Ab Initio Coupled-cluster Research Articles

Excited Electronic States of Sr2: Ab Initio Predictions and Experimental Observation of the 21Σu+ State.

Despite its apparently simple nature with four valence electrons, the strontium dimer constitutes a challenge for modern electronic structure theory. Here we focus on excited electronic states of Sr2, which we investigate theoretically up to 25000 cm-1 above the ground state, to guide and explain new spectroscopic measurements. In particular, we focus on potential energy curves for the 11Σu+, 21Σu+, 11Πu, 21Πu, and 11Δu states computed using several variants of ab initio coupled-cluster and configuration-interaction methods to benchmark them. In addition, a new experimental study of the excited 21Σu+ state using polarization labeling spectroscopy is presented, which extends knowledge of this state to high vibrational levels, where perturbation by higher electronic states is observed. The available experimental observations are compared with the theoretical predictions and help to assess the accuracy and limitations of employed theoretical models. The present results pave the way for future more accurate theoretical and experimental spectroscopic studies.

Ground and Electronically Excited States of Main-Group-Metal-Doped B20 Double Rings.

Ab initio coupled-cluster, electron propagator, and Møller-Plesset second-order perturbation theory calculations are utilized to analyze the low-lying electronic states of several metal-doped B20. In the ground state, the presently focused AB20/EB20 (A = Li, Na, and K; E = Mg and Ca) consist of charge-separated A+B20-/E2+B202- frameworks. The excited electronic states of AB20 and EB20+ were analyzed by computing the vertical electron attachment energies (VEAEs) of AB20+ and EB202+. In several excited states, the radical electron is predominantly localized on the B20 frames, which are counterparts of the low-lying states of bare B20-. A variety of basis sets were tested on obtaining VEAEs, and the aug-cc-pVDZ/A,E d-aug-cc-pVDZ/B combination provided the best accuracy-efficiency compromise on them. Furthermore, this work analyzes the Rydberg-like excited states of AB20 and EB20+ and will serve as a guide for future studies on similar metal-doped boron systems.

Photoelectron Spectroscopy of Biacetyl and Its Cluster Anions.

Photoelectron spectroscopy of the biacetyl (dimethylglyoxal) anion reveals the properties of the ground singlet and lowest triplet electronic states of the neutral biacetyl (BA) molecule. Due to the broad and congested nature of the singlet transition, which peaks at a vertical detachment energy VDE = 1.12(5) eV, only an upper bound of the adiabatic electron affinity of BA could be determined: EA(BA) < 0.7 eV. A narrower and more structured triplet band peaking at VDE = 3.17(2) eV reveals the adiabatic electron binding energy of the triplet to be 3.05(2) eV. These results are in good agreement with ab initio (coupled-cluster) calculations. The lowest-energy structures of the anion, singlet, and triplet states of biacetyl are characterized by different orientations of the methyl groups within the molecular frame. In the ground singlet state of neutral BA, the methyl torsion is offset by ∼60° compared to that of the anion, while in the triplet the methyl orientation is similar to that of the anion. Photoelectron spectra of the cluster anions reveal that the intermolecular interactions in the homogeneously solvated (BA) n- clusters are significantly stronger than the interactions of BA- with N2O or even of BA- with H2O. To account for these observations, π-π bonded structures of the dimer and trimer anions of biacetyl are proposed based on density-functional theory calculations. The analysis of the proposed structures indicates that the negative charge in the (BA) n- cluster anions, at least in the dimer and the trimer, is significantly delocalized between all BA moieties present and there is a significant degree of covalent bonding within the cluster.

Exploration of large amplitude motions in the Ca+Ar2 complex

ABSTRACTWe present a theoretical study of the ground electronic state potential of the Ca+Ar2 complex and of its photoabsorption spectra, simulated at temperatures ranging between 20 and 220 K. These calculations exploit a Monte-Carlo (MC) method, based on a one-electron pseudo-potential approach. A pairwise additive potential fitted to coupled cluster ab initio points, is used to model the Ca+Ar2 complex. Our study shows that the most stable form of Ca+Ar2 is a bent C2v structure, whereas the linear isomer is located at around 90 ± 10 cm−1 above in energy. The analysis of the photoabsorption spectra establishes that a structural transition from bent Ca+Ar2 to linear ArCa+Ar occurs at T∼100 K. Trends in binding energies of both isomers, bond lengths and bond angles are also discussed. Molecular orbital overlaps provide an explanation for the order of stability between the bent and linear structures.

