The generality of the GUGA MRCI approach in COLUMBUS for treating complex quantum chemistry.

The core part of the program system COLUMBUS allows highly efficient calculations using variational multireference (MR) methods in the framework of configuration interaction with single and double excitations (MR-CISD) and averaged quadratic coupled-cluster calculations (MR-AQCC), based on uncontracted sets of configurations and the graphical unitary group approach (GUGA). The availability of analytic MR-CISD and MR-AQCC energy gradients and analytic nonadiabatic couplings for MR-CISD enables exciting applications including, e.g., investigations of π-conjugated biradicaloid compounds, calculations of multitudes of excited states, development of diabatization procedures, and furnishing the electronic structure information for on-the-fly surface nonadiabatic dynamics. With fully variational uncontracted spin-orbit MRCI, COLUMBUS provides a unique possibility of performing high-level calculations on compounds containing heavy atoms up to lanthanides and actinides. Crucial for carrying out all of these calculations effectively is the availability of an efficient parallel code for the CI step. Configuration spaces of several billion in size now can be treated quite routinely on standard parallel computer clusters. Emerging developments in COLUMBUS, including the all configuration mean energy multiconfiguration self-consistent field method and the graphically contracted function method, promise to allow practically unlimited configuration space dimensions. Spin density based on the GUGA approach, analytic spin-orbit energy gradients, possibilities for local electron correlation MR calculations, development of general interfaces for nonadiabatic dynamics, and MRCI linear vibronic coupling models conclude this overview.

Electronic spectrum of the UO and UO(+) molecules.

Electronic theory calculations are applied to the study of the UO molecule and the UO(+) ion. Relativistic effective core potentials are used along with the accompanying valence spin-orbit operators. Polarized double-ς and triple-ς basis sets are used. Molecular orbitals are obtained from state-averaged multiconfiguration self-consistent field calculations and then used in multireference spin-orbit configuration interaction calculations with a number of millions of terms. The ground state of UO has open shells of 5f(3)7s(1), angular momentum Ω = 4, and a spin-orbit-induced avoided crossing near the equilibrium internuclear distance. Many UO excited states are studied with rotational constants, intensities, and experimental comparisons. The ground state of UO(+) is of 5f(3) nature with Ω = 9/2. Many UO(+) excited states are also studied. The open-shell nature of both UO and UO(+) leads to many low-lying excited states.

Early Experiences with Computational Quantum Chemistry.

A description is given of the progress in computational quantum chemistry in the early 1960s, the time of the early mainframe computers. In particular, the first calculation of the barrier to internal rotation in ethane and the first molecular application of perturbed self-consistent-field equations are described with the accompanying developments, trials, and tribulations.

Theoretical investigations of the time-resolved photodissociation dynamics of IBr(-).

The role of laser pulse width as well as other quantum mechanical effects in the interpretation of the observed time-resolved photoelectron spectra (TRPES) of IBr(-) are investigated using conditions that are chosen to reproduce those used for the experimental study of Mabbs et al. [ J. Chem. Phys. 2005 , 122 , 174305 ]. In that study, it was shown that one could correlate shifts in the frequency of the maximum in signal as a function of time to differences between the potential energies of the electronic states that are accessed by the pump and probe lasers. While this classical picture is attractive, it is based on a single trajectory with an initial I-Br separation that is ∼0.3 Šlonger than the equilibrium value. In addition, it does not include the role of the pulse widths and other possible quantum effects. In the present work, the six lowest energy electronic states of IBr(-) were calculated at the MR-SO-CISD/aug-cc-pVDZ level of theory/basis set as a function of the I-Br distance. The TRPES of IBr(-) were calculated in three pulse regimes: an infinitesimally short pulse, an intermediate pulse that has a temporal full width at half-maximum (fwhm) of 300 fs, which was chosen to match the experimental value, and one that is 3 times longer than the experimental value. The resulting spectra are qualitatively different, and the sources of these differences are discussed. The intermediate pulse provides very good agreement with experiment with the introduction of no adjustable parameters. The origins of the features of the experimental signal are discussed in terms of this fully quantum mechanical picture.

Resonant X-ray enhancement of the Auger effect in high-Z atoms, molecules, and nanoparticles: potential biomedical applications.

It is shown that X-ray absorption can be considerably enhanced at resonant energies corresponding to K-shell excitation into higher shells with electron vacancies following Auger emissions in high-Z elements and compounds employed in biomedical applications. We calculate Auger resonant probabilities and cross sections to obtain total mass attenuation coefficients with resonant cross sections and detailed resonance structures corresponding to Kalpha, Kbeta, Kgamma, Kdelta, and Keta complexes lying between 6.4-7.1 keV in iron and 67-80 keV in gold. The basic parameters were computed using the relativistic atomic structure codes and the R-matrix codes. It is found that the average enhancement at resonant energies is up to a factor of 1000 or more for associated K --> L, M, N, O, P transitions. The resonant energies in high-Z elements such as gold are sufficiently high to ensure significant penetration in body tissue, and hence the possibility of achieving X-radiation dose reduction commensurate with resonant enhancements for cancer theranostics using high-Z nanoparticles and molecular radiosensitizing agents embedded in malignant tumors. The in situ deposition of X-ray energy, followed by secondary photon and electron emission, will be localized at the tumor site. We also note the relevance of this work to the development of novel monochromatic or narrow-band X-ray emission sources for medical diagnostics and therapeutics.

Assessment of the accuracy of shape-consistent relativistic effective core potentials using multireference spin-orbit configuration interaction singles and doubles calculations of the ground and low-lying excited states of U(4+) and U(5+).

