Electron Dissociation Energies Research Articles

Probing the Effects of Size and Charge on the Monohydration and Dihydration of SiF5- and SiF62- via Comparisons with BF4- and PF6.

This study systematically examines the interactions of the trigonal bipyramidal silicon pentafluoride and octahedral silicon hexafluoride anions with either one or two water molecules, (SiF5-(H2O)n and SiF62-(H2O)n, respectively, where n = 1, 2). Full geometry optimizations and subsequent harmonic vibrational frequency computations are performed using the CCSD(T) ab initio method with a triple-ζ correlation consistent basis set augmented with diffuse functions on all non-hydrogen atoms (cc-pVTZ for H and aug-cc-pVTZ for Si, O, and F; denoted as haTZ). Two monohydrate and six dihydrate minima have been identified for the SiF5-(H2O)n systems, whereas one monohydrate and five dihydrate minima have been identified for the SiF62-(H2O)n systems. Both monohydrated anions have a minimum in which the water molecule adopts a symmetric double ionic hydrogen bond (DIHB) motif with C2v symmetry. However, a second unique monohydrate minimum has been identified for SiF5- in which the water molecule adopts an asymmetric DIHB motif along the edge of the trigonal bipyramidal anion between one axial and one equatorial F atom. This Cs structure is more than 2 kcal mol-1 lower in energy than the C2v local minimum at the CCSD(T)/haTZ level of theory. While the interactions between the solvent and ionic solute are quite strong for the monohydrated anions (electronic dissociation energies of ≈12 and ≈24 kcal mol-1 for the SiF5-(H2O)1 and SiF62-(H2O)1 global minima, respectively), these values are nearly perfectly doubled for the dihydrates, with the lowest-energy SiF5-(H2O)2 and SiF62-(H2O)2 minima exhibiting dissociation energies of ≈24 and ≈47 kcal mol-1, respectively. Structures that form hydrogen bonds between the solvating water molecules also exhibit the largest shifts in the harmonic OH stretching frequencies for the waters of hydration. These shifts can exceed -100 cm-1 for the SiF5-(H2O)2 minimum and -300 cm-1 for the SiF62-(H2O)2 minimum relative to an isolated H2O molecule at the CCSD(T)/haTZ level of theory. This work also corrects the OH stretching frequency shifts for two dihydrate minima of PF6- that were previously erroneously reported ( J. Phys. Chem. A 2020, 124, 8744-8752, DOI: 10.1021/acs.jpca.0c06466).

Systematic analysis of electronic barrier heights and widths for concerted proton transfer in cyclic hydrogen bonded clusters: (HF)n, (HCl)n and (H2O)n where n = 3, 4, 5.

The MP2 and CCSD(T) methods are paired with correlation consistent basis sets as large as aug-cc-pVQZ to optimize the structures of the cyclic minima for (HF)n, (HCl)n and (H2O)n where n = 3-5, as well as the corresponding transition states (TSs) for concerted proton transfer (CPT). MP2 and CCSD(T) harmonic vibrational frequencies confirm the nature of each minimum and TS. Both conventional and explicitly correlated CCSD(T) computations are employed to assess the electronic dissociation energies and barrier heights for CPT near the complete basis (CBS) limit for all 9 clusters. Results for (HF)n are consistent with prior studies identifying Cnh and Dnh point group symmetry for the minima and TSs, respectively. Our computations also confirm that CPT proceeds through Cs TS structures for the C1 minima of (H2O)3 and (H2O)5, whereas the process goes through a TS with D2d symmetry for the S4 global minimum of (H2O)4. This work corroborates earlier findings that the minima for (HCl)3, (HCl)4 and (HCl)5 have C3h, S4 and C1 point group symmetry, respectively, and that the Cnh structures are not minima for n = 4 and 5. Moreover, our computations show the TSs for CPT in (HCl)3, (HCl)4 and (HCl)5 have D3h, D2d, and C2 point group symmetry, respectively. At the CCSD(T) CBS limit, (HF)4 and (HF)5 have the smallest electronic barrier heights for CPT (≈15 kcal mol-1 for both), followed by the HF trimer (≈21 kcal mol-1). The barriers are appreciably higher for the other clusters (around 27 kcal mol-1 for (H2O)4 and (HCl)3; roughly 30 kcal mol-1 for (H2O)3, (H2O)5 and (HCl)4; up to 38 kcal mol-1 for (HCl)5). At the CBS limit, MP2 significantly underestimates the CCSD(T) barrier heights (e.g., by ca. 2, 4 and 7 kcal mol-1 for the pentamers of HF, H2O and HCl, respectively), whereas CCSD overestimates these barriers by roughly the same magnitude. Scaling the barrier heights and dissociation energies by the number of fragments in the cluster reveals strong linear relationships between the two quantities and with the magnitudes of the imaginary vibrational frequency for the TSs.

