Molcas 8: New capabilities for multiconfigurational quantum chemical calculations across the periodic table.

In this report, we summarize and describe the recent unique updates and additions to the Molcas quantum chemistry program suite as contained in release version 8. These updates include natural and spin orbitals for studies of magnetic properties, local and linear scaling methods for the Douglas-Kroll-Hess transformation, the generalized active space concept in MCSCF methods, a combination of multiconfigurational wave functions with density functional theory in the MC-PDFT method, additional methods for computation of magnetic properties, methods for diabatization, analytical gradients of state average complete active space SCF in association with density fitting, methods for constrained fragment optimization, large-scale parallel multireference configuration interaction including analytic gradients via the interface to the Columbus package, and approximations of the CASPT2 method to be used for computations of large systems. In addition, the report includes the description of a computational machinery for nonlinear optical spectroscopy through an interface to the QM/MM package Cobramm. Further, a module to run molecular dynamics simulations is added, two surface hopping algorithms are included to enable nonadiabatic calculations, and the DQ method for diabatization is added. Finally, we report on the subject of improvements with respects to alternative file options and parallelization.

Magnetic properties and electronic structure of neptunyl(VI) complexes: wavefunctions, orbitals, and crystal-field models.

The electronic structure and magnetic properties of neptunyl(VI), NpO2(2+), and two neptunyl complexes, [NpO2(NO3)3](-) and [NpO2Cl4](2-), were studied with a combination of theoretical methods: ab initio relativistic wavefunction methods and density functional theory (DFT), as well as crystal-field (CF) models with parameters extracted from the ab initio calculations. Natural orbitals for electron density and spin magnetization from wavefunctions including spin-orbit coupling were employed to analyze the connection between the electronic structure and magnetic properties, and to link the results from CF models to the ab initio data. Free complex ions and systems embedded in a crystal environment were studied. Of prime interest were the electron paramagnetic resonance g-factors and their relation to the complex geometry, ligand coordination, and nature of the nonbonding 5f orbitals. The g-factors were calculated for the ground and excited states. For [NpO2Cl4](2-), a strong influence of the environment of the complex on its magnetic behavior was demonstrated. Kohn-Sham DFT with standard functionals can produce reasonable g-factors as long as the calculation converges to a solution resembling the electronic state of interest. However, this is not always straightforward.

Magnetic resonance properties of actinyl carbonate complexes and plutonyl(VI)-tris-nitrate.

Electronic structures and magnetic properties of actinyl ions AnO2(n+) (An = U, Np, and Pu) and the equatorially coordinated carbonate complexes [UO2(CO3)3](5­), [NpO2(CO3)3](4­), and [PuO2(CO3)3](4­) are investigated by ab initio quantum chemical calculations. The complex [PuO2(NO3)3](−) is also included because of experimentally available g-factors and for comparison with a previous study of [NpO2(NO3)3](−) (Chem.­Eur. J. 2014, 20, 7994-8011). The results are rationalized with the help of crystal-field (CF)-type models with parameters extracted from the ab initio calculations, and with the help of natural orbitals and natural spin orbitals contributing to the magnetic properties and the unpaired spin distribution, generated from the spin­orbit wave functions. These orbitals resemble textbooklike representations of the actinide 5f orbitals. Calculated paramagnetic susceptibilities are used to estimate dipolar 13C chemical shifts for the carbonate ligands. Their signs and order of magnitude are compared to paramagnetic effects observed experimentally in NMR spectra. The results indicate that the experimental spectra are also influenced by contact shifts.

Theoretical investigation of paramagnetic NMR shifts in transition metal acetylacetonato complexes: analysis of signs, magnitudes, and the role of the covalency of ligand-metal bonding.

Ligand chemical shifts are calculated and analyzed for three paramagnetic transition metal tris-acetylacetonato (acac) complexes, namely high-spin Fe(III) and Cr(III), and low-spin Ru(III), using scalar relativistic density functional theory (DFT). The signs and magnitudes of the paramagnetic NMR ligand chemical shifts are directly related to the extent of covalent acac oxygen-to-metal σ donation involving unoccupied metal valence d(σ) acceptor orbitals. The role of delocalization of metal-centered spin density over the ligand atoms plays a minor secondary role. Of particular interest is the origin of the sign and magnitude of the methyl carbon chemical shift in the acac ligands, and the role played by the DFT delocalization error when calculating such shifts. It is found that the α versus ß spin balance of oxygen σ donation to metal valence d acceptor orbitals is responsible for the sign and the magnitude of the ligand methyl carbon chemical shift. A problematic case is the methyl carbon shift of Fe(acac)(3). Most functionals produce shifts in excess of 1400 ppm, whereas the experimental shift is approximately 279 ppm. Range-separated hybrid functionals that are optimally tuned for Fe(acac)(3) based on DFT energetic criteria predict a lower limit of about 2000 ppm for the methyl carbon shift of the high-spin electronic configuration. Since the experimental value is based on a very strongly broadened signal it is possibly unreliable.

