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

THE ELECTRONIC SPECTRA OF THE AMERICYL AND CURYLIONS

$^{a}$S. Matsika and R. M. Pitzer, unpublished, 1999. $^{b}$S. Brozell and R. M. Pitzer. 53rd Ohio State University International symposium, Paper RB05, p. 207, 1998.Author Institution: Department of Chemistry, The Ohio State UniversityLarge graphical unitary group approach (GUGA) spin-orbit multireference configuration interaction (SOCI) singles and doubles calculations have been performed on $Am^{2+}$, $AmO^{2+}$, $Cmo_{2}{^{2+}}$, and $CmO^{2+}$. The ground state of the americyl cation is $\delta^{2}_{u} {\phi^{1}_{u}} {^{4}\Phi_{3/2u}}$. The first ligand to metal charge transfer (LMCT) state is $3\sigma^{1}_{u} \delta^{2}_{u} \phi ^{2}_{u} {^{6}\Sigma}_{5/2u}$. The ground and first LMCT states of the isoelectronic dioxoamericium (V) and curyl cations are $\delta^{2}_{u} \phi^{2}_{u} {^{5}\Sigma}^{+}_{0+g}$ and $3\sigma^{1}_{u} \phi ^{2}_{u} 3\pi ^{1}_{u} {^{7}\Pi}_{0+g}$. The dioxocurium (V) cation ground state is $\delta^{2}_{u} \phi ^{2}_{u} 3\pi^{1}_{u} {^{6}\Pi}_{3/2u}$. Agreement with experimental LMCT excitation energies is good. Other intense electronic transitions are $f \leftarrow f$. Recent software developments have enabled the calculation of electric dipole transition moments from GUGA SOCI $wavefunctions^{a}$. Tentative $assignments^{b}$ of the $f \leftarrow f$ transitions have been reevaluated with the new soft ware. Calculations are planned to search for nonlinear minima of these ions. The actinides are modeled with relativistic effective core potentials and Gaussian correlation consistent double-zeta plus polarization (ce-PVDZ) basis sets

AB INITIO STUDY OF THE AMERICYL AND CURYL IONS

$^{a}$ G.M. Begun et al., Inorg. Chem. 23, 1914 (1984).Author Institution: Department of Chemistry, The Ohio State UniversityRestricted Hartree-Fock (RHF) and spin-orbit configuration-interaction (SOCI) calculations were performed on the ground and low-lying excited states of the americyl ion, $AmO^{2+}_{2}$. The RHF $\delta_{u}^{2}\phi_{u}^{1},^{4}\Phi_{u}$ ground state symmetric stretch potential energy curve yields a frequency of $900 cm^{-1}$ compared to an experimental value of $730 cm^{-1}$ from Raman spectroscopy on aqueous $solutions^{a}$ of $AmO^{1+}_{2}$. The SOCI ground state is $\delta_{a}^{2}\phi_{u}^{1},{^{4}}\Phi_{3/2u}$; the excited states are regular components from the above term followed by irregular components from the $\delta_{u}^{1}\phi_{u}^{2}m{^{1}}\Delta_{u}$ term. The low energy transitions are $f \to f$. Analogous calculations are planned for the curyl ion, $CmO^{2+}_{2}$. The actinides are modeled with relativistic effective core potentials and Gaussian correlation consistent double-zeta plus polarization (cc-p VDZ) basis sets

THE ENERGY LEVELS OF LANTHANIDE IONS, FREE AND DOPED INTO CALCIUM FLUORIDE

Author Institution: Department of Chemistry, The Ohio State UniversityLanthanide ions, both free and in $CaF_{2}$ crystals, have been studied extensively via optical spectroscopy and electron paramagnetic resonance. Ab initio spin-orbit configuration interaction calculations are performed on some of these systems. The primary properties of interest are the energy levels and magnetic moments. The $CaF_{2}$ host is modeld with a large finite cluster of ions which approximate the Madelung potential of the crystal lattice. Lanthanide ions are modeled with relativistic effective core potentials and Gaussian cc-p VDZ basis sets. The results for the lanthanide ions are compared and contrasted with similar studies on actinide ions

THE ENERGY LEVELS OF URANIUM V

$^{a}$ J.F. Wyart, V Kauf***, and J. Sugar Phys. Scr. 22, 389 (1980)Author Institution: Department of Chemical Physics, The Ohio State UniversitySpin-orbit configuration-interaction calculations are performed on $U^{4+}$. Uranium is modeled with relativistic effective core potentials and Gaussian cc-pVDZ and cc-pVTZ basis sets. This $5f^{2}$ ion has been studied extensively, and the calculations are compared with the experimental $spectrum^{a}$

GROUND-STATE SPLITTING OF Am2+Am^{2+} IN CaF2CaF_{2}

$^{a}$N. W. Winter, M. Ross and R. M. Pitzer J. Phys. Chem. 94, 1172 (1990).Author Institution: Department of Chemical Physics, The Ohio state UniversityThe energy-level splittings of the 5$f^{7}$ nominal $^{8}S_{7/2}$ ground-state of $Am^{2+}$ in an octahedral site of $CaF_{2}$ were studied using ab initio quantum chemical methods. The $CaF_{2}$ host was modeled with a large finite cluster of ions which approximate the Madelung potential of the crystal $lattice^{a}$. The potential energy curve for the symmetric stretch of the nearest-neighbour fluoride ions was calculated. The actinide dopant was treated with relativistic core potentials and Gaussian double-zeta basis sets