Bonding, Thermodynamics, and Spectroscopy of the Metal Borides UB0/+/– and WB0/+/–

The bonding and spectroscopy of the UB0/+/- and WB0/+/- molecules were examined by performing high-level electronic structure calculation on their low-lying electronic states. The calculations were performed at the SO-CASPT2 level to obtain the low-lying excited states and at the FPD level to calculate the adiabatic electronic affinities (AEA), ionization energies (IE), and bond dissociation energies (BDE). Compared to UC and UN, UB has a much denser manifold of states below 1.7 eV. The ground state of UB is predicted to be 8I5/2, and that of WB is 6Π7/2. The calculated IEs of UB and WB are 6.241 and 7.314 eV, respectively, and the corresponding AEAs are 1.160 and 1.422 eV. The BDE of UB is predicted to be 223.1 kJ/mol, which is considerably lower than those predicted for UC and UN and ∼35 kJ/mol lower than the BDE of WB. NBO calculations show that the U and B are connected by two 1-electron π bonds and one 1-electron σ bond with substantial ionic character and a bond order of 1.5. There are three unpaired electrons in the 5f on U. WB has less ionic character than UB with a doubly occupied π bond and a singly occupied σ bond for a bond order of ∼1.5. The results show that the U in UB behaves more like an actinide and the W in WB more like a transition metal.

Diffusion Monte Carlo investigation of electronic structure properties for 13-atom alkali metal clusters

Electronic structure properties of 13-atom icosahedral alkali metal clusters (X13, X = Li, Na, K, Rb, and Cs) were investigated with the Diffusion Monte Carlo (DMC) and Density Functional Theory (DFT) methods. The overall results of vertical ionization energies, vertical electron affinities and dissociation energies are in good agreement with theoretical/experimental values found in the literature for these clusters. However, the DFT approach investigated, B3LYP, provides larger discrepancies than DMC for dissociation energies. In this case, empirical dispersion corrections seem important to improve the binding energy data from DFT. We also considered heteronuclear alkali metal clusters such as YX12, where the Y atom was placed at the center and also at the face of the initial icosahedral structure of X13. To our knowledge, the results for such heteronuclear clusters are being reported for the first time, providing an important initial step for understanding these systems. This work demonstrates that face-type YX12 clusters are usually more strongly bonded than the respective center-type structures, except for LiNa12, CsRb12, and KCs12 (according to DMC results). In addition, the trends evidenced in binding energies are analysed in details.

Benchmarking ANO-R basis set for multiconfigurational calculations

The selection of basis sets is very important for multiconfigurational wave function calculation, due to a balance between a desired accuracy and computational costs. Recently, the atomic natural orbital-relativistic (ANO-R) basis set was published as a suggested replacement for the ANO-RCC basis set for scalar-relativistic correlated calculations Zobel et al (2021 J. Chem. Theory Comput. 16 278–294). Benchmarking ANO-R basis set against ANO-RCC for atoms (from H to Rn) and their compounds is the goal of this study. Many of these compounds (for instance, diatomic molecules containing transition metals) have open shells, for which reason a multiconfigurational approach is necessary and was primarily used throughout this project. Performance of the ANO-R basis set in multiconfigurational calculations is similar to the ANO-RCC basis set for the ionisation potential of atoms, and the bond distance in diatomic molecules. Deficiencies are noted for atomic electron affinities and dissociation energies of fluoride diatomic molecules. ANO-R basis sets are more compact in comparison to the corresponding ANO-RCC basis sets leading to smaller computational costs, which was demonstrated by chloroiron corrole molecule as an example.

