Michel Pelissier

Ab initio molecular calculations including spin-orbit coupling. I. Method and atomic tests

Abstract Ab initio pseudopotentials accounting for monoelectronic spin-orbit effects are derived. The spin-orbit coupling is introduced through an effective hamiltonian in a basis of L - S states. Results are presented for some states of the argon and indium atoms.

Testing the arbitrariness and limits of a pseudopotential technique through calculations on the series of diatoms HF, AlH, HCl, AlF, AlCl, F2, Cl2

The pseudopotential techniques present some degrees of freedom, the influence of which on molecular calculations must be tested to assess the stability and accuracy of the results. The present work uses a semi-local pseudopotential extracted from near Hartree-Fock atomic calculations; the shape of the inner part of the pseudoorbital, the analytic form of the pseudopotential are shown to have less influence than the choice of the valence basis set which must be optimized. The calculated molecular constants perfectly agree with the large basis set all-electron calculations, even for polar molecules.

Bonding between transition metal atoms. Abinitio effective potential calculations of Cu2

Valence Hartree–Fock calculations using an ab inito effective potential and a Gaussian basis set of triple zeta quality followed by extensive CI’s have been carried out for several states of the copper atom and for the ground state of diatomic copper. Correlation effects are determined to obtain a satisfactory agreement with experimental data for Cu2(re = 2.25 vs 2.22 A; ωe = 265 vs 266 cm−1; De = 15 550 vs 16 500 cm−1). The most striking effect is the 0.1 A shortening of the bond length induced by the d correlation energy. Important basis set superposition errors are shown to be possible (especially in all electron calculations) and the use of six Gaussian primitives for the d orbitals was required to avoid them.

Inexpensive determinations of valence virtual MOs for CI calculations

Abstract The full treatment of the non-dynamical correlation energy (i.e. the correlation within a shell of valence occupied and virtual MOs) is hihgly desirable since it is strongly shape and distance dependent, and ensures a proper dissociation into the HF ground state of the free atoms. The ideal way to achieve this goal is the valence CAS SCF procedure, which is rapidly very expensive. The present paper tries to define valence virtual MOs to perform full CAS CI or to use them in other approximate CI expansions. Very limited MC SCF procedures involving a few pair-wise excitations are sufficient to generate well-defined valence MOs. One may sometimes use virtual valence MOs from the upper valence multiplet but in many cases this procedure fails, the high-spin arrangement tending to avoid the σ bond region where it becomes too repulsive. The projection of the SCF atomic orbitals of the free atom in the virtual molecular space (Levys PAO) is very efficient; this procedure may be improved by a proper definition of the hybridized AOs, i.e. the linear combination of the AOs which are most occupied in the molecular Fock space. The MOs defined from these four procedures prove to give CAS CI energies which are very close to the CAS SCF result, at a very low price. On the contrary, the usual improved virtual MOs defined from SCF calculations of the cation, or with an increased nuclear charge to compensate the excess repulsion of the Fock operator, fail to define valence virtual MOs even if they concentrate the spatial extension of the lowest virtual MOs. The quality of our MO sets is compared at the valence CAS CI level and through the convergence of the iterative multireference second-order Moller-Plesset CIPSI algorithm. Three comparisons concern a triply bonded molecule (N2), an non-Lewis structure (Na4), and an intermolecular complex Cu…CO, which is proved to be weakly bound. To obtain reliable potential curves in selected CI algorithms, it is recommended to give an (almost) constant physical meaning to all MOs.

Theoretical study of an unusual reactive collision Cs(7p)+H2→CsH+H. Diabatic approach of the collinear collision potential energy surfaces

The Cs(7p)+(X 1∑+g, v=0) H2→(X 2∑+, v=0) CsH+H reactive collision was recently experimentally observed from a crossed beam experiment by Crepin et al. This reaction is rather unusual since it starts from a highly excited state of the system (11th potential surface) and must reach the ground state surface in the product channel without energy loss. Accurate nonempirical CI calculations of the potential energy surface for the collinear collision are reported, using large basis sets and a nonempirical relativistic pseudopotential for the Cs atom. The adiabatic potential surfaces, which exhibit irregular behavior, are reinterpreted in terms of an ab initio nearly diabatic effective Hamiltonian spanned by five neutral repulsive channels Cs(6s)×H2, Cs(6p)×H2,..., Cs(7p)×H2 and an ionic very flat Cs+H−2 channel which tends to the product wave function (Cs+H−)×H. The intersection of these diabatic potential surfaces are accessible from the entrance energy and this picture supports a harpooning mechanism, the 7p e...

Origins of the poor convergence of many-body perturbation theory expansions from unrestricted Hartree--Fock zeroth-order descriptions

Besides unavoidable discontinuities of the finite‐order potential curves derivatives at the symmetry‐breaking point, the UHH Moller–Plesset perturbation series show very slow convergence at intermediate and large interatomic distances. This bad behavior has two origins: (i) the MP denominators tend towards a constant value instead of vanishing so that each contribution is decreasing too rapidly with the interatomic distance; this remark condemns the use of MP zeroth‐order Hamiltonian and forces to adopt an Epstein–Nesbet definition; (ii) the coupling between the singly and doubly excited determinants is very strong at intermediate distances, and the effect of the singly excited configurations, which only appears at the fourth order, slows down the convergence rate. These trends are both analytically demonstrated and numerically illustrated in the model minimal basis sets He++2 and N2 problems. A proposal to include the effect of singly excited determinants through a low‐dimensional effective intermediate‐...

Three-electron approach of HgH using relativistic effective core, core polarization and spin-orbit operators: the low-lying states

Abstract The lowest states of HgH are calculated using a non-empirical relativistic effective core potential considering the mercury atom as a two-electron system. The configuration interaction is done within the CIPSI algorithm. Core-valence polarization and correlation energy are included through a perturbation treatment. The spin-orbit coupling is introduced through an effective hamiltonian in a basis of LS states. Results obtained for the lowest states of HgH are in fair accordance with experimental results. The “atom in molecule” approximation for SO coupling is discussed.

Theoretical calculation of the valence excited states of the AlH molecule

The ground and lowest excited state potential curves of the AlH molecule have been calculated in a double‐zeta‐plus polarization basis set of Gaussian type orbitals. The core electrons of the Al atom are taken into account through a nonempirical pseudopotential. Large scale CI’s are performed according to the CIPSI technique. The calculated transition energies, bond distances, and force constants are in good agreement with experiment for the observed excited states. Three new states with dissociative or long equilibrium distance potential curves have been identified.

Relativistic effects in Cu2 bonding

The use of effective core potentials derived from relativistic and nonrelativisitic atomic calculations exhibits an important relativisitic bond shortening in the case of Cu2.

Une extension de la méthode CNDO/2 à l'étude des complexes d'éléments de transition@@@An extension of the CNDO/2formalism for the study of transition metal complexes: II. Cr(CO)6 et Fe(CO)5@@@II. Cr(CO)6 and Fe(CO)5

The previously proposed extension of the CNDO/2 formalism is used for investigating the electronic structure of Cr(CO)6 and Fe(CO)5. Thus, the ordering and the eigen values of molecular energy levels agree well with results provided byab initio calculations and photoelectron spectroscopy. The formal charge on metal is found to be in any case positive, as in Ni(CO)4 and Ni(PF3)4. Moreover, the validity of our parametrization is supported by the pretty agreement which exists between energy levels distribution and electronic structure provided by our technique and Veillardsab initio results for Ni(CN)42−. The parameters for the (Cr, Fe, Ni) set are now available and will allow to study large series of complexes in order to interpret their physical and chemical behaviour.