A theoretical investigation of the hydrolysis of uranium hexafluoride: the initiation mechanism and vibrational spectroscopy.

Depleted uranium hexafluoride (UF6), a stockpiled byproduct of the nuclear fuel cycle, reacts readily with atmospheric humidity, but the mechanism is poorly understood. We compare several potential initiation steps at a consistent level of theory, generating underlying structures and vibrational modes using hybrid density functional theory (DFT) and computing relative energies of stationary points with double-hybrid (DH) DFT. A benchmark comparison is performed to assess the quality of DH-DFT data using reference energy differences obtained using a complete-basis-limit coupled-cluster (CC) composite method. The associated large-basis CC computations were enabled by a new general-purpose pseudopotential capability implemented as part of this work. Dispersion-corrected parameter-free DH-DFT methods, namely PBE0-DH-D3(BJ) and PBE-QIDH-D3(BJ), provided mean unsigned errors within chemical accuracy (1 kcal mol-1) for a set of barrier heights corresponding to the most energetically favorable initiation steps. The hydrolysis mechanism is found to proceed via intermolecular hydrogen transfer within van der Waals complexes involving UF6, UF5OH, and UOF4, in agreement with previous studies, followed by the formation of a previously unappreciated dihydroxide intermediate, UF4(OH)2. The dihydroxide is predicted to form under both kinetic and thermodynamic control, and, unlike the alternate pathway leading to the UO2F2 monomer, its reaction energy is exothermic, in agreement with observation. Finally, harmonic and anharmonic vibrational simulations are performed to reinterpret literature infrared spectroscopy in light of this newly identified species.

Advanced concepts in electronic structure (ACES) software programs.

The advanced concepts in electronic structure (ACES) programs are products of the Bartlett research group at the University of Florida. They consist of ACES II, which is serial, and ACES III and Aces4, which are massively parallel. All three programs are publically available free of charge. The focus of the ACES implementations is coupled cluster theory and many-body-perturbation theory. We give an overview of the ACES programs, discuss the many features of the program systems, and document the number of benchmarks.

Correlation correction to configuration interaction singles from coupled cluster perturbation theory.

A new state specific correlation correction to configuration interaction singles (CIS) excitation energies is presented using coupled cluster perturbation theory (CCPT). General expressions for CIS-CCPT are derived and expanded explicitly to first-order in the wavefunction and second-order in the energy. By virtue of the nature of CCPT this method is a priori size extensive and incorporates infinite-order effects into the wavefunction. This results in a balanced singles space excited state theory that at second-order is an improvement over the ubiquitous CIS(D) method and comparable in quality to equation of motion coupled cluster (EOM-CC). A modest test set composed of the first four excited states from nine small organic molecules was used to quantify the accuracy and consistency of the CIS-CCPT2 excitation energies and density of states. We find that CIS-CCPT2 has a standard deviation error of 0.18 eV for excitation energies and 0.14 eV for density of states compared to EOM-CC, a factor of two better than CIS(D) with a significant reduction in the maximum deviation as well.

Benchmarking for perturbative triple-excitations in EE-EOM-CC methods.

Perturbative triples corrections ((T) and (T̃)) to the equation of motion coupled cluster singles and doubles (EOM-CCSD) are rederived and implemented in a pilot parallel code. The vertical excitation energies of molecules in the test set of Sauer et al. [J. Chem. Theor. Comput. 2009, 5, 555] are reported and compared to the iterative EOM-CCSDT-3 method. The average absolute deviations of EOM-CCSD(T) and EOM-CCSD(T̃) from EOM-CCSDT-3 over this wide test set are 0.06 and 0.18 eV, respectively. The poor performance of the latter suggests misbalanced handling of the (T̃) terms. Scaling curves for EOM-CCSD(T) are also presented to demonstrate the performance across multiple compute nodes, thus enabling the routine and accurate study of excited states for ever larger molecular systems.

Benchmark studies on the building blocks of DNA. 1. Superiority of coupled cluster methods in describing the excited states of nucleobases in the Franck-Condon region.

