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The potential energy curve for the ground-state of radium dimer (Ra2) is provided by means of atomic and molecular relativistic coupled cluster calculations. The short-range part of this curve is defined by an equilibrium bond length of 5.324 Å, a dissociation energy of 897 cm-1, and a harmonic vibrational frequency of 20.5 cm-1. The asymptotic behavior at large interatomic distances is characterized by the van der Waals coefficients C6 = 5.090 × 10³, C8 = 6.978 × 105, and C10 = 8.786 × 107 atomic units. The two regions are matched in an analytical potential to provide a convenient representation for use in further calculations, for instance, to model cold collisions between radium atoms. This might become relevant in future experiments on ultracold, optically trapped, radioactive radium atoms that are used to search for a permanent electric dipole moment. [ABSTRACT FROM AUTHOR]

We present a study of the electronic structure of the [UO2]+, [UO2]2 +, [UO2]3+, NUO, [NUO]+, [NUO]2+, [NUN]-, NUN, and [NUN]+ molecules with the intermediate Hamiltonian Fock-space coupled cluster method. The accuracy of mean-field approaches based on the eXact-2-Component Hamiltonian to incorporate spin-orbit coupling and Gaunt interactions are compared to results obtained with the Dirac-Coulomb Hamiltonian. Furthermore, we assess the reliability of calculations employing approximate density functionals in describing electronic spectra and quantities useful in rationalizing Uranium (VI) species reactivity (hardness, electronegativity, and electrophilicity). [ABSTRACT FROM AUTHOR]

We report the development of a general order relativistic coupled-cluster (CC) code. Our implementation is based on Kramers-paired molecular spinors, utilizes double group symmetry, and is applicable with the full Dirac-Coulomb and several approximate relativistic Hamiltonians. The available methods include iterative and perturbative single-reference CC approaches with arbitrary excitations as well as a state-selective multi-reference CC ansatz. To illustrate the performance of the new code, benchmark calculations have been performed for the total energies, bond lengths, and vibrational frequencies of the monoxides of Group IVa elements. The trends due to the simultaneous inclusion of relativity as well as higher-order electron correlation effects are analyzed. The newly developed code significantly widens the scope of the ab initio relativistic calculations, for both molecules and atoms alike, surpassing the accuracy and reliability of the currently available implementations in the literature. [ABSTRACT FROM AUTHOR]

An implementation of a two-component all-electron (jj/ωω) treatment of both scalar and spin–orbit relativistic effects in the MOLFDIR program suite is presented. Relativity is accounted for by Douglas–Kroll transformed one-electron operators: scalar (spin-free) and so called mean-field spin–orbit terms. The interelectronic interaction is represented by the nonrelativistic Coulomb operator. High-level correlated calculations of properties of several systems (FO, ClO, Cl, O[sub 2][sup +], O[sub 2][sup -], Tl, and TlH) where spin–orbit effects play a dominant role are presented and compared with other data. Agreement with Dirac–Coulomb(–Gaunt) reference values is in general very good. © 2001 American Institute of Physics. [ABSTRACT FROM AUTHOR]

We present a robust and non-heuristic algorithm that finds all extremum points of the error distribution function of numerically Laplace-transformed orbital energy denominators. The extremum point search is one of the two key steps for finding the minimax approximation. If pre-tabulation of initial guesses is supposed to be avoided, strategies for a sufficiently robust algorithm have not been discussed so far. We compare our non-heuristic approach with a bracketing and bisection algorithm and demonstrate that 3 times less function evaluations are required altogether when applying it to typical non-relativistic and relativistic quantum chemical systems. [ABSTRACT FROM AUTHOR]

Relativistic four-component random phase approximation (RPA) calculations of indirect nuclear spin-spin coupling constants in MH[sub 4] (M=C, Si, Ge, Sn, Pb) and Pb(CH[sub 3])[sub 3]H are presented. The need for tight s-functions also in relativistic four-component calculations is verified and explained, and the effect of omission of (SS-LL) and (SS-SS) two-electron integrals is investigated. Already in GeH[sub 4] we see a relativistic increase in the coupling constant by 12%, and for PbH[sub 4] the effect is a 156% increase for the one-bond coupling. Large relativistic effects are also computed for the two-bonds couplings. We find that the relativistic effects on the one-bond couplings are mainly due to scalar relativistic factors rather than spin-orbit corrections. © 2000 American Institute of Physics. [ABSTRACT FROM AUTHOR]