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We have investigated the structure and spectra (NMR, UV/visible) of several Yariv reagents and models for these species, using both density functional theory (DFT) and approximate coupled‐cluster methods (ADC(2)). We have also studied the formation of dimers and model trimers as a first step to understand the aggregation of these species in aqueous solution. One question that our calculations help resolve is the origin of two strong bands in the visible region of the spectrum: these are confirmed as resulting from enol‐keto tautomerism, and are not a result of aggregation, which was a hypothesis that had been advanced previously. The UV/visible spectrum of the Yariv reagents can be explained entirely in terms of tautomerism within a single molecule – no dimers or clusters are involved. [ABSTRACT FROM AUTHOR]

Methylketene (CH 3 CHCO) and acrolein (CH 2 CHCHO) have received considerable attention from both theoreticians and experimentalists. In a number of computational studies, it has been identified that both isomers sit alongside their dissociation products C 2 H 4 + CO , at the bottom of the potential energy surface with energies very close to each another. The most striking feature from preliminary calculations is that the relative energy difference between methylketene and trans-acrolein ( Δ E 0.8–3.4 kJ/mol) changes with respect to choice of reference geometry, correlation treatment, and basis set expansion. While the thermodynamic profile favours trans-acrolein as the lowest-lying isomer, it is difficult to estimate the quality of the currently available results, which are determined with a variety of methods. At higher levels of theory together with reducing basis set incompleteness methylketene might be the true minimum on the C 3 H 4 O potential energy surface. In addition to their being almost isoenergetic, much interest in these C 3 H 4 O isomers stems from an astrochemical standpoint. The discovery of both cis and trans-acrolein in the interstellar medium has led to speculation that methylketene may also be present. While thermodynamically methylketene proves to be a good candidate for extra-terrestrial observation, as yet there have been no reports of its detection. To this end, by applying the best correlation treatment that has been used for this species, using by far the most elaborate and accurate basis sets (up to sextuple-zeta quality), we establish the relative ordering and energetic difference between methylketene and trans-acrolein to within chemical accuracy (within 1 kJ/mol). We find the inclusion of zero-point energy, explicit core-correlation, and augmentation with diffuse functions have significant effects in determining the relative energy difference of the isomers, and assignment of the C 3 H 4 O global minimum. [ABSTRACT FROM AUTHOR]

To date, four $ \hbox {C}_2\hbox {H}_4\hbox {O}_2 $ C 2 H 4 O 2 isomers have been identified in the interstellar medium: acetic acid, glycolaldehyde, methyl formate, and (Z)-1,2-ethenediol. The detection of these species raises the question of whether other related carbohydrates could be present in similar astrophysical environments. In this study, we report a systematic investigation of six $ \hbox {C}_2\hbox {H}_4\hbox {O}_2 $ C 2 H 4 O 2 isomers not previously observed spectroscopically in space: anti-(1)-, (2)- and (3)- $ \hbox {CH}_2=\hbox {CHOOH} $ CH 2 = CHOOH , methyldioxirane, and anti- and syn-(2S)-oxiran-2-ol. For each species, we employ coupled-cluster theory at the AE-CCSD(T)/cc-pCVQZ level to provide accurate equilibrium parameters, rotational constants, quartic centrifugal distortion parameters, and permanent electric dipole moment components. Anharmonic force fields were determined within vibrational perturbation theory to second order and evaluated using the MP2 method, thus allowing the evaluation of vibrational corrections. These computations provided, as a by-product, sextic centrifugal distortion constants. Rotational spectra have subsequently been simulated in the millimeter/submillimeter-wave frequency for the first time. It is hoped and anticipated that the accurate theoretical rovibrational spectral data presented herein will aid in experimental investigations of these title molecules in the interstellar medium. [ABSTRACT FROM AUTHOR]

We present the implementation of two-electron spin-spin coupling as a quasidegenerate perturbative treatment of the Breit–Pauli spin-spin Hamiltonian. The evaluation is based on a multireference CI treatment and constitutes one of the first efforts in the calculation of this effect within a highly sophisticated consideration of both nondynamical and dynamical correlation. The extension of existing schemes for efficient calculation, in particular, of the spin-coupling elements necessitated some involved derivations, the outline of which is presented herein. Application of the program to calculations of diagonal as well as off-diagonal spin-coupling elements is illustrated with the test cases O2 and NH. [ABSTRACT FROM AUTHOR]

