Relativistic configuration interaction study of the electronic spectrum of SnTe and SnTe+

Ab initio based relativistic configuration interaction calculations have been performed to study the electronic spectrum of the heaviest tin chalcogenide and its monopositive ion. Potential energy curves and spectroscopic constants of low-lying states of both species within 7 eV are reported. The ground-state dissociation energies of SnTe and SnTe+ are computed to be 3.48 and 2.50 eV, respectively. The spin-orbit splitting between the two components of the X 2Pi state of SnTe+ is about 3030 cm(-1). Effects of the strong spin-orbit coupling on the potential curves and spectroscopic properties of both the species are investigated in detail. The electric dipole moments of some of the low-lying states of SnTe and SnTe+ are reported. Transition moments of some important spin-allowed and spin-forbidden transitions are calculated from the configuration interaction wave functions. The radiative lifetime of the excited E 1sigma0+(+) state of SnTe is about 39 ns. The X2-X1 transition in SnTe+ is found to be more probable than the similar transition in the lighter ions. The vertical ionization energy of SnTe in the ground state is estimated to be 8.22 eV.     

Electronic states and spectroscopic properties of SiTe and SiTe+

Ab initio based configuration interaction calculations have been carried out to study the low-lying electronic states and spectroscopic properties of the heaviest nonradioactive silicon chalcogenide molecule and its monopositive ion. Spectroscopic constants and potential energy curves of states of both SiTe and SiTe+ within 5 eV are reported. The calculated dissociation energies of SiTe and SiTe+ are 4.41 and 3.52 eV, respectively. Effects of the spin-orbit coupling on the electronic spectrum of both the species are studied in detail. The spin-orbit splitting between the two components of the ground state of SiTe+ is estimated to be 1880 cm(-1). Transitions such as 0+ (II)-X1Sigma(+)0+, 0+ (III)-X1Sigma(+)0+, E1Sigma(+)0+ -X1Sigma(+)0+, and A1Pi1-X1Sigma(+)0+ are predicted to be strong in SiTe. The radiative lifetime of the A1Pi state is less than a microsecond. The X(2)2Pi(1/2)-X(1)2Pi(3/2) transition in SiTe+ is allowed due to spin-orbit mixing. However, it is weak in intensity with a partial lifetime for the X2 state of about 108 ms. The electric dipole moments of both SiTe and SiTe+ in their low-lying states are calculated. The vertical ionization energies for the ionization of the ground-state SiTe to different ionic states are also reported.     

Extensive theoretical study on the low-lying electronic states of silicon monofluoride cation including spin-orbit coupling

Ab initio calculations on the low-lying electronic states of SiF+ are performed using the internally contracted multireference configuration interaction method with the Davidson correction and entirely uncontracted aug-cc-pV5Z basis set. The effects of spin-orbit coupling are accounted for by the state interaction approach with the full Breit-Pauli Hamiltonian. The entire 23 Omega states generated from the 12 valence Lambda-S states, which correlate with the first dissociation channel are studied for the first time. Good agreement is found between the calculated results and the available experimental data. The spin-orbit coupling effects on the potential energy curves and spectroscopic properties are studied. Various curve crossings are revealed, which could lead to the predissociation of the a3Pi, A1Pi, and (2)3Sigma+ states and the predissociation pathways are analyzed based upon the calculated spin-orbit matrix elements. The calculated ionization potentials of the ground-state SiF to a few states of SiF+ are in good agreement with the available experimental measurements. Moreover, the transition dipole moments of the dipole-allowed transitions and the transition properties for the A3Pi0+ -X1Sigma+ 0+ and B3Pi1-X1Sigma+ 0+ transitions are predicted, including the Franck-Condon factors and the radiative lifetimes.     

Ab initio MRD-CI study of the spectrum of the TeO molecule employing relativistic effective core potentials

Potential energy curves and properties of the low-lying electronic states of tellurium oxide have been computed using a configuration interaction treatment that includes the spin-orbit coupling interaction. Relativistic effective core potentials (RECPs) are used to describe the inner shells of both the Te and O atoms. Good agreement is obtained for the spectroscopic constants of the X1-X2(3)sigma-, a1delta, and b1sigma+ states for which experimental data are available. The ratio of the parallel and perpendicular b-X transition moments, as well as the radiative lifetime of the b state, was computed, and both results were also found to be in good agreement with measurement. The energetic order of the electronic states in TeO appears to be very similar to that observed for the isovalent O2 molecule, but the Rydberg valence-mixing effects that are so prominent in the latter's spectrum (e.g., for the Schumann-Runge bands) are totally absent in TeO.     