When Fluxionality Beats Size Selection: Acceleration of Ostwald Ripening of Sub‐Nano Clusters

Size selection was demonstrated to suppress Ostwald ripening of supported catalytic nanoparticles. We show here that when the supported clusters are sub-nanometer in size and highly fluxional, such as Pt clusters on the rutile TiO 2 (110) surface, this paradigm breaks down, and the established theory of sintering needs a revision. At temperatures characteristic of catalysis (i.e. 700 K), sub-nano clusters thermally populate many low-energy metastable isomers. As these isomers all have different geometric and electronic structures, and thus, formation and dissociation energies (in lieu of surface energy), Ostwald ripening is not suppressed, despite the size-selection. However, some clusters arise as magic numbers in terms of sintering stability at the ensemble level. The acceleration of sintering by metastable species persists though weakens in polydisperse cluster systems. Lastly, we propose a competing pathways theory for sintering, which at the atomistic level, describes the found size-specific sintering behavior.

When Fluxionality Beats Size Selection: Acceleration of Ostwald Ripening of Sub‐Nano Clusters

AbstractSize selection was demonstrated to suppress Ostwald ripening of supported catalytic nanoparticles. When the supported clusters are subnanometer in size and highly fluxional, such as Pt clusters on the rutile TiO2(110) surface, this paradigm breaks down, and the established theory of sintering needs a revision. At temperatures characteristic of catalysis (i.e. 700 K), sub‐nano clusters thermally populate many low‐energy metastable isomers. As these isomers all have different geometric and electronic structures, and thus, formation and dissociation energies (in lieu of surface energy), Ostwald ripening is not suppressed, despite the size‐selection. However, some clusters arise as magic numbers in terms of sintering stability at the ensemble level. Acceleration of sintering by metastable species persists though weakens in polydisperse cluster systems. We propose a competing pathways theory for sintering, which at the atomistic level describes the found size‐specific sintering behavior.

Astrochemical Significance of the P + SO Reaction: Spectroscopic Characterization of SPO, PSO, and SOP Isomers

Abstract The doublet and quartet potential energy surfaces for the P + SO → PO + S reaction are explored using the highly accurate explicit correlation multireference configuration interaction method, and the reaction is found to be thermodynamically favorable, with an exothermicity value of 70.5 kJ mol−1. The electronic structure, spectra, and bond dissociation energies of the intermediates involved in the reaction, such as SPO, PSO, and SOP, are calculated using high-level quantum chemistry methods. An explanation of the nondetection of the diatomic SP is proposed. At pressures relevant to astrochemistry, the reaction is overwhelmingly bimolecular, with a predicted rate coefficient of 2.1 × 10−10 (T/300)−0.23 cm3 molecule−1 s−1. The results from this study are expected to aid in the spectroscopic detection of these new species in the laboratory and the interstellar medium.

2-body:Many-body QM:QM study of structures, energetics, and vibrational frequencies for microhydrated halide ions

ABSTRACTThe optimised structures, electronic dissociation energies and harmonic vibrational frequencies of microhydrated halide ions have been probed with the MP2 and CCSD(T) quantum mechanical (QM) electronic structure techniques as well as with the 2-body:Many-body QM:QM method (2b:Mb CCSD(T):MP2) in which the 1- and 2-body interactions are computed at the CCSD(T) level of theory while all higher-order contributions are captured at the MP2 level of theory. In general, the accuracy and efficiency of the 2b:Mb approach previously demonstrated for neutral, homogeneous HF and HO clusters extends to these negatively charged heterogeneous systems. Results indicate the MP2 method struggles to provide a reliable description of the vibrational frequencies for hydrated fluoride clusters even though it performs well for the other hydrated halides (chloride, bromide and iodide). Consequently, the accuracy of the 2b:Mb CCSD(T):MP2 approach for fluoride clusters is poorer than the other halides, and MP2 is not a recommended low-level method for high-accuracy frequency analyses of hydrated fluoride complexes with the 2b:Mb method or related QM:QM approaches.

Five ab initio potential energy and dipole moment surfaces for hydrated NaCl and NaF. I. Two-body interactions.

