Ab Initio MRD CI Research Articles

Ab initio MRD-CI calculations of the electronic structure and hydrogen-bond energy of ClHBr −

Ab initio MRD-CI calculations have been carried out on the ClHBr − ion in order to determine its equilibrium structural parameters and hydrogen-bond energy. The equilibrium Cl-H/H⋯Br distances are predicted to be 2.496/4.304, 2.551/3.915 and 2.564/3.902 bohr by the SCF, MRD-CI and estimated full CI (FCI) calculations respectively. At the highest (FCI) level of the theory a value of 14.0 ± 1.0 kcal mol −1 is obtained for the hydrogen-bond energy in ClHBr −, which is in excellent agreement with the modified experimental value (13.8 kcal mol −1).

Ab initiostudy of NO2

We report ab initio MRD-CI calculations for the two lowest 2A′ states ([Xtilde]2A1 and à 2B2) of the NO2 molecule. A diabatic representation has been obtained by transforming the adiabatic CI wavefunctions in such a way that the resulting states have equal diagonal terms for the dipole moments perpendicular to the bisector of the bond angle. The method is tested for three different cross-sections of the potential-energy surfaces and is found to yield the desired small and constant coupling elements in the diabatic basis, even in the region of the conical intersection itself.

Stability and structure of singly-charged argon clusters Ar n + ,n=3?27. A Monte-Carlo simulation

Monte-Carlo calculations have been performed for positively charged argon clusters in the temperature range between 10 K and 40 K using two different models (one with a dimer ion core, the other one with a trimer ion). The argon-argon interaction potential stems from empirical data, the ion-neutral atoms potential is determined by ab initio MRD-CI calculations. Special stability is found for clusters sizesn=13, 19, 23 and 25/26 atoms using the ‘trimeric core model’ and for those withn=14,n=17,n=20 using the ‘dimeric core model’. The geometrical structure of the clusters is given and the construction principles are discussed in light of the interactions among neutral argon atoms and the ion-neutral atoms interaction.

The GaAs and GaAs + radicals : An ab initio MRD-Cl study

Low-lying electronic states of GaAs and GaAs + are studied by nonrelativistic ab initio MRD-CI methods and compared with results from pseudopotential calculations. The ground state of GaAs is confirmed to be X 3Σ −, with a close-lying 1 3Π state at T e = 0.17 eV. The states 1 1Π, 1 1Σ +, and 1 1Δ lie in the narrow energy region from 0.74 to 0.94 eV, whereas 2 1Σ + is placed ∼-1.0 eV higher. The 1 5Π state, with T e ∼- 1.40 eV, has a shallow potential curve and a small dissociation energy. The 2 3Π state ( T e = 2.19 eV) arises mainly from the same configuration as 1 5Π. The strongly repulsive 1 5Σ − state might cause predissociation of 2 3Π as well as other high-lying states. Assuming for GaAs the same 5 s term value as experimentally observed for Ga, molecular Rydberg series are expected above 4,10 eV. Low-lying states of GaAs show a Ga +As − polarity with dipole moments increasing in the order 1 3Σ − < 1 3Π < 1 1Σ +. In general, the electronic spectra of GaAs and AlP are alike, yet both are quite different from that of BN. Except for the 2Π states, the ordering of the lowest lying states of GaAs + resembles the situation found in BN + and AlP +, all having a 4Σ − ground state. The adiabatic ionization potential (IP) of GaAs from X 3Σ − to X 4Σ −(15 σ → ∞) is calculated to be 6.85 eV, with an estimated error of −0.35 to −0.55 eV. The ionization spectrum into 1 2Π(7 π → ∞) starts at energies about 1.30 eV higher than the adiabatic IP into X 4Σ −. It is expected to show a rather complicated structure due to the existence of satellite lines caused by a strong mixing between 1 2Π and 2 2Π around the vertical ionization region.