Theoretical characterization of the low-lying electronic states of NbC

We have studied the potential-energy curves and the spectroscopic constants of the ground and low-lying excited states of NbC by employing the complete active space self-consistent field method with relativistic effective core potentials followed by multireference configuration-interaction calculations. We have identified 23 low-lying electronic states of NbC with different spin multiplicities and spatial symmetries within 40,000 cm(-1). At the multireference single and double configuration interaction level of theory the 2sigma+ and 2delta states are nearly degenerated, with the 2delta state located 187 cm(-1) lower than the 2sigma+ state. The estimated spin-orbit splitting for the 2delta state results in a 2delta(3/2) ground state and A 2sigma+ which is placed 650 cm(-1) above the ground state, in reasonable agreement with the experimental result, 831 cm(-1). Our computed spectroscopic constants are in good agreement with experimental values although our results differ from those of a previous density-functional investigation of the excited states of NbC, mainly due to the strong multiconfigurational character of NbC. In the present work we have not only suggested assignments for the observed states but also computed more electronic states that are yet to be observed experimentally.     

Theoretical studies of the electronic spectrum of SiC

Ab initio based multireference singles and doubles configuration interaction calculations have been carried out to study the electronic structure and spectroscopic properties of the SiC⁺ ion. Potential energy curves and spectroscopic constants (r_e, T_e, ω_e) of 14 low-lying doublet and quartet states of the ion are studied. The spin-orbit coupling has been included to see its effects on the spectroscopic properties. Transition probabilities of some quartet–quartet transitions are computed, while the spin-forbidden transitions are very weak. Dipole moments of all low-lying states are estimated by keeping the origin at the center of mass. The vertical and adiabatic ionization energies of SiC are also reported.     

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.     

Spectroscopic properties and potential energy curves of low-lying electronic states of RuC

The RuC molecule has been a challenging species due to the open-shell nature of Ru resulting in a large number of low-lying electronic states. We have carried out state-of-the-art calculations using the complete active space multiconfiguration self-consistent field followed by multireference configuration interaction methods that included up to 18 million configurations, in conjunction with relativistic effects. We have computed 29 low-lying electronic states of RuC with different spin multiplicities and spatial symmetries with energy separations less than 38,000 cm(-1). We find two very closely low-lying electronic states for RuC, viz., 1Sigma+ and 3Delta with the 1Sigma+ being stabilized at higher levels of theory. Our computed spectroscopic constants and dipole moments are in good agreement with experiment although we have reported more electronic states than those that have been observed experimentally. Our computations reveal a strongly bound 1Sigma+ state with a large dipole moment which is most likely the experimentally observed ground state and an energetically close 3Delta state with a smaller dipole moment. Overall our computed spectroscopic constants of the excited states with energy separations less than 18,000 cm(-1) agree quite well with those of the corresponding observed states.     

Extensive theoretical studies on the low-lying electronic states of indium monochloride cation, InCl+

The global potential energy curves for the 14 low-lying doublet and quartet Lambda-S states of InCl+ are calculated at the scalar relativistic MR-CISD+Q (multireference configuration interaction with single and double excitations, and Davidson's correction) level of theory. Spin-orbit coupling is accounted for via the state interaction approach with the full Breit-Pauli Hamiltonian, which leads to 30 Omega states. The computed spectroscopic constants of nine bound Lambda-S states and 17 bound Omega states are in good agreement with the available experimental data. The transition dipole moments and Franck-Condon factors of selected transitions are also calculated, from which the corresponding radiative lifetimes are derived.     

Theoretical study on potential energy curves and spectroscopy properties of ground and low-lying excited electronic states of BrCl<Superscript>+</Superscript>

The calculations on the potential energy curves and spectroscopic constants of the ground and low-lying excited states of BrCl⁺, one of the important molecular ions in environment science, have been performed by using the multireference configuration interaction method at high level of theory in quantum chemistry. Through analyses of the effects of the spin-orbit coupling interaction on the electronic structures and spectroscopic properties, the multiconfiguration characteristic of the X²Π ground state and low-lying excited states was established. The spin-orbit coupling splitting energy of the X²Π ground state was calculated to be 1814 cm⁻¹, close to the experimental value 2070 cm⁻¹. The spin-orbit coupling splitting energy of the ²Π(II) exited state was predicted to be 766 cm⁻¹. The transition dipole moments and Frank-Condon factors of the 3/2(III)-X3/2 and 1/2(III)-1/2(I) transitions were estimated, and the radiative lifetimes of the two transitions were briefly discussed. Supported by the National Basic Research Program of China (Grant No. 2006CB601102) and the National Natural Science Foundations of China (Grant Nos. 20490210 and 20503001)     

