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An investigation of the potential energy surfaces, which include characterization of equilibrium points and transition states, is presented. It is concluded that the 1,4-prototropic mechanisms, mainly considered as a one step concerted exchange of protons, is the most favored from an energetic point of view.

Optimal geometries, harmonic vibrational frequencies, and relative energies of different sets of metal complexes are calculated using the effective group potentials (EGP) method. All of the systems under consideration contain the cyclopentadienyl (Cp) ligand.

In this work we present all-electron fixed-node diffusion Monte Carlo (FN-DMC) calculations of the low-lying electronic states of the copper atom and its cation. The states considered are those which are the most relevant for the organometallic chemistry of copper-containing systems, namely, the 2S, 2D, and 2P electronic states of Cu and the 1S ground state of Cu+. We systematically compare our FN-DMC results to CCSD(T) calculations using very large atomic-natural-orbital-type all-electron basis sets. The FN-DMC results presented in this work provide, to the best of our knowledge, the most accurate nonrelativistic all-electron correlation energies for the lowest-lying states of copper and its cation. To compare our results to experimental data we include the relativistic contributions for all states through numerical Dirac-Fock calculations, which for copper (Z=29) provide almost the entire relativistic effects. It is found that the fixed-node errors using Hartree-Fock nodes for the lowest transition energies of copper and the first ionization potential of the atom cancel out within statistical fluctuations. The overall accuracy achieved with quantum Monte Carlo for the nonrelativistic correlation energy (statistical fluctuations of about 1600 cm-1 and near cancelation of fixed-node errors) is good enough to reproduce the experimental spectrum when relativistic effects are included. These results illustrate that, despite the presence of the large statistical fluctuations associated with core electrons, accurate all-electron FN-DMC calculations for transition metals are nowadays feasible using extensive but accessible computer resources. [ABSTRACT FROM AUTHOR]

A comparison between 'static' and 'dynamic' determination of the thermodynamic [delta(sub t)F(super o)] and kinetic data [delta(sub t)F(super #)] for the reaction of Cl2LaR (R=H, CH3) and H2 is presented. A difference is obtained in the case of the reaction between Cl2LaH and H2 and can be attributed to a failure of the 'static' approach based on the harmonic approximation.

The effective group potentials (EGP) approach is used for the computation of the ground and excited state energies of the mixed valence compound. It is the first time that for a system as big as the complex presented above, the ground and excited states are computed with their own orbitals and studied in such a detailed way.

Quantum chemical calculations are carried out at different levels of theory to verify the validity of an effective group pseudopotential (EGP) for carbonyl. Three pseudocarbonyl groups, extracted from different systems are discussed, and two are produced from the isolated carbonyl molecule and the other from BH3CO.

One proposes a state-specific self-consistent dressing of the configuration interaction (CI) matrix built on a multireference space and all the singly and doubly substituted determinants. The dressing insures size consistency [and separability when localized molecular orbitals (MOs) are used]. In the here-proposed solution, which generalizes a previous single reference method [(SC)2CI, J. Chem. Phys. 99, 1240 (1993)] valid only for the research of the ground state, all the reference determinants play an equal role and the method is applicable to excited states. The implementation will be simpler if the reference space is a complete active space, but this restriction is not compulsory. [ABSTRACT FROM AUTHOR]

Based on the principle of intermediate effective Hamiltonians, a simple procedure is proposed in order to eliminate the unlinked contributions of any truncated or selected configuration interaction (CI). The corrections are diagonal energy shifts, easily calculated. A self-consistent version is proposed, which insures separability if localized molecular orbitals (MO) are used. In the special case of double CI, the present method is an improved version of the coupled electron pair approximation (CEPA), but it may be applied to any selected model space, involving configurations of various degrees of excitation. The efficiency of the proposed algorithms is illustrated on a series of test calculations performed on Be2, F2, N2, and NH3. [ABSTRACT FROM AUTHOR]

We report a perturbation calculation of the ground state energy of the Be atom starting from a multiconfigurational model space and using the new concept of an intermediate Hamiltonian. The traditional MC-MBPT is known to diverge for this problem, whereas our method strongly converges. This convergence is explained by the fact that this method is free from any intruder state problem. The second-order calculation yields 140% of the correlation energy with a single reference model space, 93% with a multireference, and 98% with our method demonstrating the efficiency of the intermediate Hamiltonian. [ABSTRACT FROM AUTHOR]