We report full-dimensional, ab initio-based potentials and dipole moment surfaces for NaCl, NaF, Na(+)H2O, F(-)H2O, and Cl(-)H2O. The NaCl and NaF potentials are diabatic ones that dissociate to ions. These are obtained using spline fits to CCSD(T)/aug-cc-pV5Z energies. In addition, non-linear least square fits using the Born-Mayer-Huggins potential are presented, providing accurate parameters based strictly on the current ab initio energies. The long-range behavior of the NaCl and NaF potentials is shown to go, as expected, accurately to the point-charge Coulomb interaction. The three ion-H2O potentials are permutationally invariant fits to roughly 20,000 coupled cluster CCSD(T) energies (awCVTZ basis for Na(+) and aVTZ basis for Cl(-) and F(-)), over a large range of distances and H2O intramolecular configurations. These potentials are switched accurately in the long range to the analytical ion-dipole interactions, to improve computational efficiency. Dipole moment surfaces are fits to MP2 data; for the ion-ion cases, these are well described in the intermediate- and long-range by the simple point-charge expression. The performance of these new fits is examined by direct comparison to additional ab initio energies and dipole moments along various cuts. Equilibrium structures, harmonic frequencies, and electronic dissociation energies are also reported and compared to direct ab initio results. These indicate the high fidelity of the new PESs.

Samarium doped silicon clusters SmSin (n = 3–10) and their anions: Structures, thermochemistry, electron affinities, and magnetic moments

The structures, adiabatic electron affinities (AEAs), dissociation energies (DEs), hardness, and magnetic moments of SmSin (n=3–10) and their anions were examined by means of B2PLYP, TPSSh, PBE0 and B3LYP methods. Basis sets adopted are of segmented (SEG) Gaussian valence basis sets and relativistic small-core effective potentials (ECP) with additional diffuse 2dfg functions, denoted aug-SEG/ECP for Sm atoms and aug-cc-pVTZ for Si atoms. The most stable structures can be viewed as replacing a Si of the ground-state structure of Sin+1 with a Sm atom. The theoretical AEAs are in excellent agreement with experimental data, especially the AEAs at B2PLYP and TPSSh levels. The average absolute errors from experiment are by 0.04, 0.06, 0.10 and 0.11eV at the B2PLYP, the TPSSh, the PBE0 and the B3LYP levels, respectively. The DEs of Sm from SmSin (n=3–10) and their anions were estimated to examine their relative stabilities. Natural population analysis (NPA) reveals that the Sm atom in all the neutral and anionic clusters acts as an electron donor. Hardness analysis reveals that doping Sm atom to Sin (n=3–10) clusters raises the photochemical sensitivity. Analysis of magnetic moments reveals that the total magnetic moments are contributed by Samarium atom.

Relationship between electronic properties and drug activity of seven quinoxaline compounds: A DFT study

The quantum chemical calculations at the DFT/B3LYP level of theory were carried out on seven quinoxaline compounds, which have been synthesized as anti-Mycobacterium tuberculosis agents. Three conformers were optimized for each compound and the lowest energy structure was found and used in further calculations. The electronic properties including EHOMO, ELUMO and related parameters as well as electron density around oxygen and nitrogen atoms were calculated for each compound. The relationship between the calculated electronic parameters and biological activity of the studied compounds were investigated. Six similar quinoxaline derivatives with possible more drug activity were suggested based on the calculated electronic descriptors. A mechanism was proposed and discussed based on the calculated electronic parameters and bond dissociation energies.

Probing the electronic structures and properties of neutral and anionic ScSi n (0,−1) (n = 1–6) clusters using ccCA-TM and G4 theory

The geometries, electronic structures and energies of small ScSi n species (n = 1-6) and their anions have been systematically investigated by means of the higher level of the ccCA-TM, G4, and G4(MP2) schemes. The global minima of these clusters have been presented. The global minima of neutral ScSi n (n = 1-6) and their anions are "substitutional structure" which is derived from Si n+1 by replacing a Si atom with a Sc atom. The adiabatic electron affinities for ScSi n have been estimated. Compared with limited experimental data, the average absolute deviations from experiment for ccCA-TM, G4, and G4(MP2) are 0.21eV, 0.22eV, and 0.25eV, respectively. The dissociation energies of Sc atom from the lowest-energy structure of ScSi n clusters have been evaluated to examine relative stabilities. The electron affinities and dissociation energies predicted by ccCA-TM, G4, and G4(MP2) methods, especially for ccCA-TM and G4, differ little from each other. The agreement may indicate they are reliable.