Electronic spectrum and photodissociation of ClONO in comparison to BrONO

A theoretical study of the low-lying singlet and triplet states of ClONO is presented. Calculations of excitation energies and oscillator strengths are reported using multireference configuration interaction, MRD-CI, methods with the cc-pVDZ + sp basis set. The calculations predict the dominant transition, 4(1)A' <-- 1(1)A', at 5.70 eV. The transition 2(1)A' <-- 1(1)A', at 4.44 eV, with much lower intensity nicely matches the experimental absorption maximum observed around 290 nm (4.27 eV). The potential energy curves for both states are found to be highly repulsive along the Cl-O coordinate implying that direct and fast dissociation to the Cl + NO2 products will occur. Photodissociation along the N-O coordinate is less likely because of barriers on the order of 0.3 eV for low-lying excited states. A comparison between the calculated electronic energies related to the two dominant excited states of ClONO and BrONO indicates that the transitions lie about 0.6 eV higher if bromine is replaced by chlorine. The stratospheric chemistry implications of ClONO and BrONO are discussed.     

Ab initio configuration interaction study of the low-lying 1Sigma+ electronic states of LiCl

Ab initio configuration interaction calculations have been performed for the X 1Sigma+ and B 1Sigma+ electronic states of LiCl. Potential energy curves, dipole moment functions, and dipole transition moments have been computed for internuclear distances between R = 2.5a0 and 50a0. Single- and double-excitation configuration interaction wave functions were constructed using molecular orbitals obtained from a two-state averaged multiconfiguration self-consistent-field calculation. This procedure yielded an accurate energy splitting between the covalent and ionic separated-atom limits. The calculated avoided crossing of the X and B state curves occurs at R = 16.2a0, in close agreement with previous calculations using a semiempirical covalent-ionic resonance model. X 1Sigma+ state spectroscopic constants are in excellent agreement with experimental values.     

Electronic states and potential energy surfaces of rhodium carbide (RhC)

Potential energy curves and spectroscopic constants of 23 electronic states of the rhodium carbide (RhC) have been studied using the complete-active-space multi-configuration self-consistent field (CASMCSCF) followed by first-order configuration interaction (FOCI) calculations. Multi-reference singles + doubles configuration interaction (MRSDCI) were used to determine the properties of low-lying electronic states. The nature of chemical bond formation in different states is discussed in terms of their wave function and Mulliken populations.     

Time-dependent four-component relativistic density-functional theory for excitation energies. II. The exchange-correlation kernel

We extend our previous formulation of time-dependent four-component relativistic density-functional theory [J. Gao, W. Liu, B. Song, and C. Liu, J. Chem. Phys. 121, 6658 (2004)] by using a noncollinear form for the exchange-correlation kernel. The new formalism can deal with excited states involving moment (spin)-flipped configurations which are otherwise not accessible with ordinary exchange-correlation functionals. As a first application, the global potential-energy curves of 16 low-lying omega omega-coupled electronic states of the AuH molecule have been investigated. The derived spectroscopic parameters, including the adiabatic and vertical excitation energies, equilibrium bond lengths, harmonic and anharmonic vibrational constants, fundamental frequencies, and dissociation energies, are grossly in good agreement with those of ab initio multireference second-order perturbation theory and the available experimental data.     

The electric dipole polarity of the ground and low-lying metastable excited states of NF.

NF (nitrogen monofluoride, fluoroimidogen) is isoelectronic with O2, and, like O2, it has a triplet configuration in the ground state, with two low-lying metastable singlet excited states. The dipole moment of the a 1Delta excited state was measured in 1973 to be 0.37 +/- 0.06 D; at the time its polarity was assumed to be normal (i.e., with the negative charge on the fluorine). However, high-level electronic structure calculations, which reproduce with high accuracy the known spectroscopic constants of the ground and excited states of NF, predict a dipole moment of -0.388 D for a 1Delta NF, indicating that, despite the electronegativities, this molecule carries a positive charge on fluorine. The other singlet state is predicted to have an even larger negative dipole moment; the ground-state triplet should have a very small positive moment. Singlet NF resembles in this respect CO and BF, from the N2 isoelectronic series, both of which also have negative dipole moments.     

