Ab initio calculations on low-lying electronic states of PdH

Potential energy curves for the low-lying electronic states of PdH have been calculated using the MRCI method with scalar relativistic and spin-orbit corrections, and all electronic states correlating to the 4d¹⁰ (¹S), 4d⁹ 5s¹ (³D), 4d⁹ 5s¹ (¹D) and 4d⁸ 5s² (³F) states of Pd were included. Potential energy curves for the individual Ω states have been obtained, and the experimentally observed spectra of both PdH and PdD isotopologues have been assigned appropriately based on the ab initio results. Einstein A coefficients were calculated for other possible transitions from the low-lying electronic states to the X²Σ⁺ ground state. Diagonal and off-diagonal matrix elements of the spin-orbit Hamiltonian were calculated for all vibrational levels of the X²Σ⁺, 1²Δ, 1²Π, 2²Σ⁺ and 3²Σ⁺ states, and it was found from the eigenvectors that the vibrational wavefunctions of the 1²Δ_3/2 and 1²Π_3/2 states are mixed significantly in both PdH and PdD isotopologues.     

Calculation of the potential energy curves for the low-lying doublet and quartet states of the CN radical

The potential energy curves of the low-lying X²Σ⁺, A²π_i, B²Σ⁺,⁴Σ⁺, and ⁴π states of CN are calculated by the MC SCF (CAS SCF) method. Their vibrational levels and the molecular constants obtained are in good agreement with those determined in our recent experimental analysis of the CN (B²Σ⁺-X²Σ⁺) emission spectrum. several intensity anomalies in the observed spectrum are ascribed to perturbations between the B²Σ⁺ and ⁴π states with the following vibrational quantum numbers: (υ_B, υ_π)=(9,x), (11, x+2), (12, x+3), (14, x+6), (17, x+11), and (18, x+13), where x = 0 is the most probable assignment. Likewise, the perturbations between the B²Σ⁺ and ⁴Σ⁺ states with (υ_B, υ_Σ) = (11, y), (13, y+3) are interpreted as y = 8±1.     

Ab initio study of the molecular vibrations and electronic structure of the X,B, D and F2Σ+ states of AlO

Molecular vibrations and electronic structure of the X²Σ⁺, B²Σ⁺, D²Σ⁺, and F²Σ⁺ states of AlO are studied by carrying out ab initio configuration interaction calculations and molecular vibration calculations using accurate potential energy functions. An avoided crossing between the D²Σ⁺ and F²Σ⁺ potential energy curves occurs in the neighborhood of 4.0 a₀ and results in irregular vibrational levels of the D and F²Σ⁺ states. The vibrational constants for the F²Σ⁺ state are predicted from the vibrational levels not involved in the irregularity. Configuration mixing is important in describing the B, D, and F²Σ⁺ states. The F²Σ⁺ state at and around its well minimum and the D and F²Σ⁺ states in the avoided crossing region are characterized in terms of their main configurations and dipole moment functions.     

Theoretical spectroscopy and metastability of BeS and its cation

Multiconfiguration self-consistent field and multiconfiguration reference interaction including the Davidson’s correction techniques were employed to calculate the potential energy curves (PECs) of the BeS/BeS⁺ electronic states correlating to the 4/5 lowest dissociation limits. After nuclear motion treatment, we deduced reliable spectroscopic data for the neutral and cationic bound states. For BeS, the transition moments and spin-orbit couplings were also evaluated and used later with the PECs to deduce the rovibronic transition probabilities and the radiative lifetimes in the low-lying states, and to investigate the unimolecular decomposition processes of BeS (X¹Σ⁺, A¹Π, ³Σ⁺ and B¹Σ⁺) leading to Be(¹S_g) + S(³P_g). The prominent mechanism is a spin-orbit induced predissociation via the repulsive BeS(1³Σ⁻) state. Finally, we give the single ionization spectrum of BeS (X¹Σ⁺) populating the BeS⁺ (X²Π, 1²Σ⁻, 1²Σ⁺, 1²Δ, 2²Σ⁺, 2²Π and 3²Π) electronic states. The adiabatic ionisation energy of BeS is estimated to be ∼9.15 eV.     