Effects from spin-orbit coupling on electron-nucleus hyperfine coupling calculated at the restricted active space level for Kramers doublets.

Calculations of electron-nucleus hyperfine coupling were implemented at the restricted active space state interaction (RASSI) level to treat spin-orbit (SO) coupling, based on scalar relativistic restricted active space wave functions. The current implementation is suitable for light atomic systems, for light ligand atoms in heavy metal complexes, and for spin-orbit coupling-induced hyperfine coupling of heavy atoms if the unpaired electrons are described by orbitals with high angular momentum. Spin polarization is reasonably well treated by allowing one hole and one electron in a window of active orbitals ('ras1', 'ras3') surrounding the principal active space ('ras2'). A benchmark set of Kramers doublet states of molecules with light and heavy atoms is used to evaluate the approach and verify the implementation. For NpF6, the impact of SO coupling on the Np and F hyperfine coupling tensors is investigated in detail. It is demonstrated that the Np hyperfine coupling is strongly dominated by SO effects, that there is a large SO effect on the F hyperfine tensor components, and that SO coupling causes the fluorine dipolar term to acquire an isotropic component.

Scalar Relativistic Computations and Localized Orbital Analyses of Nuclear Hyperfine Coupling and Paramagnetic NMR Chemical Shifts.

A method is reported by which calculated hyperfine coupling constants (HFCCs) and paramagnetic NMR (pNMR) chemical shifts can be analyzed in a chemically intuitive way by decomposition into contributions from localized molecular orbitals (LMOs). A new module for density functional calculations with nonhybrid functionals, global hybrids, and range-separated hybrids, utilizing the two-component relativistic zeroth-order regular approximation (ZORA), has been implemented in the parallel open-source NWChem quantum chemistry package. Benchmark results are reported for a test set of few-atom molecules with light and heavy elements. Finite nucleus effects on (199)Hg HFCCs are shown to be on the order of -11 to -15%. A proof of concept for the LMO analysis is provided for the metal and fluorine HFCCs of TiF3 and NpF6. Calculated pNMR chemical shifts are reported for the 2-methylphenyl-t-butylnitroxide radical and for five cyclopentadienyl (Cp) sandwich complexes with 3d metals. Nickelocene and vanadocene carbon pNMR shifts are analyzed in detail, demonstrating that the large carbon pNMR shifts calculated as +1540 for Ni (exptl.: +1514) and -443 for V (exptl.: -510) are caused by different spin-polarization mechanisms. For Ni, Cp to Ni π back-donation dominates the result, whereas for vanadocene, V to Cp σ donation with relaxation of the carbon 1s shells can be identified as the dominant mechanism.

Calculation of Hyperfine Tensors and Paramagnetic NMR Shifts Using the Relativistic Zeroth-Order Regular Approximation and Density Functional Theory.

Density functional theory (DFT) calculations of molecular hyperfine tensors were implemented as a second derivative property within the two-component relativistic zeroth-order regular approximation (ZORA). Hyperfine coupling constants were computed for systems ranging from light atomic radicals to molecules with heavy d and f block elements. For comparison, computations were also performed with a ZORA first-order derivative approach. In each set of computations, Slater-type basis sets have been used. The implementation allows for nonhybrid and hybrid DFT calculations and incorporates a Gaussian finite nucleus model. A comparison of results calculated with the PBE nonhybrid and the PBE0 hybrid functional is provided. Comparisons with differing basis sets and incorporation of finite-nucleus corrections are discussed. The second derivative method is applied to calculations of paramagnetic NMR ligand chemical shifts of three Ru(III) complexes. The results are consistent with those calculated using a first-order derivative method, and the results are consistent for different functionals used. A comparison of two different methods of calculating pseudo-contact shifts, one using the full hyperfine tensor and one assuming a point-charge paramagnetic center, is made for the Ru(III) complexes.