All-electron first principles calculations of the ground and some low-lying excited states of BaI

The electronic structure of the heavy diatomic molecule BaI has been examined for the first time by ab initio multiconfigurational configuration interaction (MRCI) and coupled cluster (RCCSD(T)) methods. The effects of special relativity have been taken into account through the second-order Douglas-Kroll-Hess approximation. The construction of Omega(omega,omega) potential energy curves allows for the estimation of "experimental" dissociation energies (De) of the first few excited states by exploiting the accurately known De experimental value of the X2Sigma+ ground state. All states examined are of ionic character with a Mulliken charge transfer of 0.5 e- from Ba to I, and this is reflected to large dipole moments ranging from 6 to 11 D. Despite the inherent difficulties of a heavy system like BaI, our results are encouraging. With the exception of bond distances that on the average are calculated 0.05 A longer than the experimental ones, common spectroscopic parameters are in fair agreement with experiment, whereas De values are on the average 10 kcal/mol smaller.     

Spectroscopy of the ground and low-lying excited states of ThO+

The ThO(+) cation is of interest as it is a useful prototype for experimental and theoretical studies of bonding in a simple actinide compound. Formally the ground state of ThO(+) has the configuration Th(3+)(7s)O(2-), where there is a single unpaired electron associated with a closed-shell Th(4+)-ion core. The first tier of excited states above the X (2)Sigma(+) ground state is expected to be 1 (2)Delta, 1 (2)Pi, and 2 (2)Sigma(+) derived from the Th(3+)(6d)O(2-) configuration. Spectroscopic observations of ThO(+) using the pulsed field ionization-zero kinetic-energy photoelectron technique are reported here. Rotationally resolved spectra were recorded for the X (2)Sigma(+), 1 (2)Delta, and 1 (2)Pi states. Extensive vibrational progressions were observed. Surprisingly, it was found that ionization of ThO decreases the dissociation energy, while increasing the vibrational frequency and decreasing the bond length. Accurate values for the ionization energies of ThO [53 253.8(2) cm(-1)] and Th [50 868.71(8) cm(-1)] were determined as part of this investigation.     

Density functional and multiconfigurational ab initio study of the ground and excited states of Os2

Multiconfigurational ab initio methods predict that the ⁵Π_u state as the ground state instead of the ⁷Δ_u state. Although multiconfigurational perturbation theory correctly predicts the ground state, they overestimate the bond dissociation energy (BDE). Only multireference configuration interaction method can reasonably calculate the BDE. The spin‐orbit effect on the spectroscopic constants is not significant. The results calculated by density functional theory (DFT) vary significantly depending on the selection of a DFT functional. No DFT functional gives the same energy ordering as calculated by the second‐order multiconfigurational perturbation theory (CASPT2). The old generalized gradient approximations functionals are well suited for predicting the ground state and calculating the bond length and the vibrational frequency of Os₂. According to the CASPT2 calculation, the ground state of Os₂ has a quadruple bond. © 2014 Wiley Periodicals, Inc.     

An ab initio study of the ground and low-lying excited states of the permanganate ion

The results of ab-initio self-consistent field calculations for the ground state and configuration interaction calculations for the excited states of the permanganate ion are presented and discussed. The calculations were performed using two large basis sets of contracted gaussian functions, and singly excited configurations were used in the calculations of the excited states. Fair agreement is obtained between these results and the experimental absorption spectra.     

Characterization of ground and low-lying excited states of CoO4: a combined matrix isolation and DFT study