Ab initio study of the elusive HO3(X2A″) radical and the [formula omitted] reaction

Electronic structures, vibrational properties, and dissociation energies of the elusive HO3(X2A″) radical have been calculated using various benchmark ab initio methods, including multireference RS2, MRCI+Q, MR-AQCC, NEVPT2, and the explicitly correlated RCCSD(T)-F12, RS2-F12, and MRCI-F12. The RS2 results strongly depend on the semiempirical level shifts applied to modify the zeroth-order Hamiltonian and the basis sets. The sizes of active spaces are crucial to the MRCI+Q data. The calculated dissociation energy (De) of HO3 into OH+O2 ranges from 4.6 to 6.2kcal/mol. Theoretical intermolecular vibrational frequencies are in excellent agreement with the experimental measurements. The zero-point energy correction to De is estimated to be 2.6kcal/mol. The formation of HO3 from OH(X2Π) and O2(X3Σg-) is determined to be a barrierless process. Temperature and pressure dependent kinetics for the association reaction are calculated variationally using the Master equations. It is revealed that the rate constants exhibit steep negative temperature dependence and typical non-Arrhenius behavior. The experimental low-temperature rate constants have been well reproduced theoretically.

Heats of formation and thermochemical parameters of small silicon clusters and their ions, [formula omitted] with n = 2–13

Total atomization energies and heats of formation of small silicon clusters Sin and their ions are calculated using G4 (n=2–13) and CCSD(T)/CBS (aug-cc-pV(n+d)Z for n=2–6) methods. Experimental data for Sin were available with large uncertainties. A new ground state structure for Si11- was located. Using ΔfH∘(Si,298K)=451.5kJ/mol, ΔfH∘(Sin,298K) are computed as: Si2: 588/588kJ/mol (G4/CBS), Si3: 625/632, Si4: 633/639, Si5: 669/692, Si6: 675/701, Si7: 698, Si8: 866, Si9: 872, Si10: 833, Si11: 996, Si12: 1051 and Si13: 1158. Adiabatic electron affinities, ionization, binding and dissociation energies of Sin are determined.

Theoretical studies on the thermodynamic properties, densities, detonation properties, and pyrolysis mechanisms of trinitromethyl-substituted aminotetrazole compounds

Trinitromethyl-substituted aminotetrazoles with -NH₂, -NO₂, -N₃, and -NHC(NO₂)₃ groups were investigated at the B3LYP/6-31G(d) level of density functional theory. Their sublimation enthalpies, thermodynamic properties, and heats of formation were calculated. The thermodynamic properties of these compounds increase with temperature as well as with the number of nitro groups attached to the tetrazole ring. In addition, the detonation velocities and detonation pressures of these compounds were successfully predicted using the Kamlet-Jacobs equations. It was found that these compounds exhibit good detonation properties, and that compound G (D = 9.2 km/s, P = 38.8 GPa) has the most powerful detonation properties, which are similar to those of the well-known explosive HMX (1,3,5,7-tetranitro-1,3,5,7-tetrazocine). Finally, the electronic structures and bond dissociation energies of these compounds were calculated. The BDEs of their C-NO₂ bonds were found to range from 101.9 to 125.8 kJ/mol(-1). All of these results should provide useful fundamental information for the design of novel HEDMs.

Theoretical investigation of M≡E bonds in transition metal–ylidyne complexes trans-[H(PMe3)4M≡ER] (M = Mo, W; E = Si, Ge, Sn, Pb; R = Mes, Xylyl)

The electronic structures and bond dissociation energies of the hydrido- transition metal–ylidyne complexes trans-[H(PMe3)4M≡ER] (M = Mo, W; E = Si, Ge, Sn, Pb; R = Mes, Xylyl) have been investigated using density functional theory at BP86/TZ2P/ZORA level. The calculated geometries are in excellent agreement with available experimental values. Pauling bond order shows that the M-E bonds in these complexes are nearly M≡E multiple bonds. The EDA results indicate that the absolute values of ∆Eint., ∆Eelstat, ∆Eorb, ∆EPauli and bond dissociation energies decrease in the order Si > Ge > Sn > Pb, and tungsten complexes have stronger bonding than the molybdenum complexes. The contribution of electrostatic interaction ∆Eelstat to the M–ER bonding is comparable to, or in some cases even slightly larger than the covalent bonding ∆Eorb. The σ-bonding interactions are significantly poor (less than 20%) and [H(PMe3)4M]− → ER+ π back-donation is dominant interaction in the studied metal–ylidyne complexes. The WBI values of the M-E bonds are significantly large (1.78–2.06). The NBO analysis indicates that the σ-bonding and both π-bonding orbitals are well occupied (>1.703e). The M-E σ-bonding orbitals are slightly polarized towards the heavier group 14 element while the π-bonding orbitals are highly polarized towards the metal atom.