Large amplitude vibrations of acetyl isocyanate, methyl cyanoformate, and acetyl cyanate.

The far infrared region of three detectable molecules sharing the empirical formula C3H3O2N, acetyl isocyanate CH3CONCO (AISO), methyl cyanoformate NC-COOCH3 (MCN) and acetyl cyanate CH3COOCN (ACN), is explored using explicitly correlated coupled cluster ab initio methods and a variational procedure designed for non-rigid species and large amplitude motions. The three isomeric forms display two conformers, cis and trans, of Cs symmetry that intertransform through the torsion of the central bond. This internal rotation interacts with the methyl group torsion generating a ground electronic state potential energy surface of six minima. Accurate rotational constants, centrifugal distortion constants, potential energy barriers, and surfaces, as well as, the low energy levels and their splittings, are provided. Far infrared energies are calculated up to 600 cm-1 which represent excitations of the torsional and the skeletal bending modes. Below 410 cm-1, 28, 14 and 20 vibrational energy levels and their splittings have been identified and classified for acetyl isocyanate, methyl cyanoformate, and acetyl cyanate, respectively. All the methyl torsion barriers are relatively low (∼300 cm-1) generating relevant tunneling effects. Computed spectroscopic parameters can help further interpretation and assignments of experimental rotational spectra using effective Hamiltonians.

Full-dimensional quantum dynamics of rovibrationally inelastic scattering between CN and H2.

We report six-dimensional (6D) potential energy surface (PES) and rovibrational scattering calculations for the CN-H2 collision system. The PES was computed using the high-level ab initio spin-restricted coupled-cluster with single, double, and perturbative triple excitations-F12B method and fitted to an analytic function using an invariant polynomial method in 6D. Quantum close-coupling calculations are reported for rotational transitions in CN by H2 and D2 collisions in 6D as well as four-dimensional (4D) within a rigid rotor model for collision energies of 1.0-1500 cm-1. Comparisons with experimental data and previous 4D calculations are presented for CN rotational levels j1 = 4 and 11. For the first time, rovibrational quenching cross sections and rate coefficients of CN (v1 = 1,j1 = 0) in collisions with para- and ortho-H2 are also reported in full-dimension. Agreement for pure rotational transitions is found to be good, but no experimental data on rovibrational collisional quenching for CN-H2 are available. Applications of the current rotational and rovibrational rate coefficients in astrophysical modeling are briefly discussed.

Opensd-shell nuclei from first principles

We extend the ab initio coupled-cluster effective interaction (CCEI) method to deformed open-shell nuclei with protons and neutrons in the valence space, and compute binding energies and excited states of isotopes of neon and magnesium. We employ a nucleon-nucleon and three-nucleon interaction from chiral effective field theory evolved to a lower cutoff via a similarity renormalization group transformation. We find good agreement with experiment for binding energies and spectra, while charge radii of neon isotopes are underestimated. For the deformed nuclei $^{20}$Ne and $^{24}$Mg we reproduce rotational bands and electric quadrupole transitions within uncertainties estimated from an effective field theory for deformed nuclei, thereby demonstrating that collective phenomena in $sd$-shell nuclei emerge from complex ab initio calculations.