The nature of the reaction products between CoO(2) and molecular O(2), isolated in rare gas matrices, have been investigated using IR absorption spectroscopy. In this paper, we report on the vibrational spectrum of the CoO(4) molecule in its ground and first low-lying excited states. Isotopic substitutions using (16)O(2) and (18)O(2) precursors, as well as (16)O(2) + (18)O(2) and (16)O(2) +( 16)O(18)O + (18)O(2) mixtures in either excess argon or neon, enable demonstration of C(2)(v)() and C(s)() structures for the respective states. CoO(4) is formed following molecular diffusion by complexation of ground-state CoO(2) by an O(2) molecule. The molecule is first formed in the excited state and then spontaneously relaxes to the ground state after remaining in the dark. The kinetics of relaxation can be fitted to a first-order exponential decay with an excited-state lifetime estimated around 23 +/- 2 min in argon and 15 +/- 2 min in neon, indicative of a slow, spin-forbidden process. Population of the excited state is induced by photons around 4250 +/- 250 cm(-1). Experimental results are compared to density functional theory (DFT) calculations at the BPW91/6-311G(3df) level. Electronic and geometrical optimizations were carried out starting from the ground-state precursors (i.e., (3)Sigma(g)(-) for O(2) and (2)Sigma(g)(+) for CoO(2)). Calculations predict a (2)A(2) (C(2)(v)()) ground state and a (4)A' (C(s)()) first excited state 0.37 eV above, close to the 4250 +/- 250 cm(-1) experimental excitation energy. The transition pathway is found to involve two supplementary states with crossed potential energy surfaces (PESs): a (2)B(1) excited state, 0.48 eV above the ground state, reached first through an adiabatic transition with a photon around 4800 cm(-1), and a (4)B(1) transition state into which the system relaxes before finally attaining the (4)A' (C(s)()) excited state. Harmonic frequencies and absolute intensities are also calculated and compared with the experimental data, indicating however that the DFT underestimates the internuclear distances for both configurations. Force and interaction constants were obtained with a semiempirical harmonic force-field potential calculation. They were then used in an empirical rule of plot linking force constants and internuclear distances in order to obtain an estimate of the Co-O bond lengths for each state and are compared to the DFT predictions.     

Quantum Monte Carlo study of the ground state and low-lying excited states of the scandium dimer

A large set of electronic states of scandium dimer has been calculated using high-level theoretical methods such as quantum diffusion Monte Carlo (DMC), complete active space perturbation theory as implemented in GAMESS-US, coupled-cluster singles, doubles, and triples, and density functional theory (DFT). The 3 Sigma u and 5 Sigma u states are calculated to be close in energy in all cases, but whereas DFT predicts the 5 Sigma u state to be the ground state by 0.08 eV, DMC and CASPT2 calculations predict the 3 Sigma u to be more stable by 0.17 and 0.16 eV, respectively. The experimental data available are in agreement with the calculated frequencies and dissociation energies of both states, and therefore we conclude that the correct ground state of scandium dimer is the 3 Sigma u state, which breaks with the assumption of a 5 Sigma u ground state for scandium dimer, believed throughout the past decades.     

On the low-lying excited states of sym-triazine-based herbicides.

We report a joint computational and luminescence study on the low-lying excited states of sym-triazines, namely, 1,3,5-triazine (1) and the ubiquitous herbicides atrazine [6-chloro-N2-ethyl-N4-isopropyl-1,3,5-triazine-2,4-diamine (2)] and ametryn [6-methylthio-N2-ethyl-N4-isopropyl-1,3,5-triazine-2,4-diamine (3)]. Geometrical structures, energetics, and transition and state properties of I and 2 were computed at the TD-DFT, CASSCF, and CASPT2 levels of theory. The fluorescence and phosphorescence emission spectra, lifetimes, and fluorescence quantum yields were measured for the three compounds, and from these, the energies of the lowest excited states and their corresponding radiative rates were determined. The predictions from CASPT2 calculations are in good agreement with the experimental results obtained from the luminescence studies and allow the interpretation of different absorption and emission features.     

Electronic structures of permethyloligosilane radical cations at the ground and low-lying excited states

The electronic structures at the ground and low-lying excited states of permethyloligosilane radical cations, Si_n(CH₃)_2n+2⁺ (n = 4-7), have been investigated using DFT and ab initio calculations. The calculations showed that positive charge (hole) is delocalized along the Si-Si main chain at the ground and first excited states. On the other hand, the hole is transferred to the methyl side-chain at the second and higher excited states. From these results, it was concluded that hole can move along the Si-Si main chain at thermal conditions. Also, it was predicted that intermolecular hole hopping takes place by photo-irradiation to the permethyloligosilane radical cation. The mechanism of hole transfer was discussed on the basis of the results.     