Ground and valence excited states of BI: a MR-CISD+Q study

Ab initio calculations on the valence electronic states of the BI molecule have been performed by using the entirely uncontracted all-electronic aug-cc-pVQZ (for the B atom) and Sadlej-pVTZ (for the I atom) basis sets and the internally contracted multireference singles and doubles configuration interaction method with Davidson size-extensively correction and Douglas-Kroll scalar relativistic correction. The potential energy curves of all valence states and the spectroscopic constants of bound states are fitted. It is the first time that the 12 Lambda-S states of BI molecule and all of the 23 Omega states generated from the former are studied in a theoretical way. Calculation results reproduce well most of the experimental data. The effects of the spin-orbit coupling and the avoided crossing rule between Omega states of the same symmetry are analyzed. The transition properties of the A3Pi0+, B3Pi1, and C1Pi1 states to the ground-state transitions are predicted, including the transition dipole moments, the Franck-Condon factors, and the radiative lifetimes. The radiative lifetime of the C1Pi1 state of BI molecule is less than 1 micros, while that of the A3Pi0+ and B3Pi1 states are the order of 1 ms.     

The electronic structure of transition metal dihelide dications

Multi-reference configuration interaction (MRCI) calculations have been employed to characterize the low-lying states of first-row transition metal dihelide dications, He(2)TM(2+) (TM = Sc-Cu). The most important state-ordering principles were determined to be the occupation of the 4s orbital and orientation of the occupied 3d orbital. The ground states of all species are predicted to be of D(infinityh) symmetry arising from a 3d(n+1) electronic configuration. For excited states with singly occupied 4s or doubly occupied 3d(sigma) orbitals, bending to C(2v) symmetry typically lowers the energy and shortens the He-TM bond length. Coupled cluster singles and doubles with a perturbative treatment of triple excitations (CCSD(T)) results for ground state spectroscopic properties are in agreement with the MRCI predicted trends.     

Multireference configuration interaction study of the electronic states of ZrC

The potential energy curves and spectroscopic constants of the ground and 32 low-lying electronic states of ZrC have been studied by employing multireference configuration interaction methods, in conjunction with relativistic effective core potentials and 5s3p3d1f, 3s3p1d basis sets con Zr and C, respectively. We have determined that the ground state is (3)Sigma(+). However there are two low-lying (1)Sigma(+) states (below 5000 cm(-1)) which strongly interact resulting in avoided crossings. The lowest (1)Sigma(+) state corresponds to a combination of 1sigma(2) Xsigma(2) 1pi(4) configurations whereas the second is an open shell singlet 1sigma(2) 2sigma(1) 3sigma(1) 1pi(4). Several avoided crossings were observed, for (1)Pi, (3)Pi, (1)Delta, (3)Sigma(+), and (3)Delta states. We have identified (3)Pi and (1)Pi lying at 4367 and 5797 cm(-1), respectively. The results are in good agreement with the recent experimental findings of Rixon et al. [J. Mol. Spectrosc. 228, 554 (2004)], and indicate that the (3)Pi-(3)Sigma(+), and (1)Pi-(1)Sigma(+), bands located between 16 000-19 000 cm(-1) are extremely complex due to near degeneracy of several (1)Pi and (3)Pi states. We also have identified a (1)Sigma(+) state in the same region that may interfere with the (1)Pi emission bands. The present results not only shed further light into the spectra of ZrC but also predict yet to be observed systems.     

Excited electronic states, transition probabilities, and radiative lifetimes of CAs: a theoretical contribution and challenge to experimentalists

High-level CASSCF/MRCI calculations with a quintuple-ζ quality basis set are reported by characterizing for the first time a manifold of electronic states of the CAs radical yet to be investigated experimentally. Along with the potential energy curves and the associated spectroscopic constants, the dipole moment functions for selected electronic states as well as the transition dipole moment functions for the most relevant electronic transitions are also presented. Estimates of radiative transition probabilities and lifetimes complement this investigation, which also assesses the effect of spin-orbit interaction on the A (2)Π state. Whenever pertinent, comparisons of similarities and differences with the isovalent CN and CP radicals are made.