Communication: Rigorous calculation of dissociation energies (D) of the water trimer, (H2O)3 and (D2O)3

Using a recent, full-dimensional, ab initio potential energy surface [Y. Wang, X. Huang, B. C. Shepler, B. J. Braams, and J. M. Bowman, J. Chem. Phys. 134, 094509 (2011)] together with rigorous diffusion Monte Carlo calculations of the zero-point energy of the water trimer, we report dissociation energies, D(0), to form one monomer plus the water dimer and three monomers. The calculations make use of essentially exact zero-point energies for the water trimer, dimer, and monomer, and benchmark values of the electronic dissociation energies, D(e), of the water trimer [J. A. Anderson, K. Crager, L. Fedoroff, and G. S. Tschumper, J. Chem. Phys. 121, 11023 (2004)]. The D(0) results are 3855 and 2726 cm(-1) for the 3H(2)O and H(2)O + (H(2)O)(2) dissociation channels, respectively, and 4206 and 2947 cm(-1) for 3D(2)O and D(2)O + (D(2)O)(2) dissociation channels, respectively. The results have estimated uncertainties of 20 and 30 cm(-1) for the monomer plus dimer and three monomer of dissociation channels, respectively.

Relativistic effects in atomic and molecular properties

Relativistic effects in atomic and molecular propertiesWe present an overview of basic principles and methods of the relativistic quantum chemistry. Practical aspects of different methods will be discussed stressing their capability of providing accurate predictions of molecular properties, particularly in species containing a heavy metal element. We will present a series of examples showing the importance of relativistic effects in a variety of molecular properties including electron affinities, ionization potentials, reaction and dissociation energies, electric, spectroscopic and other properties. It is possible to recognize a link between these properties and behaviour of materials in some cases. Particular attention is paid to relativistic calculations of the nuclear quadrupole moments for which accurate theoretical electric field gradient is combined with data from the microwave spectra. Important aspect of the present paper is understanding of trends in electronically related atoms throughout the Mendeleev Periodic Table rather than focusing on highly accurate numbers. We will show that relativistic effects represent an unavoidable instrument for explaining some unexpected properties of heavy metal containing compounds. We will also discuss an interplay between the many-electron correlation and relativistic effects.

Structures and Thermochemistry of the Alkali Metal Monoxide Anions, Monoxide Radicals, and Hydroxides

The geometries, enthalpies of formation (DeltaH(o)(f)), separations of electronic states, electron affinities, gas-phase acidities, and bond dissociation energies associated with the alkali metal monoxide anions (MO(-)), monoxide radicals (MO(*)), and hydroxides (MOH) (M = Li, Na, and K) have been investigated using single-reference and multireference variants of the WnC procedures. Our best estimates of the DeltaH(o)(f) values for the ground states at 298 K are as follows: 8.5 ((3)Pi LiO(-)), 48.5 ((2)Pi LiO(*)), -243.4 ((1)Sigma(+) LiOH), 34.2 ((3)Pi NaO(-)), 86.4 ((2)Pi NaO(*)), -190.8 ((1)Sigma(+) NaOH), 15.1 ((1)Sigma(+) KO(-)), 55.9 ((2)Sigma(+) KO(*)), and -227.0 ((1)Sigma(+) KOH) kJ mol(-1). While the LiO(*) and NaO(*) radicals have (2)Pi ground states, for KO(*), the (2)Sigma(+) and (2)Pi electronic states lie very close in energy, with our best estimate being a preference for the (2)Sigma(+) state by 1.1 kJ mol(-1) at 0 K. In a similar manner, the ground state for MO(-) changes from (3)Pi for LiO(-) and NaO(-) to (1)Sigma(+) for KO(-). The (1)Sigma(+) state of KO(-) is indicated by the calculated T(1) diagnostic and the SCF contribution to the total atomization energy to have a significant degree of multireference character. This leads to a difference of more than 100 kJ mol(-1) between the single-reference W2C and multireference W2C-CAS-ACPF and W2C-CAS-AQCC estimates for the (1)Sigma(+) DeltaH(o)(f) for KO(-).