Ab initio coupled-cluster and multi-reference configuration interaction studies of the low-lying electronic states of 1,2,3,4-cyclobutanetetraone

ABSTRACTThe four, closely spaced, lowest energy electronic states of the challenging, D4h-symmetric, 1,2,3,4-cyclobutanetetraone (C4O4) molecule have been investigated using high-level ab initio methods. The calculated states include the closed-shell singlet 8π(1A1g) state, the singlet 10π(1A1g) state, in which the π-type lowest unoccupied molecular orbital (LUMO) of the 8π(1A1g) reference is doubly occupied and the σ-type highest occupied molecular orbital (HOMO) is empty, and the open-shell singlet and triplet states, designated as 9π(1B2u) and 9π(3B2u), respectively, originating from single occupancy of the HOMO and LUMO. Our focus is on single-reference coupled-cluster (CC) approaches capable of handling electronic near-degeneracies in diradicals, especially the completely renormalised CR-CC(2,3) and active-space CCSDt methods, along with their CCSD and EOMCCSD counterparts. The internally contracted multi-reference configuration interaction calculations with a quasi-degenerate Davidson correction are performed as well. Our computations demonstrate that the state ordering is 9π(3B2u) < 8π(1A1g) < 9π(1B2u) < 10π(1A1g) and that the 8π(1A1g) − 9π(3B2u) gap is in the 7–11 kJ/mol range, in reasonable agreement with the negative ion photoelectron spectroscopy measurements, which give 6.27 ± 0.5 kJ/mol. In addition to the theory level used, geometry relaxation and basis set play a significant role in determining the state ordering and energy spacings. In particular, it is unsafe to use lower level, non-CC geometries and smaller basis sets.

Theoretical study on ThF+, a prospective system in search of time-reversal violation

The low-lying electronic states of ThF+, a possible candidate in the search for - and -violation, have been studied using high-level correlated relativistic ab initio multi-reference coupled-cluster and configuration interaction approaches. For the state component with Ω = 1 (electron electric dipole moment ‘science state’) we obtain an effective electric field of , a - and -odd electron–nucleon interaction constant of kHz, a magnetic hyperfine interaction constant of MHz for 229Th (), and a very large molecular dipole moment of 4.03 D. The Ω = 1 state is found to be more than 300 cm−1 lower in energy than (), challenging the state assignment from an earlier theoretical study on this species (Barker et al 2012 J. Chem. Phys. 136 104305).

Vibrationally resolved photoelectron spectroscopy of electronic excited states of DNA bases: application to the ã state of thymine cation.

For fully understanding the light-molecule interaction dynamics at short time scales, recent theoretical and experimental studies proved the importance of accurate characterizations not only of the ground (D0) but also of the electronic excited states (e.g., D1) of molecules. While ground state investigations are currently straightforward, those of electronic excited states are not. Here, we characterized the à electronic state of ionic thymine (T(+)) DNA base using explicitly correlated coupled cluster ab initio methods and state-of-the-art synchrotron-based electron/ion coincidence techniques. The experimental spectrum is composed of rich and long vibrational progressions corresponding to the population of the low frequency modes of T(+)(Ã). This work challenges previous numerous works carried out on DNA bases using common synchrotron and VUV-based photoelectron spectroscopies. We provide hence a powerful theoretical and experimental framework to study the electronic structure of ionized DNA bases that could be generalized to other medium-sized biologically relevant systems.

Challenges in predicting ΔrxnG in solution: the mechanism of ether-catalyzed hydroboration of alkenes.

Ab initio (coupled-cluster and density-functional) calculations of Gibbs reaction energies in solution, with new entropy-of-solvation damping terms, were performed for the ether-catalyzed hydroboration of alkenes. The goal was to test the accuracy of continuum-solvation models for reactions of neutral species in nonaqueous solvents, and the hope was to achieve an accuracy sufficient to address the mechanism in the "Pasto case": B2H6 + alkene in THF solvent. Brown's SN2/SN1 "dissociative" mechanism, of SN2 formation of borane-ether adducts followed by SN1 alkene attack, was at odds with Pasto's original SN2/SN2 hypothesis, and while Brown could prove his mechanism for a variety of cases, he could not perform the experimental test with THF adducts in THF solvent, where the higher THF concentrations might favor an SN2 second step. Two diboranes were tested: B2H6, used by Pasto, and (9BBN)2 (9BBN = 9-borabicyclo[3.3.1]nonane, C8H15B), used by Brown. The new entropy terms resulted in improved accuracy vs traditional techniques (∼2 kcal mol(-1)), but this accuracy was not sufficient to resolve the mechanism in the Pasto case.

