Juana Vázquez

HEAT: High accuracy extrapolated ab initio thermochemistry

A theoretical model chemistry designed to achieve high accuracy for enthalpies of formation of atoms and small molecules is described. This approach is entirely independent of experimental data and contains no empirical scaling factors, and includes a treatment of electron correlation up to the full coupled-cluster singles, doubles, triples and quadruples approach. Energies are further augmented by anharmonic zero-point vibrational energies, a scalar relativistic correction, first-order spin-orbit coupling, and the diagonal Born-Oppenheimer correction. The accuracy of the approach is assessed by several means. Enthalpies of formation (at 0 K) calculated for a test suite of 31 atoms and molecules via direct calculation of the corresponding elemental formation reactions are within 1 kJ mol(-1) to experiment in all cases. Given the quite different bonding environments in the product and reactant sides of these reactions, the results strongly indicate that even greater accuracy may be expected in reactions that preserve (either exactly or approximately) the number and types of chemical bonds.

High-accuracy extrapolated ab initio thermochemistry. III. Additional improvements and overview

Effects of increased basis-set size as well as a correlated treatment of the diagonal Born-Oppenheimer approximation are studied within the context of the high-accuracy extrapolated ab initio thermochemistry (HEAT) theoretical model chemistry. It is found that the addition of these ostensible improvements does little to increase the overall accuracy of HEAT for the determination of molecular atomization energies. Fortuitous cancellation of high-level effects is shown to give the overall HEAT strategy an accuracy that is, in fact, higher than most of its individual components. In addition, the issue of core-valence electron correlation separation is explored; it is found that approximate additive treatments of the two effects have limitations that are significant in the realm of <1 kJ mol(-1) theoretical thermochemistry.

High-accuracy extrapolated ab initio thermochemistry. II. Minor improvements to the protocol and a vital simplification

The recently developed high-accuracy extrapolated ab initio thermochemistry method for theoretical thermochemistry, which is intimately related to other high-precision protocols such as the Weizmann-3 and focal-point approaches, is revisited. Some minor improvements in theoretical rigor are introduced which do not lead to any significant additional computational overhead, but are shown to have a negligible overall effect on the accuracy. In addition, the method is extended to completely treat electron correlation effects up to pentuple excitations. The use of an approximate treatment of quadruple and pentuple excitations is suggested; the former as a pragmatic approximation for standard cases and the latter when extremely high accuracy is required. For a test suite of molecules that have rather precisely known enthalpies of formation {as taken from the active thermochemical tables of Ruscic and co-workers [Lecture Notes in Computer Science, edited by M. Parashar (Springer, Berlin, 2002), Vol. 2536, pp. 25-38; J. Phys. Chem. A 108, 9979 (2004)]}, the largest deviations between theory and experiment are 0.52, -0.70, and 0.51 kJ mol(-1) for the latter three methods, respectively. Some perspective is provided on this level of accuracy, and sources of remaining systematic deficiencies in the approaches are discussed.

Simple(r) algebraic equation for transition moments of fundamental transitions in vibrational second-order perturbation theory

Rayleigh–Schrödinger perturbation theory is applied in second order to obtain an expression for the transition moment of vibrational fundamentals. The reported equation is considerably simpler than previous ones. Applications for H2O and HFCO are reported using force fields and dipole functions calculated at the CCSD(T) level of theory with different basis sets.

A new experimental absolute nuclear magnetic shielding scale for oxygen based on the rotational hyperfine structure of H2O17

The hyperfine structure in the rotational spectrum of water containing (17)O has been investigated experimentally and by means of quantum-chemical calculations. The Lamb-dip technique has been used to resolve the hyperfine structure due to spin-rotation as well as spin-spin interactions and allowed the determination of the corresponding hyperfine parameters with high accuracy. The experimental investigation and, in particular, the analysis of the spectra have been supported by quantum-chemical computations at the coupled-cluster level. The experimental (17)O isotropic spin-rotation constant of H(2)(17)O has been used in a further step for the determination of the paramagnetic part of the corresponding nuclear magnetic shielding constant, whereas the diamagnetic contribution as well as vibrational and temperature corrections have been obtained from quantum-chemical calculations. This joint procedure leads to a value of 325.3(3) ppm for the oxygen shielding in H(2)(17)O at 300 K, in good agreement with pure theoretical predictions, and in this way provides the basis for a new absolute oxygen shielding scale.