MRD CI study of the electron affinity of HO 2 and the photodetachment energy of HO 2−

Extended ab initio MRD CI electronic structure calculations are carried out on the ground state of HO 2 − and on the ground and first excited states of HO 2. Cuts of the potential energy surfaces along the OOH, bending and OOH coordinates are calculated in the neighbourhood of the equilibrium position and employed to compute the equilibrium geometry and fundamental vibrational frequencies of HO 2 −. Systematic calculations on the OO −, OHOH −, O 2O 2 − and HO 2 HO 2 − neutral-anion pairs indicate that the theoretical calculations underestimate the electron affinities by a nearly constant amount. Accordingly the EA (HO 2) is estimated to be 1.069±0.05 eV which is in good agreement with the measured value of 1.078 eV obtained from the HO 2 − photoelectron spectrum. The vertical and adiabatic electron affinities of HO 2 (X 2A″, A 2A′) and electron photodetachment energies of HO 2 − (X 1A′) to the X and A states of the neutral molecule are reported.

Ab initio MRD CI study of the FHBr − ion

Ab initio MRD CI calculations using an extended basis set have been performed on the FHBr − ion in order to determine its electronic structure and hydrogen bond energy. The equilibirum F-H/H...Br distances are predicted to be 1.739/3.344, 1.777/3.073 and 1.786/3.064 bohr by the SCF, MRD CI and estimated full-CI calculations, respectively. The calculated (full-CI estimate) hydrogen bond energy (73.6 kJ mol −1) of FHBr − is in good agreement with the corresponding experimental value (71.1 kJ mol −1).

Ab initio MRD-CI calculations of the ground state spectroscopic constants of HBr

The ground state spectroscopic constants ( D e, r e, B e, α e, w e, w ex e) of HBr have been calculated by the ab initio MRD-CI method using the (13 s 10 p7 d1 f/9 s7 p4 d1 f) basis for Br and (5 s1 p/3 s1 p) basis for H. The exponents of the outermost p, two outermost d and the f functions in the basis set of Br have been optimised at the CI level for Br and Br −. Both the basis sets yield the spectroscopic constants in good agreement with experiment and account for about 90% valence shell correlation energy of HBr.

The hyperfine coupling constants of the five lowest states of CH: An ab initio MRDCI study

The isotropic ( a iso) and the anisotropic ( a aniso) hyperfine coupling constants of the five lowest states of the CH molecule, X 2Π, a 4Σ −, A 2Δ, B 2Σ − and C 2Σ + have been calculated. The agreement with experimental data is excellent for a aniso and within about 8% for a iso. Differences in the values for the various states are discussed at the RHF and CI levels. For X 2Π and A 2Δ the effects of the doubly occupied shells are studied. It turns out that in these states the contributions of the 1σ and 2σ shells to a iso are opposite in sign.

Ab initio CI study of the electronic structure and spectrum of the dibromide ion

The ground-state spectroscopic constants, vertical excitation energies and associated oscillator strengths or lifetimes have been calculated for the dibromide ion, Br 2 − by the ab initio MRD CI method using a double-zeta Gaussian basis augmented with a set of diffuse p functions, together with an s and a set of p bond functions. For the sake of comparison, the ground-state spectroscopic constants of Br 2 have also been calculated. The calculated transition energies of Br 2 − are in good agreement with experimental data.

Stability and structure of singly-charged xenon-argon clusters [Xe1Arn?1]+,n=3?27

Monte-Carlo calculations have been performed for positively charged xenon-argon clusters in the temperature range between 10K and 40K for cluster sizes up ton=27. The argon-argon interaction potential stems from empirical data, the Xe+-Ar potential is determined by ab initio MRD-CI calculations and a semi-empirical treatment of spin-orbit effects. Special stability is found for cluster sizesn=10, 13, 19 and less pronounced forn=23 and 25 fairly independent of the temperature. The geometrical structure of the clusters are given and the construction principle is discussed in light of the interactions among neutral argon atoms and the xenon ion — argon interaction. Comparison with measured mass spectra for mixed rare-gas clusters and [Xen]+ clusters is made and shows a consistent picture for the building principle.