Spin-rotation and NMR shielding constants in XF molecules (X = B, Al, Ga, In, and Tl).

Accurate spin-rotation and absolute shielding constants in a series of XF molecules (X = (11)B, (27)Al, (69)Ga, (115)In, and (205)Tl) determined using high-level ab initio coupled-cluster and four-component relativistic density-functional theory (DFT) calculations are presented. The accuracy of the results is established by comparing the relativistically and vibrationally corrected calculated values with available experimental data; for spin-rotation and shielding constants for which no experimental data exist, we provide new and reliable values. For both properties, our results can be considered as reference values against which more approximate quantum-chemical methods can be benchmarked.

Excess Electrons Bound to Molecular Systems with a Vanishing Dipole but Large Molecular Quadrupole

Electron attachment properties of covalent molecules and ion clusters with vanishing dipole moments but large quadrupoles are studied with coupled cluster ab initio methods. Selection of the molecules studied is driven by two goals, finding a paradigm quadrupole-bound anion and investigating whether there is a correlation between the magnitude of the molecular quadrupole and the vertical attachment energy. Out of all examined species, only the ion clusters and four of the covalent molecules are found to support bound anions. The shapes and spatial extents of the associated excess electron distributions are qualitatively and quantitatively characterized, respectively. Two of the four covalent systems are especially promising as paradigm systems because of advantageous trade-offs regarding the number of isomers and conformers as well as synthetic closeness to commercial sources. No correlation was found between the vertical attachment energy and molecular quadrupole in an analysis that included the newly identified bound anions, those molecules, which were found not to support bound anions, and succinonitrile, which had been studied before. Moreover, there is clearly no such thing as a "critical quadrupole moment". There are, however, very strong electron correlation effects involved in the binding of the excess electrons, and similar to succinonitrile, for five out of six anions identified here, the molecular quadrupole of the neutral itself is too weak to bind an excess electron, and electron correlation in the form of dynamic polarization is required to do so.

Ab Initio Coupled-Cluster Effective Interactions for the Shell Model: Application to Neutron-Rich Oxygen and Carbon Isotopes

Ab Initio Coupled-Cluster Effective Interactions for the Shell Model: Application to Neutron-Rich Oxygen and Carbon Isotopes

Criegee Intermediates React with Ozone

We have investigated the reaction of the one-carbon stabilized Criegee intermediate (H2COO, formaldehyde oxide) with ozone, theoretically, using high level coupled cluster ab initio methods. Key to the reactivity of the Criegee intermediate with ozone is the strongly exothermic formation of an intermediate consisting of five oxygen and one carbon atoms (H2CO5) in a six-membered ring structure. This intermediate proceeds via a spin-allowed route over two transition states with low energy barriers to form molecular oxygen and formaldehyde. The reaction may contribute to the loss of these biradicals in the atmosphere.

CO Dimer: New Potential Energy Surface and Rovibrational Calculations

The spectrum of CO dimer was investigated by solving the rovibrational Schrödinger equation on a new potential energy surface constructed from coupled-cluster ab initio points. The Schrödinger equation was solved with a Lanczos algorithm. Several 4D (rigid monomer) global ab initio potential energy surfaces (PESs) were made using a previously reported interpolating moving least-squares (IMLS) fitting procedure specialized to describe the interaction of two linear fragments. The potential has two nonpolar minima giving rise to a complicated set of energy level stacks, which are very sensitive to the shapes and relative depths of the two wells. Although the CO dimer has defied previous attempts at an accurate purely ab initio description our best surface yields results in good agreement with experiment. Root-mean-square (rms) fitting errors of less than 0.1 cm(-1) were obtained for each of the fits using 2226 ab initio data at different levels. This allowed direct assessment of the quality of various levels of ab initio theory for prediction of spectra. Our tests indicate that standard CCSD(T) is slow to converge the interaction energy even when sextuple zeta bases as large as ACV6Z are used. The explicitly correlated CCSD(T)-F12b method was found to recover significantly more correlation energy (from singles and doubles) at the CBS limit. Correlation of the core-electrons was found to be important for this system. The best PES was obtained by extrapolation of calculations at the CCSD(T)(AE)-F12b/CVnZ-F12 (n = 3,4) levels. The calculated energy levels were compared to 105 J ≤ 10 levels from experiment. The rms error for 68 levels with J ≤ 6 is only 0.29 cm(-1). The calculated energy levels were assigned stack labels using several tools. New stacks were found. One of them, stack y1, has an energy lower than many previously known stacks and may be observable.