Basis set convergence of correlation effects on molecular atomization energies beyond the coupled cluster with singles and doubles (CCSD) approximation has been studied near the one-particle basis set limit. Quasiperturbative connected triple excitations, (T), converge more rapidly than L-3 (where L is the highest angular momentum represented in the basis set), while higher-order connected triples, T3-(T), converge more slowly—empirically, ∝L-5/2. Quasiperturbative connected quadruple excitations, (Q), converge smoothly as ∝L-3 starting with the cc-pVTZ basis set, while the cc-pVDZ basis set causes overshooting of the contribution in highly polar systems. Higher-order connected quadruples display only weak, but somewhat erratic, basis set dependence. Connected quintuple excitations converge very rapidly with the basis set, to the point where even an unpolarized double-zeta basis set yields useful numbers. In cases where fully iterative coupled cluster up to connected quintuples (CCSDTQ5) calculations are not an option, CCSDTQ(5) (i.e., coupled cluster up to connected quadruples plus a quasiperturbative connected quintuples correction) cannot be relied upon in the presence of significant nondynamical correlation, whereas CCSDTQ(5)Λ represents a viable alternative. Connected quadruples corrections to the core-valence contribution are thermochemically significant in some systems. We propose an additional variant of W4 theory [A. Karton et al., J. Chem. Phys. 125, 144108 (2006)], denoted W4.4 theory, which is shown to yield a rms deviation from experimental atomization energies (active thermochemical tables, ATcT) of only 0.05 kcal/mol for systems for which ATcT values are available. We conclude that “3σ≤=1 kJ/mol” thermochemistry is feasible with current technology, but that the more ambitious goal of ±10 cm-1 accuracy is illusory, at least for atomization energies. [ABSTRACT FROM AUTHOR]

The potential energy surfaces (PESs) and S1-T1 spin-orbit coupling matrix element (SOCME) surfaces are investigated for the toluene–X weakly bound clusters (X=Ne, Ar, and Kr). Calculations of the vibrational wave functions using a one-dimensional stretch model are presented and used to determine vibrationally averaged values of the SOCMEs. Our ab initio theoretical results compare well with intersystem crossing rates derived from recent experimental fluorescence lifetime data [Doyle et al., J. Chem. Phys. 122, 194315 (2005)]. Vibrational averaging is shown to change the absolute magnitude of the calculated SOCMEs, but the ratio between them remains very similar to that of the single-point values calculated at the minima of the PESs. [ABSTRACT FROM AUTHOR]

Highly accurate ab initio computations of the molecular structure and properties, torsional potential energy function, and harmonic force field of disilane and ethane have been carried out. Equilibrium parameters as well as vibrational corrections have been evaluated. In addition, for these systems a vibrational averaging procedure has been employed for calculating the dipole moment of molecules which have no permanent dipole moment, i.e., SiH3SiD3 and CH3CD3. The molecular and spectroscopic properties calculated for ethane and its isotopomers provide a calibration against known experimental data, allowing us to estimate the reliability of our computed results for disilane for which there is much less experimental data. The goal of the present study is to predict the molecular parameters, with estimated uncertainties, that determine the microwave spectrum of SiH3SiD3. © 2005 American Institute of Physics. [ABSTRACT FROM AUTHOR]

Extensive tests have been performed to benchmark and to compare with second-order perturbation theory based on a complete active space self-consistent field reference function (CASPT2), the recently developed n-electron valence state perturbation theory at second order (NEVPT2). Test calculations included the group fifteen diatomic molecules X[sub 2] (X=N, P, As, and Sb) and the [sup 4]S/[sup 2]D and [sup 4]S/[sup 2]P splittings for the corresponding atoms, the [sup 1]A[sub 1]–[sup 3]B[sub 1] splittings for CH[sub 2] and SiH[sub 2], and the absorption spectra of pyrrole and of Cu(Imidazole)[sub 2](SH)(SH[sub 2])[sup +], which is a model for plastocyanin. Comparisons with full configuration-interaction calculations and experimental data show that the accuracy of NEVPT2 is in most cases even better than CASPT2. Where intruder states hamper the CASPT2 calculations, NEVPT2 performs significantly better. Care is needed in the choice of active orbitals, for example in the calculation of the [sup 4]S/[sup 2]D and [sup 4]S/[sup 2]P splittings for the group fifteen atoms. This is due to the different treatment of orbitals belonging to the inactive or active spaces, making the NEVPT2 not invariant for the choice of active space, even in cases where the multiconfiguration self-consistent field energy is invariant. © 2004 American Institute of Physics. [ABSTRACT FROM AUTHOR]

Investigates the role of asymmetric molecular bonding in chemisorption of iodine trichloride molecule. Formation of partial bonds between an iodine atom and silicon adatom; Interaction between asymmetric highest occupied molecular orbits (HOMO) and silicon bonds; Purpose of asymmetric HOMO orbital and zero dipole moment.