An ab initio CI study of the electronic structure and spectrum of the trichloride ion

The equilibrium geometry of the ground state (X 1Σ g), vertical excitation energies and associated oscillator strengths or lifetimes have been calculated for the trichloride ion, Cl 3 −, by the ab initio MRD CI method using a double-zeta plus polarization basis set. The calculated transition energies are in good agreement with the experimental values.

Characterization of the excited electronic states of the SH radical byab initiomethods

Ab initio MRD-CI calculations have been performed for the SH radical in its low-lying valence states as well as vertically for the first Rydberg terms (2π24s, 2π24p and 2π23d). The lowest 1 4,2Σ-, 1 2Δ and 2 2Σ+ states are found to be of valence character in the vertical region, while a Rydberg structure was suggested by an earlier ab initio study. For the bound X 2Π i and A 2Σ+ various molecular constants such as spectroscopic parameters, dipole moments, Franck-Condon factors and spin-orbit constant are reported. The predissociation of A 2Σ+ in its low-vibrational levels observed by Ramsay is confirmed to be caused by the repulsive 1 4,2Σ- states. Furthermore, a complete reassignment of the electronic structure of the experimentally measured Rydberg levels is given: all these states are found to have a 2π24s (B 2Σ- and C 2Δ), 2π23d σ (E 2Σ-) and 2π2 nd σ, n = 3–6 (D, F, G, H 2Δ) electronic configuration respectively. The E 2Σ- and C 2Δ states are classified here for the first time, while the B 2Σ- has be...

The ab initio calculation of the band origin and vibrational frequencies of the Ã-X̃ system of HNCl using the non-rigid bender hamiltonian

A three-dimensional fit of ab initio MRD CI potential data has been made for the lowest two electronic states of the HNC1 molecule (X̃ 2A″ and à 2A'), and the corresponding vibrational frequencies and rotational energies have been computed using the non-rigid bender Hamiltonian. For the ground state the vibrational frequencies obtained are ν 1 = 2942 cm −1, ν 2 = 1232 cm −1, and ν 3 = 549 cm −1, while the corresponding values for the first excited state are 3524,947 and 836 cm −1 respectively. We calculate T c(à 2A') 16200 cm −1, T o(à 2A') = 16400 cm −1, and the Franck-Condon maximum, Ã(0,3,1)-X̃(0,0.0), is calculate at 19200 cm −1(5200 Å).

Factors involved in the accurate calculation of oscillator strengths: the A 1B 1-X 1A 1 transition of H 2O

The oscillator strength for the A 1B 1-X 1A 1 transition of water is computed by means of ab initio MRD CI calculations employing a variety of AO basis sets. The advantages of carrying out a large CI computation are emphasized in this work, withy f L and f v values of 0.0500 and 0.0576 resulting from the most extensive treatment, as compared to experimental values for these quantities of 0.046 and 0.060 ±0.006.

Ab initioMRD-CI calculations for the propagation step of cationic polymerization of oxetanes based on localized orbitals

In the propagation step of the cationic polymerization, oxetane reacts with the protonated oxetane formed in the initiation step, with concomitant ring opening of the protonated oxetane. To describe properly bond making or bond breaking, it is necessary to use MC-SCF or CI calculations. We have carried out ab initio MODPOT/VRDDO MRD-CI calculations (by the multireference double excitation–configuration interaction technique of Buenker and Peyerimhoff into which we have also meshed a number of desirable computational options for ab initio calculations on large molecules). The CI calculations were carried out on strictly orthogonalized localized occupied and virtual orbitals in the reaction region, with the remainder of the occupied molecular orbitals being folded into an effective CI Hamiltonian. The calculated potential energy surfaces indicate that a preferred pathway for this reaction resembles an SN2 reaction with the oxygen of the oxetane attacking linearly along the C4O direction of the protonated oxetane and inversion of the hydrogens around the C4 atom.