Multireference spin-orbit configuration interaction calculations were used to determine the accuracy of 60-, 68-, and 78-electron shape-consistent relativistic effective core potentials (RECPs) for uranium V and VI ground and low-lying excited states. Both 5f(n) and (5f6d)(n), (n = 1, 2) reference spaces were investigated using correlation-consistent double-zeta quality basis sets. Accuracy was assessed against gas-phase experimental spectra. The 68-electron RECP calculations yielded low relative and rms errors and predicted the empirical ordering of states most consistently.

Performance optimization of tensor contraction expressions for many-body methods in quantum chemistry.

Complex tensor contraction expressions arise in accurate electronic structure models in quantum chemistry, such as the coupled cluster method. This paper addresses two complementary aspects of performance optimization of such tensor contraction expressions. Transformations using algebraic properties of commutativity and associativity can be used to significantly decrease the number of arithmetic operations required for evaluation of these expressions. The identification of common subexpressions among a set of tensor contraction expressions can result in a reduction of the total number of operations required to evaluate the tensor contractions. The first part of the paper describes an effective algorithm for operation minimization with common subexpression identification and demonstrates its effectiveness on tensor contraction expressions for coupled cluster equations. The second part of the paper highlights the importance of data layout transformation in the optimization of tensor contraction computations on modern processors. A number of considerations, such as minimization of cache misses and utilization of multimedia vector instructions, are discussed. A library for efficient index permutation of multidimensional tensors is described, and experimental performance data is provided that demonstrates its effectiveness.

Jahn-Teller and related effects in the silver trimer. I. The ab initio calculation of spectroscopically observable parameters for the X 2E' and A 2E" electronic states.

Extensive ab initio calculations were performed for the X2E' and A2E" states of Ag3, using a newly constructed basis set for Ag. An important goal of these calculations is to guide the analysis of the experimentally observed A 2E"-X2E' electronic spectrum. Vibrational frequencies of Ag(3) for both the X and A states are reported. Spectroscopically obtainable parameters describing the Jahn-Teller effect are calculated for the X and A states. The magnitude of the spin-orbit effects for this relativistic system was also calculated for the X2E' and A 2E" states. Using all this information, the X-A electronic spectrum is predicted for Ag(3). Additionally, the geometries and symmetries of the global minima and saddle points as well as the barrier to pseudorotation around the moat of the potential energy surface are determined for both states.

Jahn-Teller and related effects in the silver trimer. II: vibrational analysis of the A 2E"-X 2E' electronic transition.

The laser-excited, jet-cooled A 2E"-X 2E' electronic spectrum of the silver trimer yields detailed information about its A- and X-state vibronic structure. Following extensive parameter fitting, the absorption and emission spectra are simulated and the bands are assigned. The Jahn-Teller analysis includes both linear and quadratic coupling terms, considered simultaneously with spin-orbit coupling. The spin-orbit splitting is shown to be largely quenched in both the A and X electronic states. The Jahn-Teller analysis of the A and X vibronic structures reveals the distortion of their corresponding potential energy surfaces.

Theoretical investigation of the binding energies of the iodide ion and xenon atom with decaborane.

The interaction of decaborane (B(10)H(14)) with the I(-) ion and the (isoelectronic) Xe atom is investigated using a number of theoretical methods: MP2, CCSD(T), CCSD, spin-orbit CISD, and DFT using the B3LYP, B3PW91, PW91PW91, and PBE0 methods. All non-DFT and some DFT methods agree that B(10)H(14)I(-) is bound by charge-dipole electrostatic forces, charge- and dipole-induced-dipole forces, and dispersion forces, while B(10)H(14)Xe is bound by dipole-induced-dipole forces and dispersion forces. Counterpoise corrections are necessary to obtain reliable results. Relativistic effective core potentials were used for the I, Xe, and B atoms. Basis sets for use with these potentials are discussed as is the question of basis set balance in molecules. We find B(10)H(14)I(-) to be bound by 19.8 kcal/mol and B(10)H(14)Xe by 1.1 kcal/mol, indicating that the charge and polarizability of I(-) play the major role in the interaction energy.

Oxidation of gas-phase protactinium ions, Pa+ and Pa2+: formation and properties of PaO2(2+)(g), protactinyl.

Oxidation reactions of bare and ligated, monopositive, and dipositive Pa ions in the gas phase were studied by Fourier transform ion cyclotron resonance mass spectrometry. Seven oxidants were employed, ranging from the thermodynamically robust N(2)O to the relatively weak CH(2)O-all oxidized Pa(+) to PaO(+) and PaO(+) to PaO(2)(+). On the basis of experimental observations, it was established that D[Pa(+)-O] and D[OPa(+)-O] > or = 751 kJ mol(-1). Estimates for D[Pa(+)-O], D[OPa(+)-O], IE[PaO], and IE[PaO(2)] were also obtained. The seven oxidants reacted with Pa(2+) to produce PaO(2+), indicating that D[Pa(2+)-O] > or = 751 kJ mol(-1). A particularly notable finding was the oxidation of PaO(2+) by N(2)O to PaO(2)(2+), a species, which formally comprises Pa(VI). Collision-induced dissociation of PaO(2)(2+) suggested the protactinyl connectivity, {O-Pa-O}(2+). The experimentally determined IE[PaO(2)(+)] approximately 16.6 eV is in agreement with self-consistent-field and configuration interaction calculations for PaO(2)(+) and PaO(2)(2+). These calculations provide insights into the electronic structures of these ions and indicate the participation of 5f orbitals in bonding and a partial "6p hole" in the case of protactinyl. It was found that PaO(2)(2+) catalyzes the oxidation of CO by N(2)O-such O atom transport via a dipositive metal oxide ion is distinctive. It was also observed that PaO(2)(2+) is capable of activating H(2) to form the stable PaO(2)H(2+) ion.