Ab initio MRD CI calculations on the cesium hydride (CsH) molecule

Ab initio multi-reference configuration interaction (MRD CI) calculations were carried out for the potential energy curves of the first 17 electronic states of the CsH molecule up to large bond distances (20 bohr). The¹Σ⁺ states were also calculated by means of relativistic all-electron SCF and CI using the spin-free no-pair operator with external field projectors. For the low-lying states, the spectroscopic parameters were determined. Dipole moments as well as the transition dipole moments: μ(X ¹ Σ⁺ →A ¹ Σ⁺), μ(X ¹ Σ⁺ →B ¹ Σ⁺), μ(A ¹ Σ⁺ →B ¹ Σ⁺), were also calculated. Non-relativistic and relativistic results are compared. An analysis of the interactions in the^1,3Σ⁺ states is also proposed.     

Electronic states and nature of bonding of the molecule PdSi by all electron ab initio HF-CI calculations and mass spectrometric equilibrium experiments

Six low-lying electronic states of the PdSi molecule have been investigated by performing all electron ab initio Hartree-Fock (HF) and configuration interaction (CI) calculations. The molecule is predicted to have a³∏ ground state and two low-lying excited states,³Σ^? and¹Σ⁺. The electronic structure of the PdSi molecule has been rationalized in a simple molecular orbital diagram. As part of the PdSi molecule the Pd atom essentially retains its (4d)¹⁰ ground term configuration. The chemical bond in the PdSi molecule has been interpreted in terms of donation and back-donation of charge. The bond is polar with charge transfer from the Pd to the Si atom. The dissociation energy of the PdSi molecule has been determined from the mass spectrometric equilibrium data in combination with the theoretical results asD ₀ ^o =257±12 kJ mol^?1.     

Electronic states and nature of bonding of the molecule NiSi by all electron ab initio HF-CI and CASSCF calculations

All electron ab initio Hartree-Fock (HF), configuration interaction (CI) and multiconfiguration self-consistent field (CASSCF) calculations have been applied to investigate the low-lying electronic states of the NiSi molecule. The ground state of the NiSi molecule is predicted to be¹Σ⁺. The chemical bond in the¹Σ⁺ ground state is a double bond composed of one σ and one π bond. The σ bond is due to a delocalized molecular orbital formed by combining the Ni 4s and the Si 3pσ orbitals. The π bond is a partly delocalized valence bond, originating from the coupling of the 3dπ hole on Ni with the 3pπ electron on Si. Withing the energy range 1 eV 18 electronic states have been identified. The lowest lying electronic states have been characterized as having a hole in either the 3dπ or the 3dδ orbital of Ni, and the respective final states are formed when either of these holes are coupled to the 3pπ valence electron of Si.     

AB initio calculations of the dipole moments in low-lying electronic states of the ccn radical

The structures and dipole moments of the four low-lying electronic states (X²Π, A²Δ, B²Σ⁻ and C²Σ⁺) of the linear CCN radical are investigated by ab initio calculations at SDCI/DZP and TZP levels. For all the electronically excited states, the dipole moments are calculated to be ≈ 3.0 D. However, a significantly smaller dipole moment, ≈ 0.6 D, is predicted for the ground state. This result is consistent with the recent experiment by Suzuki, Saito and Hirota, where the MODR signals are observed for the A state CCN but not for the X state. Electronic correlation is important in determining both equilibrium bond lengths and dipole moments.     

A theoretical investigation of the origins of the green and red spectra of Ca2

The potential energy curves and transition moments of the ground state of Ca₂ and ¹Σ⁺_u states correlating with the ¹S + ¹P and ¹S + ¹D calcium atoms have been calculated. The calculations support the assignment of the observed emission spectra of Ca₂ in the red and in the green to transitions between the ground state and the 1,2¹Σ⁺_u states. Predissociation of the 1¹Σ⁺_u state is also shown to be possible from an interaction with the 1³Π_u state.     