Equation of motion excitation energy coupled-cluster (EOMEE-CC) methods including perturbative triple excitations have been used to set benchmark results for the excitation energy and oscillator strength of the building units of DNA, i.e., cytosine, guanine, adenine and thymine. In all cases the lowest twelve transitions have been considered including valence and Rydberg ones. Triple-ζ basis sets with diffuse functions have been used and the results are compared to CC2, CASPT2, TDDFT, and DFT/MRCI results from the literature. The results clearly show that it is only the EOMEE-CCSD(T) that is capable of providing accuracy of about 0.1 eV. EOMEE-CCSD systematically overshoots the energy of all types of transitions by 0.1-0.3 eV, whereas CC2 is surprisingly accurate for ππ* transitions but fails (often badly) for nπ* and Rydberg transitions. DFT and CASPT2 seem to give reliable results for the lowest transition, but the error increases fast with the excitation level. The differences in the excitation energies often change the energy ordering of the states, which should even influence the conclusions of excited state dynamics obtained with these approximate methods. The results call for further benchmark calculations on larger building blocks of DNA (nucleosides, basis pairs) at the CCSD(T) level.

RDX geometries, excited states, and revised energy ordering of conformers via MP2 and CCSD(T) methodologies: insights into decomposition mechanism.

The geometries, harmonic frequencies, elec-tronic excitation levels, and energetic orderings of various conformers of RDX have been computed at the ab initio MP2 and CCSD(T) levels, providing more reliable results than have been previously obtained. We observe that the various local minimum-energy conformers are all competitive for being the absolute minimum and that, at reasonable temperatures, several conformers will appreciably contribute to the population of RDX. As a result, we have concluded that any mechanistic study to investigate thermal decomposition can reasonably begin from any one of the cyclohexane conformers of RDX. As such, it is necessary to consider the transition states for each RDX conformer to gauge what the activation energy is. Homolytic bond dissociation has long been speculated to be critical to detonation; we report here the most accurate estimates of homolytic BDEs yet calculated, likely to be accurate within 3 kcal mol(-1). The differences in energy for homolytic BDEs among all the possible RDR conformers are again small, such that most all of the conformers can reasonably be speculated as the next step in the mechanism starting from the RDR radical.

Parallel implementation of the equation-of-motion coupled-cluster singles and doubles method and application for radical adducts of cytosine.

The equation-of-motion coupled-cluster singles and doubles (EOM-CCSD) method has been implemented into the massively parallel ACES III program using two alternative strategies: (1) storing the entire EOM Hamiltonian matrix prior to diagonalization and (2) recomputing the four-virtual part of the matrix from integrals in a direct mode. The second is found to be far more efficient. EOM-CC shows virtually ideal scaling from 32 to 256 processors. With basis sets as large as 552 functions, the program was applied to determine vertical excitation energies for five cytosine radical adducts of -OH and -H at three sites C5, C6, and N3. These radicals are considered to play an important role in radiation induced DNA damage. The excitation energy spectrum shows two distinct patterns for the lowest transitions distinguishing the C6-OH, C6-H, and N3-H adducts from the C5-OH and C5-H. The results indicate that the two lowest transitions of the C6-OH isomer should contribute to the experimentally observed absorption maximum at 2.88 eV, while the third and fourth transitions of C6-OH and the two lowest transitions of C5-OH contribute to the 3.65 eV absorption maximum. We also report the CCSD with noniterative triples correction [CCSD(T)] relative energies of the C5-OH and C6-OH adducts using 1000 processors.

Ab initio correlation functionals from second-order perturbation theory.

Orbital-dependent exchange-correlation functionals are not limited by the explicit dependence on the density and present an attractive alternative to conventional functionals. With the successful implementation of the exact orbital-dependent exchange functional, the challenge lies in developing orbital-dependent approximations for the correlation functional. Ab initio many-body methods can provide such approximations. In particular, perturbation theory with the Kohn-Sham model as the reference [Görling and Levy, Phys. Rev. A 50, 196 (1994)] defines the exact correlation functional via an infinite perturbation series. The second-order term of these series gives the lowest-order approximation to the correlation functional. However, it has been suggested [Bartlett et al., J. Chem. Phys. 122, 034104 (2005)] that the Kohn-Sham Hamiltonian is not the optimal choice for the perturbation expansion and a different reference Hamiltonian may lead to an improved perturbation series and more accurate second-order approximation. Here, we demonstrate explicitly that the modified series can be used to define superior functional and potential. We present results of atomic and molecular calculations with both second-order functionals. Our results demonstrate that the modified functional offers a significantly improved description of the correlation effects as it does not suffer from convergence problems and results in energies and densities that are more accurate than those obtained with second-order Møller-Plesset perturbation theory or generalized-gradient approximation functionals.

Ab initio density functional theory: the best of both worlds?