Focuses on the computation of the equilibrium bond length and quartic force field for tetrahedral phosphorus molecule. Use of coupled-cluster methods and a variety of extended basis sets; Effect of correlating core and valence electrons; Differences with experimental estimates of bond length and fundamental vibrational frequencies.

Focuses on benchmark quality total atomization energies (TAE) of small polyatomic molecules. Successive coupled-cluster calculations in basis sets; Separate Schwartz-type extrapolations of the self consistent field (SCF) and correlation energies; Mean absolute error; Valence correlation contribution to the (TAE).

We investigate augmenting conventional Gaussian-type one-electron orbital basis sets with two-electron functions that have a Gaussian dependence on the interelectronic distance. We observe substantial improvements in calculated correlation energies for helium and neon atoms and for the water molecule. A feature of our approach is that there is no nonlinear optimization of the two-electron basis function parameters at all. © 1996 American Institute of Physics. [ABSTRACT FROM AUTHOR]

The reaction of vinylidene (CH2C) with acetylene may be an initiating reaction in soot formation. We report minimum energy paths and accurate energetics for a pathway leading to vinylacetylene and for a number of isomers of C4H4. The calculations use complete active space self-consistent field (CASSCF) derivative methods to characterize the stationary points and internally contacted configuration interaction (ICCI) and/or coupled cluster singles and doubles with a perturbational estimate of triple excitations [CCSD(T)] to determine the energetics. We find an entrance channel barrier of about 5 kcal/mol for the addition of vinylidene to acetylene, but no barriers above reactants for the reaction pathway leading to vinylacetylene. © 1995 American Institute of Physics. [ABSTRACT FROM AUTHOR]

The total atomization energies of the carbon clusters Cn (n=2–10) have been calculated ab initio using augmented coupled-cluster methods and basis sets of spdf and spdfg quality. The values agree to within experimental uncertainty with recent Knudsen effusion measurements for Cn (n=2–7), but suggest actual values on the high side of their experimental range. For the odd-numbered clusters, an additivity approximation holds very well, and extrapolated ∑D0 values are proposed for C11, C13, C15, C17, and C19. © 1995 American Institute of Physics. [ABSTRACT FROM AUTHOR]

The potential energy surface for the B2C molecule and the potential energy curve for the ground state of BC have been investigated using full-valence complete active space SCF (CASSCF), augmented coupled cluster [CCSD(T)] and multireference treatments. The ground state of B2C is an extraordinarily stable ring (∑ De=261.6±1 kcal/mol) with two 2-electron π systems. The first excited state is linear BCB (1Σ+g), which is essentially biconfigurational due to a (4σg)–(3σu) near degeneracy. Anharmonic spectroscopic constants were obtained from quartic force fields at the CCSD(T) level with a correlation-consistent basis set of [4s3p2d1f] quality. A severe Fermi resonance exists between the bending and symmetric stretching modes. All computed intensities are fairly weak. Spectroscopic constants for BC using elaborate multireference techniques were very well reproduced using the CCSD(T) method with a spin-restricted Hartree–Fock reference configuration, but not with an unrestricted Hartree–Fock reference. This suggests that even moderate levels of spin contamination that do not significantly affect relative energies may have a detrimental effect on computed spectroscopic constants. [ABSTRACT FROM AUTHOR]

Pulsed laser evaporation of pellets pressed from boron and graphite powder gave a new 1:4 doublet at 1232.5 and 1194.6 cm-1 in addition to the carbon cluster absorptions reported previously. The 1232.5 cm-1 band dominated boron-10 experiments. The new bands increased as carbon cluster bands decreased with increasing B/C ratio in the pellet and with increasing laser power. Augmented coupled cluster and full-valence complete active space SCF (CASSCF) calculations predict the global minimum BC2 structure to be an asymmetric triangle: however, the vibrationally averaged structure will be an isosceles triangle with a strong symmetric B–C2 stretching frequency near 1200 cm-1. The calculated boron-10/boron-11 frequency ratio (1.0323) is in excellent agreement with the observed ratio (1.0317), and confirms assignment of the 1194.6 cm-1 band to the BC2 ring. Calculations predict linear BCC to be less stable by 6.2±2 kcal/mol and to absorb in the 2000–2050 cm-1 range: the barrier towards rearrangement to the cyclic structure is very low (1.1 kcal/mol). Linear BCC was not detected in these experiments. Computed energetics explain why BC2 is abundant in B/C experiments, but absent in B+C2H2 experiments. [ABSTRACT FROM AUTHOR]