Ab initio study on the ground and low-lying excited states of cesium iodide (CsI)

Potential energy curves (PECs) for the ground and low-lying excited states of the cesium iodide (CsI) molecule have been calculated using the internally contracted multireference configuration interaction calculation with single and double excitation method with the relativistic pseudopotentials. PECs for seven Lambda-S states, X 1Sigma+, 2 1Sigma+, 3Sigma+, 1Pi, and 3Pi are first calculated and then those for 13 Omega states are obtained by diagonalizing the matrix of the electronic Hamiltonian H(el) plus the effective one-electron spin-orbit (SO) Hamiltonian H(SO). Spectroscopic constants for the calculated ground X 0+-state PEC with the Davidson correction are found to agree well with the experiment. Transition dipole moments (TDMs) between X 0 and the other Omega states are also obtained and the TDM between X 0+ and A 0+ is predicted to be the largest and that between X 0+ and B 0+ is the second largest around the equilibrium internuclear distance. The TDMs between X 0+ and the Omega=1 states are estimated to be nonzero, but they are notably small as compared with those between the 0+ states. Finally, vibrational levels of the X 0+ PEC for the two isotopic analogs, (133)CsI and (135)CsI, are numerically obtained to investigate the isotope effect on the vibrational-level shift. It has been found that the maximized available isotope shift is approximately 30 cm(-1) around nu=136.     

Theoretical investigation of the ground and low-lying excited states of nickel carbide, NiC

The electronic structure and bonding of 19 states of the diatomic nickel carbide (NiC) has been studied by multireference methods. Potential energy curves have been constructed for all states, whereas for the three lowest states of symmetries X (1)Sigma(+), a (3)Pi, and A (1)Pi well separated from the rest of the states, special attention was paid through the use of very large basis sets and the calculation of core-valence correlation and scalar relativistic effects. The recommended binding energies for these states are 91, 67, and 54 kcal/mol with respect to the ground state atoms. Our results in general can be considered in fair agreement with the limited experimental findings.     

Characterization of singlet ground and low-lying electronic excited states of phosphaethyne and isophosphaethyne

The singlet ground ((approximate)X(1)Sigma1+) and excited (1Sigma-,1Delta) states of HCP and HPC have been systematically investigated using ab initio molecular electronic structure theory. For the ground state, geometries of the two linear stationary points have been optimized and physical properties have been predicted utilizing restricted self-consistent field theory, coupled cluster theory with single and double excitations (CCSD), CCSD with perturbative triple corrections [CCSD(T)], and CCSD with partial iterative triple excitations (CCSDT-3 and CC3). Physical properties computed for the global minimum ((approximate)X(1)Sigma+HCP) include harmonic vibrational frequencies with the cc-pV5Z CCSD(T) method of omega1=3344 cm(-1), omega2=689 cm(-1), and omega3=1298 cm(-1). Linear HPC, a stationary point of Hessian index 2, is predicted to lie 75.2 kcal mol(-1) above the global minimum HCP. The dissociation energy D0[HCP((approximate)X(1)Sigma+)-->H(2S)+CP(X2Sigma+)] of HCP is predicted to be 119.0 kcal mol(-1), which is very close to the experimental lower limit of 119.1 kcal mol(-1). Eight singlet excited states were examined and their physical properties were determined employing three equation-of-motion coupled cluster methods (EOM-CCSD, EOM-CCSDT-3, and EOM-CC3). Four stationary points were located on the lowest-lying excited state potential energy surface, 1Sigma- -->1A", with excitation energies Te of 101.4 kcal mol(-1) (1A"HCP), 104.6 kcal mol(-1)(1Sigma-HCP), 122.3 kcal mol(-1)(1A" HPC), and 171.6 kcal mol(-1)(1Sigma-HPC) at the cc-pVQZ EOM-CCSDT-3 level of theory. The physical properties of the 1A" state with a predicted bond angle of 129.5 degrees compare well with the experimentally reported first singlet state ((approximate)A1A"). The excitation energy predicted for this excitation is T0=99.4 kcal mol(-1) (34 800 cm(-1),4.31 eV), in essentially perfect agreement with the experimental value of T0=99.3 kcal mol(-1)(34 746 cm(-1),4.308 eV). For the second lowest-lying excited singlet surface, 1Delta-->1A', four stationary points were found with Te values of 111.2 kcal mol(-1) (2(1)A' HCP), 112.4 kcal mol(-1) (1Delta HPC), 125.6 kcal mol(-1)(2(1)A' HCP), and 177.8 kcal mol(-1)(1Delta HPC). The predicted CP bond length and frequencies of the 2(1)A' state with a bond angle of 89.8 degrees (1.707 A, 666 and 979 cm(-1)) compare reasonably well with those for the experimentally reported (approximate)C(1)A' state (1.69 A, 615 and 969 cm(-1)). However, the excitation energy and bond angle do not agree well: theoretical values of 108.7 kcal mol(-1) and 89.8 degrees versus experimental values of 115.1 kcal mol(-1) and 113 degrees. of 115.1 kcal mol(-1) and 113 degrees.     