Treatment of Fermi resonance effects on transition moments in vibrational perturbation theory

An algebraic expression for the vibrational transition moment of fundamental transitions within the context of second-order vibrational perturbation theory (VPT2) has been modified to account for cases in which the fundamental interacts through a Fermi resonance with close-lying two-quantum levels. This procedure is applied to the formaldehyde molecule, and it is shown that explicit treatment of the 5 1 ≈ 21 61 ≈ 3161 resonance is necessary to give intensities of the ν5 transitions that agree qualitatively with that seen experimentally. In addition, use of the Almlöf–Taylor atomic natural orbital basis sets results in an extremely close correspondence between predicted and observed level positions; the largest difference found for the 23 one- and two-quantum transitions for which experimental assignments have been made is 15 cm−1. This is a remarkable result, especially when one considers that seven of these levels are involved in relatively strong resonances.

Dissociation Energy of the HOOO Radical

The dissociation of the hydrotrioxy (HOOO) radical to OH and O(2) has been studied theoretically using coupled-cluster methods. The calculated dissociation energy for the trans-HOOO isomer is 2.5 kcal mol(-1) including zero-point corrections. The minimum energy path to dissociation has been explored and an exit barrier has been revealed, which may help to rationalize the apparent disagreement between theory and experiment on the magnitude of the bond energy.

Calculated stretching overtone levels and Darling–Dennison resonances in water: a triumph of simple theoretical approaches

The coupled-cluster singles and doubles treatment with a perturbative treatment of triple excitations known as CCSD(T) has been used in conjunction with a hierarchy of atomic natural orbital basis sets to study stretching levels in water up to 2 eV above the zero-point level. Agreement with experiment obtained with perturbation theory augmented by a simple treatment of resonances suggested by Lehmann is quite remarkable in spite of the well-known and strong Darling–Dennison resonances in this system. With the largest basis sets, deviation from experiment is ca. 20 cm−1 at energies in the 15,000–16,000 cm−1 range.

High-Accuracy Extrapolated ab Initio Thermochemistry of the Propargyl Radical and the Singlet C3H2 Carbenes

Total atomization energies and enthalpies of formation (0 and 298.15 K) are evaluated using the high-accuracy extrapolated ab initio thermochemistry (HEAT) scheme for the two stable singlet C(3)H(2) carbenes [cyclopropenylidene (c-C(3)H(2)) and propadienylidene (vinylidencarbene, l-C(3)H(2))], as well as for the 2-propynyl (propargyl, C(3)H(3)) radical. In the case of propargyl, the HEAT protocol predicts an enthalpy of formation of 354.9 +/- 1.0 kJ mol(-1) for 0 K; corresponding values of 498.1 +/- 1.0 and 555.6 +/- 1.0 kJ mol(-1) are estimated for c-C(3)H(2) and l-C(3)H(2). Additional consideration of temperature corrections leads to estimates of 352.2 +/- 1.0, 497.1 +/- 1.0, and 556.7 +/- 1.0 kJ mol(-1) for the heats of formation at 298.15 K of the propargyl radical, c-C(3)H(2), and l-C(3)H(2), respectively. Potential strategies for simplifying the HEAT protocol are also investigated and shown to have negligible impact on accuracy.

Towards highly accurate ab initio thermochemistry of larger systems: Benzene

The high accuracy extrapolated ab initio thermochemistry (HEAT) protocol is applied to compute the total atomization energy (TAE) and the heat of formation of benzene. Large-scale coupled-cluster calculations with more than 1500 basis functions and 42 correlated electrons as well as zero-point energies based on full cubic and (semi)diagonal quartic force fields obtained with the coupled-cluster singles and doubles with perturbative treatment of the triples method and atomic natural orbital (ANO) triple- and quadruple-zeta basis sets are presented. The performance of modifications to the HEAT scheme and the scaling properties of its contributions with respect to the system size are investigated. A purely quantum-chemical TAE and associated conservative error bar of 5463.0 ± 3.1 kJ mol(-1) are obtained, while the corresponding 95% confidence interval, based on a statistical analysis of HEAT results for other and related molecules, is ± 1.8 kJ mol(-1). The heat of formation of benzene is determined to be 101.5 ± 2.0 kJ mol(-1) and 83.9 ± 2.1 kJ mol(-1) at 0 K and 298.15 K, respectively.