Spin-forbidden transitions in the presence of an intersystem crossing: application to the b 1Σ + state in OH +

A procedure to calculate spin-forbidden radiative transitions in the presence of an intersystem crossing occurring in either the initial or final state has been developed and applied to determine the lifetime of the b 1Σ + state in OH +. Potential curves for many states and dipole moments and dipole transition moments between those as well as spin—orbit matrix elements have been evaluated as a function of OH + separations by employing ab initio MRD CI wavefunctions. It is found that the higher vibronic levels (υ > 5) of the b 1Σ + state are predissociated by the A 3Π state, whereby the lifetime is in the order of 2 × 10 −12 s, i.e. the transition probability for intersystem crossing is around 4 × 10 11 s −1. In the lower levels depopulation occurs via the spin-forbidden b 1Σ + 0−X 3Σ ± 1 mechanism whereby the transition probability is in the order of 10 4 s −1. The corresponding parallel transition is weaker by a factor of roughly 10 3. The probability for the dipole-allowed A 3Π− 3Σ − process is 3 × 10 5 s −1 or τ = 3.3 × 10 −6 s.

The quenching of Na*(3p) in collisions with N2: A classical trajectory study

The quenching reaction Na(2P)+N2(1Σ+g,v,J)→Na(2S) +N2(1Σ+g,v′,J′) has been studied. Scattering calculations have been performed using the surface hopping trajectory method for the two energetically lowest potential energy surfaces of NaN2. The latter have been determined in an ab initio MRD-CI treatment. They exhibit a (avoided) crossing where quenching is likely to occur. Model potentials, which are constructed along the lines given by the ab initio surfaces, are used to investigate the influence of the shape of the potential on the scattering process. Cross sections and final translational energy distributions are compared with experimental data. Alignment and orientation as expressed by the collision induced density matrix have also been considered. Theoretical and experimental results show good agreement. The detailed analysis of the scattering calculations have provided with a better understanding of the quenching process.

AB initio MRD CI potential curves, dipole moments and zero-field splittings for the X 2Π ground states of the CF and CCl molecules

A series of ab initio MRD CI calculations at various levels of theoretical treatment is carried out for the X 2Π ground states of the CF and CCl molecules. The resulting potential energy curves lead to quite good agreement with known spectroscopic constants for these systems and also allow for the accurate computation of the corresponding vibrational wavefunctions. Particular attention is given to the dependence of the electric dipole moments and spin-orbit splittings on the choice of the one-electron basis in the CI calculations. Best agreement with experimental values for these quantities is obtained by employing the natural orbitals of X 2Π states, in which case errors of only 0.1–0.2 D and 2.0–4.0 cm −1 respectively in the computed dipole moments and zero-field splittings result.

Spin-orbit splitting of the A2Π and D2Π states of BeF by ab initio MRD CI calculations

Abstract The electronic structure and energetics of the X 2 Σ + ground state and several excited states of beryllium-monofluoride have been investigated by means of the MRD CI method using a GTO DZP AO basis set. The potential curves are characterized by avoided crossings between bound ionic and repulsive covalent configurations leading to double minimum potentials in case of the second and fourth excited states, B 2 Σ + and D 2 Π. Concerning the first-order spin-orbit splitting of the A 2 Π state, the choice of one-particle functions is found to be essential for a proper description of the highly ionic charge distribution. Using parent natural orbitals the calculated spin-orbit splitting A 0 21.2 cm −1 is in excellent agreement with experiment results.

Photoionization mass spectrometry of gaseous ethene and fluoroethenes in the energy range 280–300 eV

The photoionization mass spectra of the parent cations of ethene, fluoroethene, 1,1′-difluoroethene, cis-difluoroethene, trifluoroethene and tetrafluoroethene have been measured in the energy range 280–300 eV. Below, the C1s ionization limits strong resonances are observed, which are assigned to C(1s) → π * and C(1s) → R transitions. The term values of the core excited valence states are strongly increased compared to those of π → π * transitions. The 1s core hole causes negligible shifts in the Rydberg states. Ab initio MRD CI calculations are used to interpret the experimental results.