Ab initio calculation of some low-lying electronic excited states of methane

The energies of some low-lying electronic excited states of methane are calculated by using wave functions built up in terms of plane waves modulated by multicenter Gaussian factors. The wave functions of the various states are evaluated by a two steps iterative process. In the first step, each excited orbital is determined while keeping all other rigid; in the second, rearrangement effects are introduced. Final results are in good agreement with experimental data and allow to enhance an assignement hypothesis for the first electronic transitions.     

CASPT2 study of the decomposition of nitrosomethane and its tautomerization reactions in the ground and low-lying excited states

The dissociation reaction of nitrosomethane into methyl and nitric oxide and the tautomerization reactions to formaldehyde oxime, nitrone, and methoxy nitrene have been studied with the second-order multiconfigurational perturbation theory (CASPT2) by the computation of numerical energy gradients. The prevailing reactions in both the ground and the excited states are dissociations. The structures of the ground and excited states are compared with the corresponding complete active space SCF (CAS-SCF) geometries. It is found that changes in the individual bond lengths are rather large (0.01-0.02 A), while the character and energetics of the CASPT2 optimizations remain similar to the CAS-SCF values.     

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.     

Theoretical study on potential energy curves and spectroscopy properties of ground and low-lying excited electronic states of BrCl~

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 elec-tronic 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 X2 Π 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.     

MRCI potential energy curves and spectroscopic constants of the ground and low-lying excited states of HgCd

The multireference configuration interaction (MRCI) electronic energy calculations with different basis sets have been performed on the ground state (X¹Σ) and three low-lying excited states (³Σ, ¹Π and ³Π) of HgCd dimer. The obtained potential energy curves (PECs) are fit to analytical potential energy functions (APEFs) using the Murrell–Sorbie potential function. Spectroscopic constants are calculated using the APEFs. Based on the PECs, the vibrational levels of each state are predicted. Our equilibrium positions of the X¹Σ state and ³Π state are in excellent agreement with the experimental reports.     

Basis dependence of wave functions of the ground state and low-lying excited states of planar formaldehyde

By the use of three bases containing 3p STO 's possessing different diffuseness, the electronic structure of several excited states (valence- and Rydberg-type) of planar formaldehyde is investigated by limited CI calculations. The spatial extension of the valence-type state is independent of the bases, whereas a diffuse orbital is important in describing the Rydberg-type state. The natural orbital analysis shows that the Rydberg-type states can be described as an outer electron plus an electronic cloud representing the ionized state to which the Rydberg states converge.     

The low-lying electronic excited states of NiCO

Highly correlated coupled cluster methods with single and double excitations (CSSD) and CCSD with perturbative triple excitations were used to predict molecular structures and harmonic vibrational frequencies for the electronic ground state X 1Sigma+, and for the 3Delta, 3Sigma+, 3Phi, 1 3Pi, 2 3Pi, 1Sigma+, 1Delta, and 1Pi excited states of NiCO. The X 1Sigma+ ground state's geometry is for the first time compared with the recently determined experimental structure. The adiabatic excitation energies, vertical excitation energies, and dissociation energies of these excited states are predicted. The importance of pi and sigma bonding for the Ni-C bond is discussed based on the structures of excited states.     

Quantum chemical study of the electronic structure of NiCH2 + in its ground state and low-lying electronic excited states

The electronic structure of NiCH(2) (+), representative of transition metal carbene ions, is investigated by means of several methods of quantum chemistry. The relative stabilities of the four low-lying doublet electronic states ((2)A(1), (2)A(2), (2)B(1), and (2)B(2)) are determined at the coupled cluster singles and doubles level (CCSD) and triples level [CCSD(T) and CCSDT-3] with both a Hartree-Fock and density functional theory (Kohn-Sham) reference. The equation-of-motion coupled cluster for treatment of excited states in singles and doubles approximation (EOM-CCSD) is used to characterize the transition energies from the (2)A(1) electronic ground state to the low-lying doublet excited states. The (2)A(2) and (2)B(1) states are nearly degenerate, found to be separated by 940 cm(-1) at the EOM-CCSD level, in agreement with the CASSCF energy ordering. The (2)B(2) state is calculated to be higher in energy by more than 1.0 eV. The spin purity of the low-lying doublet and quadruplet states described by CCSD calculations based on the unrestricted open-shell Hartree-Fock reference is discussed.