The quartic force fields of HCO+ and HOC+ have been computed using augmented coupled cluster methods and basis sets of spdf and spdfg quality. Calculations on HCN, CO, and N2 have been performed to assist in calibrating the computed results. Going from an spdf to an spdfg basis shortens triple bonds by about 0.004 Å, and increases the corresponding harmonic frequency by 10–20 cm-1, leaving bond distances about 0.003 Å too long and triple bond stretching frequencies about 5 cm-1 too low. Accurate estimates for the bond distances, fundamental frequencies, and thermochemical quantities are given. HOC+ lies 37.8±0.5 kcal/mol (0 K) above HCO+; the classical barrier height for proton exchange is 76.7±1.0 kcal/mol. [ABSTRACT FROM AUTHOR]

We describe a basis-set-expansion Dirac–Hartree–Fock program for molecules. Bond lengths and harmonic frequencies are presented for the ground states of the group IV tetrahydrides CH4, SiH4, GeH4, SnH4, and PbH4. The results are compared with relativistic effective core potential (RECP) calculations, first-order perturbation theory (PT) calculations and with experimental data. The bond lengths are well predicted by first-order perturbation theory for all molecules, but none of the sets of RECPs considered provides a consistent prediction. Perturbation theory overestimates the relativistic correction to the harmonic frequencies; the RECP calculations underestimate the correction. [ABSTRACT FROM AUTHOR]

The static dipole polarizabilities α and γ for the noble gases helium through xenon have been determined using large flexible one-particle basis sets in conjunction with high-level treatments of electron correlation. The electron correlation methods include single and double excitation coupled-cluster theory (CCSD), an extension of CCSD that includes a perturbational estimate of connected triple excitations, CCSD(T), and second-order perturbation theory (MP2). The computed α and γ values are estimated to be accurate to within a few percent. Agreement with experimental data for the static hyperpolarizability γ is good for neon and xenon, but for argon and krypton the differences are larger than the combined theoretical and experimental uncertainties. Based on our calculations, we suggest that the experimental value of γ for argon is too low. Adjusting this value would also bring the experimental value of γ for krypton into better agreement with our computed result. The MP2 values for the polarizabilities of neon, argon, krypton, and xenon are in reasonable agreement with the CCSD and CCSD(T) values, suggesting that this less expensive method may be useful in studies of polarizabilities for larger systems. [ABSTRACT FROM AUTHOR]

Our recently proposed scheme for using natural orbitals from atomic configuration interaction wave functions as a basis set for linear combination of atomic orbitals (LCAO) calculations is extended for the calculation of molecular properties. For one-electron properties like multipole moments, which are determined largely by the outermost regions of the molecular wave function, it is necessary to increase the flexibility of the basis in these regions. This is most easily done by uncontracting the outermost Gaussian primitives, and/or by adding diffuse primitives. A similar approach can be employed for the calculation of polarizabilities. Properties which are not dominated by the long-range part of the wave function, such as spectroscopic constants or electric field gradients at the nucleus, can generally be treated satisfactorily with the original atomic natural orbital sets. [ABSTRACT FROM AUTHOR]

A general contraction scheme for Gaussian basis sets is presented. The contraction coefficients are defined by the natural orbitals obtained from an atomic configuration-interaction calculation. Such atomic natural orbitals provide an excellent basis for molecular electronic structure calculations. Large primitive sets can be contracted to only a few functions without significant loss in either the SCF or correlation energy. Polarization functions can be included using the same approach. [ABSTRACT FROM AUTHOR]

Full CI (FCI) wave functions are used to compute energies for several electronic states of the same symmetry for SiH2, CH2, and CH+2. It is found that CASSCF/multireference CI (MRCI) wave functions yield results very similar to FCI, irrespective of whether the CASSCF MOs are optimized independently for each state or using an average of the CASSCF energies for all desired states. The ionization potentials (IPs) and excitation energies obtained from the FCI calculations should help calibrate methods such as Green’s function approaches, equations of motion and propagator methods, and cluster expansions, in which energy differences are computed directly. [ABSTRACT FROM AUTHOR]

