaya i Éksperimental'naya Khimiya, Vol. 21, No. 1, pp. 94–97, January–February, 1985.     

A diagrammatic valence bond method for configuration interaction calculations in atoms and molecules

A diagrammatic valence bond method based on Rumer-Pauling rules for configuration interaction calculations is described. The advantages of this method are that it is simple and flexible and is expected to be computationally efficient as the basis functions can be coded as increasing integers. Evaluation of Hamiltonian matrix elements involves simple bit manipulations and binary searches. The basis, being represented pictorially, should also help in utilizing spatial symmetries for further block-diagonalizing the Hamiltonian matrix. The eigenfunctions of the Hamiltonian can also be used to compute matrix elements between different electronic states.     

Ab initio calculations of the static electric polarizability of infinite polymer chains

The results of recent investigations on the potentialities of the Genkin-Mednis approach for calculating the electric polarizability of infinite stereoregular polymers at the ab initio level are reported. The working expressions are deduced and some of the computation implications are exemplified with results obtained in two study cases: the infinite chain of hydrogen molecules and trarns-polyacetylene.     

Giant enhancement of photodissociation of polar diatomic molecules in electric fields

We explore the photodissociation of polar diatomic molecules in static electric fields in the rotationally cold regime using the example of the LiCs molecule. A giant enhancement of the differential cross section is found for laboratory electric field strengths, and analyzed with varying rovibrational bound states, continuum energies as well as field strengths.