Spectroscopic properties and potential energy curves of some low-lying electronic states of AlO,AlO+, LaO,and LaO+: An ab initio CASSCF study

Results of CASSCF state-averaged calculations on the lowest electronic states of LaO and LaO⁺ are reported in this work. For comparison, some low-lying electronic states of AlO and AlO⁺ are also reported. The ground state of LaO was found to be the X²Σ⁺ (R_e = 1.987 Å, ω_e = 794 cm^?1) with a low-lying A²Δ excited state. Five more excited states below 26000 cm^?1 were found. The first ionization potential (IP ) is found at 5.16 eV, resulting in an X¹Σ⁺ ground state for the LaO⁺ diatom, in opposition to AlO⁺ for which an X³ Π ground state has been found. Analysis of the wave functions, dipole moments, and Mulliken populations reveal that the bonding is quite ionic in both systems. © 1994 John Wiley & Sons, Inc.     

The potential energy curves of BH+

Potential energy curves for low-lying states of BH⁺ dissociating to B⁺(¹S) + H, B⁺(³P) + H and B(²P) + H⁺ have been determined by ab initio calculations. Agreement between experimental and calculated values of the spectroscopic constants for the X²Σ⁺and A²Π states supports the theoretical predictions concerning the bound B' ²Σ⁺ state. The 3²∑⁺ and 2²Π states are predicted to be repulsive.     

Theoretical study of electronic structure of rhodium mononitride and interpretation of experimental spectra

Potential energy curves of 22 electronic states of RhN have been calculated by the complete active space second‐order perturbation theory method. The X¹Σ₀⁺ is assigned as the ground state, and the first excited state a³Π₀+ is 978 cm^?1 higher. The ¹Δ(I) and B¹Σ⁺ states are located at 9521 and 13,046 cm^?1 above the ground state, respectively. The B¹Σ⁺ state should be the excited state located 12,300 cm^?1 above the ground state in the experimental study. Moreover, two excited states, C¹Π and b³Σ⁺, are found 14,963 and 15,082 cm^?1 above the X¹Σ⁺ state, respectively. The transition C¹Π₁–X¹Σ₀⁺ may contribute to the experimentally observed bands headed at 15,071 cm^?1. There are two excited states, D¹Δ and E¹Σ⁺, situate at 20,715 and 23,145 cm^?1 above the X¹Σ⁺ state. The visible bands near 20,000 cm^?1 could be generated by the electronic transitions D¹Δ₂–a³Π₁ and E¹Σ⁺₀–X¹Σ⁺₀ because of the spin–orbit coupling effect. © 2013 Wiley Periodicals, Inc.     

Molecular ion LiHe: ab initio study

High level ab initio calculations are performed on the molecular ion LiHe⁺. Potential energy curves for the low-lying singlet and triplet electronic states are calculated using the multi-reference configuration interaction and single-reference coupled cluster methods with large basis sets. The corresponding dipole moments and transition dipole moments functions are also determined. The basic spectroscopic properties and excitation energies of the electronic states are derived from rovibrational bound state calculations.     

On the low-lying states of TiC

The ground and low-lying excited states of TiC are investigated using a CASSCF—externally contracted Cl approach. The calculations yield a ³Σ⁺ ground state, but the ¹Σ⁺ state is only 780 cm^?1 higher and cannot be ruled out. The low-lying states have some triple bond character. The nature of the bonding and origin of the states are discussed.     

Relativistic all electron configuration interaction calculation of ground and excited states of the gold hydride molecule

We present relativistic configuration interaction calculations with the spin-free no-pair hamiltonian on the gold hydride molecule, treating the ground state as well as the eleven lowest excited states. The calculations provide a picture of the bonding in theX ¹Σ⁺ ground state consistent with previous work on this species using four-component spinors: compared to non-relativistic calculations, the dipole moment is reduced by a factor of two, hybridization (and thus participation ofd orbitals at the bonding) is greatly enhanced, the bond length is shortened by 20 pm, and the dissociation energy is increased by 50%. Comparison of the spin-averaged potential curves of the excited states with experiment suggests a reinterpretation of theC ¹Σ⁺ as the 0⁺ fine structure component of 2³Π and the prediction of a weakly bound³Σ⁺ state with weak transitions to the ground state in the range of 2.9–3.1 eV.     