Ab initio determination of the ionization potentials of water clusters (H2O)n (n = 2−6)

High-level quantum-chemical ab initio coupled-cluster and multiconfigurational perturbation methods have been used to compute the vertical and adiabatic ionization potentials of several water clusters: dimer, trimer, tetramer, pentamer, hexamer book, hexamer ring, hexamer cage, and hexamer prism. The present results establish reference values at a level not reported before for these systems, calibrating different computational strategies and helping to discard less reliable theoretical and experimental data. The systematic study with the increasing size of the water cluster allows obtaining some clues on the structure and reductive properties of liquid water.

Valence photoionization and photoelectron–photoion coincidence (PEPICO) study of molecular LiCl and Li2Cl2

Molecular LiCl and Li2Cl2 have been studied in the vapor phase with valence photoelectron and photoelectron–photoion coincidence spectroscopies. These two techniques determine the binding energies in fundamentally different ways. Binding energies obtained from photoelectron spectra are usually taken as the vertical ionization energies of the corresponding electronic states. In cases with several overlapping bands, corresponding to different electronic states, the coincidence measurement can separate the bands if the respective final states fragment differently. This applies well to the monomer case. To facilitate the determination of state-specific ionization energies in the dimeric molecule, a theoretical Franck–Condon analysis has been carried out. Moreover, ab initio coupled-cluster and density-functional-theory calculations have been used to analyze the fragmentation pattern based on asymptotic dissociation energies. The fragmentation pattern is largely common to all the accessible valence-ionized states of the dimer, consistent with rapid conversion to the ionic ground state before fragmentation. However, the highest-lying state of Li2Cl2+, 2Ag, shows enhanced propensity for Li+ as dissociation product.

Electronic Absorption Spectra of Protonated Pyrene and Coronene in Neon Matrixes

Protonated pyrene and coronene have been isolated in 6 K neon matrixes. The cations were produced in the reaction of the parent aromatics with protonated ethanol in a hot-cathode discharge source, mass selected, and co-deposited with neon. Three electronic transitions of the most stable isomer of protonated pyrene and four of protonated coronene were recorded. The strongest, S(1) ← S(0) transitions, are in the visible region, with onset at 487.5 nm for protonated pyrene and 695.6 nm for protonated coronene. The corresponding neutrals were also observed. The absorptions were assigned on the basis of ab initio coupled-cluster and time-dependent density functional theory calculations. The astrophysical relevance of protonated polycyclic aromatic hydrocarbons is discussed.

Ab initio characterization of the Ne–I2 van der Waals complex: Intermolecular potentials and vibrational bound states

A theoretical study of the potential energy surface and bound states is performed for the ground state of the NeI(2) van der Waals (vdW) complex. The three-dimensional interaction energies are obtained from ab initio coupled-cluster, coupled-cluster single double (triple)/complete basis set, calculations using large basis sets, of quadruple- through quintuple-zeta quality, in conjunction with relativistic effective core potentials for the heavy iodine atoms. For the analytical representation of the surface two different schemes, based on fitting and interpolation surface generation techniques, are employed. The surface shows a double-minimum topology for linear and T-shaped configurations. Full variational quantum mechanical calculations are carried out using the model surfaces, and the vibrationally averaged structures and energetics for the NeI(2) isomers are determined. The accuracy of the potential energy surfaces is validated by a comparison between the present results and the corresponding experimental data available. In lieu of more experimental measurements, we also report our results/predictions on higher bound vibrational vdW levels, and the influence of the employed surface on them is discussed.