Full CI calculations are presented for the 1A1,3B1, and 1B1 states of SiH2 at their respective equilibrium geometries and at geometries with the SiH bonds stretched. These results are compared with those obtained from single reference and multireference CI calculations. The computed Te values agree well with the full CI results provided that the effects of higher than double excitations are accounted for either by the Davidson correction or by a multireference approach. When the SiH bonds are stretched the single reference methods are not sufficiently flexible, and only CASSCF/MRCI achieves chemical accuracy (i.e., agrees with the full CI to 1 kcal/mol). Overall, the accuracy of the various approximate methods is very similar to that found for H2O, NH2, and CH2. [ABSTRACT FROM AUTHOR]

We have performed full CI calculations on the collinear transition state for the reaction F+H2→HF+H. The full CI results are compared with those obtained from single reference and multireference CI calculations and from single reference CPF calculations. In general, only those methods which attempt to account for the effects of higher excitations, such as CPF or CI plus the Davidson correction, yield a transition state location and barrier height in good agreement with the full CI. In an extended basis, the effect of higher excitations is estimated to lower the barrier by as much as 1.5 kcal/mol; such an effect would essentially eliminate the present discrepancy between theory and experiment. [ABSTRACT FROM AUTHOR]

Full configuration-interaction calculations are reported, and compared to other methods, for H2O at its equilibrium geometry and at two geometries with the H–O bonds stretched. Since the percentage of the SCF reference in the FCI wave function decreases greatly with the bond elongation, the accuracy of techniques based on a single reference do not compare well with the FCI results. However, the results from a CASSCF/MRCI treatment are in good agreement with the FCI. Correlation effects in F compared to Ne are far more similar than for F- compared to Ne, despite F- and Ne being isoelectronic. Since the importance of higher than double excitations is greater for F- than F, a very high percentage of the correlation must be obtained to accurately compute the electron affinity. In a CASSCF/MRCI treatment the higher than quadruple excitations contribute 0.02 eV to the E.A., even for modest basis sets. [ABSTRACT FROM AUTHOR]

In this essay, I review some of the advances made by Jan Almlöf in the calculation of molecular integrals over Gaussian-type basis functions, and in the use of molecular symmetry to both produce integrals over symmetry-adapted combinations of basis functions and to ensure that these latter were constructed from the minimum possible number of distinct integrals over atom-centred functions. The survey is, as already noted, an essay, historical in nature, and I crave the reader's indulgence that it is structured as an essay and not as a conventional scientific manuscript. [ABSTRACT FROM AUTHOR]

Electronic transitions are observed for the O 2 + –Ar and O 2 + –N2 complexes over the 225–350 nm range. The transitions are not associated with recognized electronic band systems of the respective atomic and diatomic constituents (Ar+, Ar, O 2 + , O2, N 2 + , and N2) but rather are due to charge transfer transitions. Onsets of the O 2 + –Ar and O 2 + –N2 band systems occur at 3.68 and 3.62 eV, respectively, corresponding to the difference in the ionization potentials of Ar and O2 (3.69 eV), and N2 and of O2 (3.51 eV), suggesting the band systems arise from intramolecular charge transfer transitions to states correlating with O2 (X 3 Σ g − ) + Ar+ (2Pu) and O2 (X 3 Σ g − ) + N 2 + (X 2 Σ g + ) limits, respectively. The dominant vibronic progressions have ωe values of 1565 cm−1 for O 2 + –Ar and 1532 cm−1 for O 2 + –N2, reasonably close to the value for the neutral O2 molecule in its X 3 Σ g − state (1580 cm−1). Higher energy band systems for O 2 + –Ar and O 2 + –N2 are assigned to transitions to states correlating with the O2 (a1Δg) + Ar+ (2Pu) and O2 (a1Δg) + N 2 + (X 2 Σ g + ) limits, respectively. [ABSTRACT FROM AUTHOR]

Vibrational frequencies have been computed for TiFnCl4-n, for n=0,4 at the SCF level. The computed frequencies are in good agreement with the known experimental values for TiF4 and TiCl4. The calculations suggest that the band at 772 cm-1 observed by DeVore and Gallaher is due to the Ti–F stretching mode of TiFCl3. [ABSTRACT FROM AUTHOR]

Full CI calculations have been performed on the CH3 radical. The full CI results are compared to those obtained using CASSCF/multireference CI and coupled-pair functional methods, both at the equilibrium CH distance and at geometries with the three CH bonds extended. In general, the performance of the approximate methods is similar to that observed in calculations on other molecules in which one or two bonds were stretched. [ABSTRACT FROM AUTHOR]