The singlet–triplet splitting (2J=EST) is calculated in bis-[bromo-(N,N-diethylaminoethanolato)copper(II)], belonging to the group of alkoxo-bridged copper(II) dimers, by a recently proposed ab initio method. The singlet–triplet splitting energy is obtained by a perturbation expansion closely related to Anderson’s formalism. The role of all valence electrons is taken into account and it is shown to be very important in order to obtain a satisfying agreement with the experimental result. Two important facts, from a computational point of view, are tested: influence of the atomic basis set size, and influence of modifying of the ligands. Changes in the basis set of the bridging atoms can induce variations in the results from 10% to 20%. As a final result, the theoretical value of the real complex is calculated to 2J(calc)=-1095 cm-1 and agrees with the experimental one 2J(exp)=-817 cm-1. [ABSTRACT FROM AUTHOR]

The effective group potential (EGP) methodology allows one to reduce the number of electrons and nuclei explicitly implied in an ab initio calculation. EGP scheme could be integrated to QM/MM methods to resolve lack of static interactions and the problem of frontier bonds in hybrid methods.

Effective group potentials (EGP) are aimed at simplifying molecular ab initio calculations for large systems involving bulky ligands, as long as these ligands are supposed to play the role of spectator groups. The most important step is the definition of modified valence pseudo-orbitals (MVPO), which plays the same role as valence pseudo-orbitals in the derivation of EGP.

A nonrotating coordinated dihydrogen ligand is responsible for the exchange couplings in the d2 systems. Nuclear magnetic resonance spectroscopy characterizes the electronic structures of two tantalum derivatives and their isotopomers having different ground states. The tantalum derivative (Cp2TaH2CO)+ is a dihydrogen complex whereas (Cp2TaH2POMe3)+ exhibits exchange coupling. The quantum mechanical exchange observed in d2 systems is due to the rotational tunneling of a coordinated dihydrogen ligand.

The protonation of Cp2-TaH(CO) by aqueous HCl at room temperature facilitates the formation of the first tantalum and the first thermally stable group 5 dihydrogen complexes. The complexes are the kinetic isomers of the protonation reaction. The poor back-bonding on CO indicates that these dihydrogen derivative complexes are electrophilic and suggests that the coordinated dihydrogen molecule is highly acidic.

Time-dependent density-functional theory (TDDFT) is an increasingly popular approach for calculating molecular excitation energies. However, the TDDFT lowest triplet excitation energy, ω[sub T], of a closed-shell molecule often falls rapidly to zero and then becomes imaginary at large internuclear distances. We show that this unphysical behavior occurs because ω[sub T][sup 2] must become negative wherever symmetry breaking lowers the energy of the ground state solution below that of the symmetry unbroken solution. We use the fact that the ΔSCF method gives a qualitatively correct first triplet excited state to derive a "charge-transfer correction" (CTC) for the time-dependent local density approximation (TDLDA) within the two-level model and the Tamm-Dancoff approximation (TDA). Although this correction would not be needed for the exact exchange-correlation functional, it is evidently important for a correct description of molecular excited state potential energy surfaces in the TDLDA. As a byproduct of our analysis, we show why TDLDA and LDA ΔSCF excitation energies are often very similar near the equilibrium geometries. The reasoning given here is fairly general and it is expected that similar corrections will be needed in the case of generalized gradient approximations and hybrid functionals. © 2000 American Institute of Physics. [ABSTRACT FROM AUTHOR]

By use of optical steady state and time resolved spectroscopy, we studied the evolution of the triplet excited state in a series of six ethynylenic polymers of the structure [-Pt(PBu[sub 3][sup n])[sub 2]-C=C-R-C=C-][sub n] where the spacer unit R is systematically varied to give optical gaps from 1.7-3.0 eV. The inclusion of platinum in the polymer backbone induces a strong spin-orbit coupling such that triplet state emission (phosphorescence) associated with the conjugated system can be detected. Throughout the series we find the S[sub 1]-T[sub 1] (singlet-triplet) energy splitting to be independent of the spacer R, such that the T[sub 1] state is always 0.7±0.1 eV below the S[sub 1] state. With decreasing optical gap, the intensity and lifetime of the triplet state emission were seen to reduce in accordance with the energy gap law. © 2000 American Institute of Physics. [ABSTRACT FROM AUTHOR]