Density functional theory (DFT), in its current local, gradient corrected, and hybrid implementations and their extensions, is approaching an impasse. To continue to progress toward the quality of results demanded by today's ab initio quantum chemistry encourages a new direction. We believe ab initio DFT is a promising route to pursue. Whereas conventional DFT cannot describe weak interactions, photoelectron spectra, or many potential energy surfaces, ab initio DFT, even in its initial, optimized effective potential, second-order many-body perturbation theory form [OEP (2)-semi canonical], is shown to do all well. In fact, we obtain accuracy that frequently exceeds MP2, being competitive with coupled-cluster theory in some cases. Furthermore, this is accomplished within a relatively fast computational procedure that scales like iterative second order. We illustrate our results with several molecular examples including Ne2,Be2,F2, and benzene.

Jahn-Teller effect in van der Waals complexes; Ar-C6H6 + and Ar-C6D6 +.

The two asymptotically degenerate potential energy surfaces of argon interacting with the X (2)E(1g) ground state benzene(+) cation were calculated ab initio from the interaction energy of the neutral Ar-benzene complex given by Koch et al. [J. Chem. Phys. 111, 198 (1999)] and the difference of the geometry-dependent ionization energies of the complex and the benzene monomer computed by the outer valence Green's function method. Coinciding minima in the two potential surfaces of the ionic complex occur for Ar on the C(6v) symmetry axis of benzene(+) (the z axis) at z(e)=3.506 A. The binding energy D(e) of 520 cm(-1) is only 34% larger than the value for the neutral Ar-benzene complex. The higher one of the two surfaces is similar in shape to the neutral Ar-benzene potential, the lower potential is much flatter in the (x,y) bend direction. Nonadiabatic (Jahn-Teller) coupling was taken into account by transformation of the two adiabatic potentials to a two-by-two matrix of diabatic potentials. This transformation is based on the assumption that the adiabatic states of the Ar-benzene(+) complex geometrically follow the Ar atom. Ab initio calculations of the nonadiabatic coupling matrix element between the adiabatic states with the two-state-averaged CAS-SCF(5,6) method confirmed the validity of this assumption. The bound vibronic states of both Ar-C(6)H(6) (+) and Ar-C(6)D(6) (+) were computed with this two-state diabatic model in a basis of three-dimensional harmonic oscillator functions for the van der Waals modes. The binding energy D(0)=480 cm(-1) of the perdeuterated complex agrees well with the experimental upper bound of 485 cm(-1). The ground and excited vibronic levels and wave functions were used, with a simple model dipole function, to generate a theoretical far-infrared spectrum. Strong absorption lines were found at 10.1 cm(-1) (bend) and 47.9 cm(-1) (stretch) that agree well with measurements. The unusually low bend frequency is related to the flatness of the lower adiabatic potential in the (x,y) direction. The van der Waals bend mode of e(1) symmetry is quadratically Jahn-Teller active and shows a large splitting, with vibronic levels of A(1), E(2), and A(2) symmetry at 1.3, 10.1, and 50.2 cm(-1). The level at 1.3 cm(-1) leads to a strong absorption line as well, which could not be measured because it is too close to the monomer line. The level at 50.2 cm(-1) gives rise to weaker absorption. Several other weak lines in the frequency range of 10 to 60 cm(-1) were found.

Theoretical study of the He-HF+ complex. I. The two asymptotically degenerate ground state potential energy surfaces.

Two three-dimensional potential energy surfaces (PESs) are reported for the cationic complex He-HF+; they are degenerate for linear geometries of the complex and correlate with the doubly degenerate X2Pi ground state of the HF+monomer. The PESs are computed from the interaction energies of the neutral dimer and the ionization potentials of the He-HF complex and the HF molecule. Ionization potentials are obtained from the outer valence Green's function (OVGF) method, while the energies of the neutral species are computed by means of the single and double coupled-cluster method with perturbative triples [CCSD(T)]. For comparison, interaction energies of the ionic complex were computed also by the use of the partially spin-restricted variant of the CCSD(T) method. After asymptotic scaling of the OVGF results, good agreement is found between the two methods. A single global minimum is found in the PES, for the linear He-HF+ geometry. The well depth and equilibrium separation are 2.240 A and 1631.3 cm(-1), respectively, at an HF+ bond length r=1.0012 A, in rather good agreement with results of Schmelz and Rosmus [Chem. Phys. Lett. 220, 117 (1994)]. The well depth depends much more strongly on the internuclear H-F separation than in the neutral He-HF complex and the global minimum in a full three-dimensional PES occurs at r=1.0273 A.