Full CI calculations are presented for the 1A1–3B1 separation in CH2 as a calibration for other levels of theory. Compared to the full CI result, the SCF/SDCI separation, after applying the Davidson correction for higher excitations, is found to be in error by only 0.38 kcal/mol, while the error in a CASSCF/MRSDCI treatment is less than 0.01 kcal/mol. [ABSTRACT FROM AUTHOR]

High levels of theory have been used to compute quartic force fields (QFFs) for the cyclic and linear forms of the C3H3+ molecular cation, referred to as c-C3H3+ and 1-C3H3+. Specifically, the singles and doubles coupled-cluster method that includes a perturbation a! estimate of connected triple excitations, CCSD(T), has been used in conjunction with extrapolation to the one-particle basis set limit, and corrections for scalar relativity and core correlation have been included. The QFFs have been used to compute highly accurate fundamental vibrational frequencies and other spectroscopic constants by use of both vibrational second-order perturbation theory and variational methods to solve the nuclear Schrödinger equation. Agreement between our best computed fundamental vibrational frequencies and recent infrared photodissociation experiments is reasonable for most bands, but there are a few exceptions. Possible sources for the discrepancies are discussed. We determine the energy difference between the cyclic and linear forms of C3H3+, obtaining 27.9 kcal/mol at 0 K, which should be the most reliable available. It is expected that the fundamental vibrational frequencies and spectroscopic constants presented here for c-C3H3+ and 1-C3H3+ are the most reliable available for the free gas-phase species, and it is hoped that these will be useful in the assignment of future high- resolution laboratory experiments or astronomical observations. [ABSTRACT FROM AUTHOR]

Abstract: We present the results of correlated calculations on a variety of small carbon rings. Equilibrium structures and vibrational frequencies are calculated and transition states connecting symmetry-equivalent minima are considered in detail. We show that neither single-reference coupled-cluster nor multiconfigurational self-consistent field methods (even after perturbational inclusion of dynamical correlation effects) give qualitatively correct potential surfaces in the vicinity of the minima, suggesting that there is little recourse for these systems other than a multireference coupled-cluster treatment. Density-functional theory using the B3LYP functional produces results broadly in agreement with single-reference coupled-cluster methods and is thus no more reliable, but considerably more economical. [Copyright &y& Elsevier]

Abstract: We briefly review different approaches to calculating the Heisenberg exchange coupling between two weakly coupled metal centres in a complex, with particular emphasis on using multiconfigurational methods to treat individually all the states that arise, without the use of symmetry breaking to obtain a mixed state in which the spins are coupled antiferromagnetically. We illustrate the multiconfigurational approach first using very accurate calculations on two weakly interacting high-spin nitrogen atoms, and then we use these very accurate results to calibrate broken-symmetry single configuration approaches, including density-functional theory with various different functionals. We then consider a dinuclear iron complex chosen to reproduce qualitatively the environment around the diiron–dioxygen “diamond core” in soluble methane monooxygenase hydroxylase. Agreement between high-level multiconfigurational methods and broken-symmetry density-functional theory is only fair. [Copyright &y& Elsevier]

Using new atomic natural orbital basis sets constructed to treat correlation of all M-shell electrons for the first transition row, we have calculated the energies of the reaction M(CO)n→M+nCO for M=Ni,Fe,Cr and n=4,5,6, respectively. Both coupled-cluster and multiconfigurational reference perturbation theory are used as correlation treatments, and although they give qualitatively similar results once basis set superposition error is accounted for, multireference perturbation theory gives binding energies that are larger than both coupled-cluster theory and experiment. The agreement between our best calculated results and experiment is excellent for Ni(CO)4 and Fe(CO)5, but very poor for Cr(CO)6. The experimental result for the latter is old and we suggest that revisiting this system experimentally would be valuable. [ABSTRACT FROM AUTHOR]

Although DFT is the unrivaled method of choice for quantum chemical studies of bioinorganic problems, little is known about its ability to predict the energetics of the low-lying electronic states of transition metal complexes. The first high-level ab initio calculations aimed at calibrating DFT vis-a-vis this issue indicate that, despite its many successes, DFT is far from infallible. In the short term, additional calibration of DFT against more elaborate ab initio methods remains an important goal for computational bioinorganic researchers. In the longer term, we are optimistic that high-level ab initio methods such as CASPT2 and CCSD(T) will be regularly used to study realistic molecules of genuine biochemical interest. [Copyright &y& Elsevier]

Part I. Provides the professional reflections of school principals in England. Phases of school leadership; Involvement in the creation of autonomous learning organizations; Reinvention of self in a climate of change, competition and customer demand.

