A quantum Monte Carlo method is presented for determining multideterminantal Jastrow-Slater wave functions for which the energy is stationary with respect to the simultaneous optimization of orbitals and configuration interaction coefficients. The approach is within the framework of the so-called energy fluctuation potential method which minimizes the energy in an iterative fashion based on Monte Carlo sampling and a fitting of the local energy fluctuations. The optimization of the orbitals is combined with the optimization of the configuration interaction coefficients through the use of additional single excitations to a set of external orbitals. A new set of orbitals is then obtained from the natural orbitals of this enlarged configuration interaction expansion. For excited states, the approach is extended to treat the average of several states within the same irreducible representation of the pointgroup of the molecule. The relationship of our optimization method with the stochastic reconfiguration technique by Sorella et al. is examined. Finally, the performance of our approach is illustrated with the lowest states of ethene, in particular with the difficult case of the 1(1)B(1u) state.

Despite significant advances in electronic structure methods for the treatment of excited states, attaining an accurate description of the photoinduced processes in photoactive biomolecules is proving very difficult. For the prototypical photosensitive molecules, formaldimine, formaldehyde, and a minimal protonated Schiff base model of the retinal chromophore, we investigate the performance of various approaches generally considered promising for the computation of excited potential energy surfaces. We show that quantum Monte Carlo can accurately estimate the excitation energies of the studied systems if one constructs carefully the trial wave function, including in most cases the reoptimization of its determinantal part within quantum Monte Carlo. While time-dependent density functional theory and quantum Monte Carlo are generally in reasonable agreement, they yield a qualitatively different description of the isomerization of the Schiff base model. Finally, we find that the restricted open shell Kohn-Sham method is at variance with quantum Monte Carlo in estimating the lowest-singlet excited state potential energy surface for low-symmetry molecular structures.

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The zerovalent bis(η6-benzene) f-metal sandwich complexes M(C6H6)2 (M = La, Ce, Nd, Gd, Tb, Lu, Th, U) were investigated with state-of-the-art quantum chemical ab initio approaches taking into account the effects of electron correlation and relativity. Ground state assignments, optimized metal−ring distances, symmetric metal−ring stretching frequencies, and metal−ring bonding energies are reported. The effects of ring substitution on the metal−ring binding energies are discussed. The complexes of Th and U are predicted to be at least as stable as the corresponding lanthanide systems and form possible synthetic targets. Whereas the lanthanide systems have a 4f n ground state configuration (n = 1, 3 for Ce, Nd), the corresponding actinide compounds should possess, as a consequence of stronger relativistic effects, a 5f n -1 ground state configuration, with a possible strong admixture of 5f n -1 (n = 1, 3 for Th, U). The back-donation from the occupied metal d± 2 to the empty π orbitals of the benzene ligands is found to be the dominant bonding interaction. Whereas the lanthanide 4f orbitals are essentially localized on the metals and chemically inactive, the actinide 5f shell is partially delocalized and its f± 2 components may also take part in the back-donation.

Pure diusion quantum Monte Carlo calculations have been carried out for Be 2 and the weakly bound group 12 dimers Zn 2 , Cd 2 and Hg 2 . We have applied relativistic energy-consistent large-core pseudopotentials and corresponding core-polarization potentials for the group 12 atoms. The derived spectroscopic constants (R e , D e , x e for Zn 2 and Cd 2 (Zn 2 : 3X88 AE 0X05 Ê A, 0X024 AE 0X007 eV, 25 AE 2 cm À1 ; Cd 2 : 4X05 AE 0X03 Ê A, 0X031 AE 0X005 eV, 21 AE 1 cm À1 ) are in good agreement with corresponding coupled-cluster results (Zn 2 : 4X11 Ê A, 0.022 eV, 21 cm À1 ; Cd 2 : 4.23 Ê A, 0.029 eV, 18 cm À1 ) and available experimental data (Zn 2 : 0.034 eV, 26 cm À1 ; Cd 2 : 0.039 eV, 23 cm À1 ). A comparison with previous results for the heavier homologue Hg 2 is made. Using a multireference trial wavefunction for Be 2 we achieved a suciently accurate description of the nodes of the wavefunction to obtain a bonding interaction within the ®xed-node approximation. The applicability of this approach has been justi®ed in pseudopotential and allelectron calculations. Covalent bonding contributions which appear in addition to pure van der Waals interactions for these molecules are analysed in terms of local occupation number operators and the associated interatomic charge¯uctuations. Static dipole polarizabilities for group 12 atoms and dimers are calculated using a dierential quantum Monte Carlo method for ®nite external electric ®elds. We have extended this method to pseudopotential calculations by taking into account the electric ®eld dependence of the localized pseudopotentials. Within the statistical uncertainties our results agree with those from coupled-cluster calculations.