One‐electron pseudopotential study of the alkali hydride cation NaH+: Structure,spectroscopy, transition dipole moments,and radiative lifetimes

The structure and spectroscopic properties of the ground and the lowest excited electronic states of the alkali hydride cation NaH⁺ have been investigated using an ab initio approach. In this approach, a nonempirical pseudopotential for the Na⁺ core has been used and a core–core and a core‐valence correlation corrections have been added. The adiabatic potential energy curves and the molecular spectroscopic constants for numerous electronic states of ²Σ⁺, ²Π, and ²Δ symmetries, dissociating up to Na (4d) + H⁺ and Na⁺ + H (3d), have been calculated. As no experimental data are available, we discuss our results by comparing with the available theoretical calculations. A satisfying agreement has been found for the ground state with previous works. However, a clear disagreement between this study and the model potential work of Magnier (Magnier, J. Phys. Chem. A 2005, 109, 5411) has been observed for several excited states. Numerous avoided crossings between electronic states of ²Σ⁺ and ²Π symmetries have been found and analysed. They are related to the interaction between the potential energy curves and to the charge transfer process between the two ionic systems Na⁺H and NaH⁺. Furthermore, we provide an extensive set of data concerning the transition dipole moments from X²Σ⁺ and the 2²Σ⁺ states to higher excited states of ²Σ⁺ and ²Π symmetries. Finally, the adiabatic potential energy curves of the ground (X²Σ⁺) and the first (2²Σ⁺) excited states and the transition dipole moments between these states are used to evaluate the radiative lifetimes for the vibrational levels of the 2²∑⁺ state for the first time. In addition to the bound–bound contribution, the bound‐free term has been evaluated and added to the total radiative lifetime. © 2012 Wiley Periodicals, Inc.     

Ab initio calculations of doublet states of NH+

The eight low-lying doublet states of the NH⁺ ion are investigated with an ab initio configuration interaction method including all single and double excitations from a multi-reference configuration space (MRSD CI). The spectroscopic constants for the X²Π, A²Σ⁻,B²Δ and C²Σ⁺ states and the transition moments for X²Π-A ²Σ⁻¹ and X²Π-B²Δ are calculated. The results are compared with experiments and other calculations.     

The potential energy curves and probability density distributions of the X1Σ+ and A1Σ+ states of RBH

The potential energy curves PMO—RKR—van der Waals of the electronic A¹Σ⁺ and X₁Σ⁺ states of RbH have been determined. The potentials obtained are self-consistent with the experimental data because they have been tested by direct numerical solution of the radial Schrödinger equation. From exact vibrational eigenfunctions probability density distributions and Franck—Condon factors have been calculated over the range of vibrational levels observed. It is observed that the anomalous behaviour of the A¹Σ⁺ state arises in the υ′ = 1, 2 and 3 levels with probability density functions similar to those of a harmonic oscillator.     

Ab initio potential energy curves and transition dipole moments for the interaction of a ground state He with Na(3s–3p)

Potential energy curves of the states X ²Σ⁺, B (1)²Σ⁺ and A (1)²Π of the NaHe molecule have been calculated accurately in a large range of internuclear distances R from SA-CASSCF-MRCI calculations, using molecular orbitals expanded in cc-pV5Z basis sets. Transition dipole moments have also been calculated for the X–B, X–A and A–B transitions, in the same range of R. Their long-range behaviour have been considered.     

Electronic states of Al2

Ab initio multi-configuration self-consistent field and first-order configuration interaction (FOCI) calculations in an extended basis set have been carried out for the lower energy electronic states of Al₂. The ten core electrons of each Al atom were replaced by an accurate compact effective core potential. The FOCI calculated T_o value for the ³Σ_g^?-³Σ_u^? transition agrees with the experimentally observed emission band to within 90 cm^?1. ³Π_u is calculated to be the electronic ground state of Al₂. Based on FOCI energies and qualitative intensity arguments, the reported optical absorption spectrum of matrix isolated Al₂ also agrees best with a ³Π_u ground state. The ³Σ_g^?1 state is calculated (T_e) at only 324 cm^?1 above the ³Π_u state, and the ¹ΣE_g⁺ state is predicted to lie higher.