Focuses on primary schools in Great Britain. Development of autonomous schools; Role of local education authority; Role of the board of governors.

A benchmark ab initio study on the thermochemistry of the trans -HNNH, cis -HNNH, and H2NN isomers of diazene has been carried out using the CCSD(T) coupled cluster method, basis sets as large as [7s6p5d4f3g2h/5s4p3d2f1g], and extrapolations towards the 1-particle basis set limit. The effects on inner shell correlation and of anharmonicity in the zero-point energy were taken into account: thus accurate geometries and anharmonic force fields were obtained as by-products. The best computed DeltaHof0 for trans-HNNH, 49.2-0.3kcal mol-1, is in very good agreement with a recent experimental lower limit of 48.8 0.5kcal mol-1. CCSD(T) basis set limit values for the isomerization energies at 0K are 5.2 0.2kcal mol-1 (cis-trans) and 24.1 0.2kcal mol-1 (iso-trans). Our best computed geometry for trans-HNNH, reNN = 1.2468A, (NH) = 1.0283A, and thetae = 106.17o, reproduces the precisely known ground state rotational constants of trans-HNNH to within better than 0.1%. The rotation-vibration spectra of both cis-HNNH and H2NN are dominated by very strong Coriolis and Fermi resonances. In addition, the NH stretches in H2NN are so strongly anharmonic that vibrational perturbation theory breaks down, and the molecule appears to be an excellent test case for variational treatments of the vibrational Schrodinger equation. [ABSTRACT FROM AUTHOR]

We propose the use of the singular value decomposition to decrease the storage required for wave function information. The specific case considered is determinantal full configuration interaction, but the same technique is readily applicable to truncated configuration interaction and coupled-cluster calculations of various types; as we discuss this is a reformulation of approximate methods that have been in use for some time, but our approach eliminates those approximations. Numerical examples support the contention that considerable compression of the wave function is possible without significant loss of accuracy: as expected a considerable amount of the information contained in the full CI wave function is redundant. [ABSTRACT FROM AUTHOR]

Focuses on the debate about the subject of school autonomy and the stages of school leadership. Requirements for the adoption of the philosophy of continuous improvement; Necessity for school leaders to review career strategies; Maintenance of school organization effectiveness.

We present the results of an ab initio quantum-chemical investigation of the structure of the anion O[sub 4][sup -] and potential energy surfaces for several electronic states. In addition to ground-state vibrational frequencies, which are in good agreement with other calculations and with matrix isolation experiments, we have investigated both photodissociation of O[sub 4][sup -] into various states of O[sub 2] and O[sub 2][sup -] and dissociative photodetachment leading to a free electron and various states of two O[sub 2] molecules. Our surfaces lead to predictions for both photodissociation and photodetachment processes that are in excellent agreement with experiment. © 2001 American Institute of Physics. [ABSTRACT FROM AUTHOR]

In this work, the resolution of the identity (RI) approximation is developed for the calculation of the electron-electron spin-spin coupling (SSC) interaction that is a central component of the zero-field splitting (ZFS) term in the effective spin Hamiltonian. The approximated integrals are then used in large-scale multireference configuration interaction treatments of the SSC interaction. The SSC contribution to the ZFS is treated using the Breit–Pauli spin-spin Hamiltonian in conjunction with first-order perturbation theory. Test calculations on a set of diatomic molecules reveal that the error of the RI approximation does not exceed 0.01 cm-1 even if standard auxiliary basis sets are used. This error of less than 1% is considered to be negligible compared to the presently achievable accuracy of the SSC calculations relative to experimental data. The present development allows the correlated ab initio calculation of ZFS parameters of larger systems such as linear polyenes and linear polyacenes. The basis set convergence of the calculated ZFS values was investigated, and the effect of electronic correlation on the calculated ZFS parameters is discussed. [ABSTRACT FROM AUTHOR]

We present a combined experimental and computational study of the external heavy atom effect in van der Waals clusters of para-difluorobenzene (pDFB) with rare-gas atoms. Experimentally, clustering with rare-gas atoms is observed to shorten significantly the S1 fluorescence lifetime compared with that of the pDFB monomer, an effect we interpret in terms of an enhancement of the S1-T1 intersystem crossing rate. In order to test the validity of this widely held assumption, we have calculated the S1-T1 spin–orbit coupling matrix elements in the X-pDFB complexes (X=Ne,Ar,Kr) using a multiconfigurational linear response approach. © 2004 American Institute of Physics. [ABSTRACT FROM AUTHOR]

Presents a correction on an article concerning ab initio quartic force fields for bicarbonates ions.

The quartic force field of ethylene, C2H4, has been calculated ab initio using augmented coupled cluster, CCSD(T), methods and correlation consistent basis sets of spdf quality. For the 12C isotopomers C2H4, C2H3D, H2CCD2, cis-C2H2D2, trans-C2H2D2, C2HD3, and C2D4, all fundamentals are reproduced to better than 10 cm-1, except for three cases where the error is 11 cm-1. Our calculated harmonic frequencies suggest a thorough revision of the accepted experimentally derived values. Our computed and empirically corrected re geometry differs substantially from experimentally derived values: Both the predicted rz geometry and the ground-state rotational constants are, however, in excellent agreement with experiment, suggesting revision of the older values. Anharmonicity constants agree well with experiment for stretches, but differ substantially for stretch–bend interaction constants, due to equality constraints in the experimental analysis that do not hold. Improved criteria for detecting Fermi and Coriolis resonances are proposed and found to work well, contrary to the established method based on harmonic frequency differences that fails to detect several important resonances for C2H4 and its isotopomers. Surprisingly good results are obtained with a small spd basis at the CCSD(T) level. The well-documented strong basis set effect on the ν8 out-of-plane motion is present to a much lesser extent when correlation-optimized polarization functions are used. Complete sets of anharmonic, rovibrational coupling, and centrifugal distortion constants for the isotopomers are available as supplementary material to the paper via the World-Wide Web. © 1995 American Institute of Physics. [ABSTRACT FROM AUTHOR]

A very accurate ab initio quartic force field for CH4 and its isotopomers is presented. The quartic force field was determined with the singles and doubles coupled-cluster procedure that includes a quasiperturbative estimate of the effects of connected triple excitations, CCSD(T), using the correlation consistent polarized valence triple zeta, cc-pVTZ, basis set. Improved quadratic force constants were evaluated with the correlation consistent polarized valence quadruple zeta, cc-pVQZ, basis set. Fundamental vibrational frequencies are determined using second-order perturbation theory anharmonic analyses. All fundamentals of CH4 and isotopomers for which accurate experimental values exist and for which there is not a large Fermi resonance, are predicted to within ±6 cm-1. It is thus concluded that our predictions for the harmonic frequencies and the anharmonic constants are the most accurate estimates available. It is also shown that using cubic and quartic force constants determined with the correlation consistent polarized double zeta, cc-pVDZ, basis set in conjunction with the cc-pVQZ quadratic force constants and equilibrium geometry leads to accurate predictions for the fundamental vibrational frequencies of methane, suggesting that this approach may be a viable alternative for larger molecules. Using CCSD(T), core correlation is found to reduce the CH4 re by 0.0015 Å. Our best estimate for re is 1.0862±0.0005 Å. © 1995 American Institute of Physics. [ABSTRACT FROM AUTHOR]

We describe an analytic representation of the ArH2 potential energy surface which well reproduces the results of extensive ab initio electronic structure calculations. We also give an empirical modification of the function designed to improve agreement with experimental estimates of the van der Waals minimum. The analytic representation smoothly interpolates between the H+H and strong bonding H2 limits. In the fitting process, an accurate reproduction of regions of the potential expected to be important for high temperature (∼3000 K) collision processes is emphasized. Overall, the analytic representation well reproduces the anisotropy and H2 bond length dependence of the input data. [ABSTRACT FROM AUTHOR]

The quartic force field of ammonia is computed using basis sets of spdf/spd and spdfg/spdf quality and an augmented coupled cluster method. After correcting for Fermi resonance, the computed fundamentals and ν4 overtones agree on average to better than 3 cm-1 with the experimental ones except for ν2. The discrepancy for ν2 is principally due to higher-order anharmonicity effects. The computed ω1, ω3, and ω4 confirm the recent experimental determination by Lehmann and Coy but are associated with smaller error bars. The discrepancy between the computed and experimental ω2 is far outside the expected error range, which is also attributed to higher-order anharmonicity effects not accounted for in the experimental determination. Spectroscopic constants are predicted for a number of symmetric and asymmetric top isotopomers of NH3. [ABSTRACT FROM AUTHOR]