Mats Svensson

PCI-X, a parametrized correlation method containing a single adjustable parameter X

Abstract A new method intended for the calculation of bond strengths is suggested and tested on a large variety of systems. The method is based on the simple fact that in a balanced treatment about the same percentage X of the correlation effects is obtained for every system. The total energy is then obtained by a simple extrapolation to 100 percent. Using a double zeta plus polarization basis set in a coupled cluster type calculation it is found that the percentage X is about 80. Small changes of the basis sets or methods do not change this percentage significantly. Since the method is parametrized and the underlying method is some type of configuration interaction the method is termed PCI- X . For a selection of 13 representative simple first-row molecules the present version of the PCI-80 method gives an average absolute deviation from experiment of 1.8 kcal/mol compared to 10.3 kcal/mol for the unparametrized calculations. For some of these bond strengths a Hartree-Fock limit correction is required. For transition metal complexes, for which the method is primarily aimed, the improvement can be quite dramatic compared to a normal standard treatment. The method is tested on essentially all small second-row transition metal systems studied experimentally, for 38 systems, and the average absolute deviation from these sometimes rather uncertain experiments is 5.1 kcal/mol (0.22 eV).

Comparisons of results from parametrized configuration interaction (PCI‐80) and from hybrid density functional theory with experiments for first row transition metal compounds

Different methods and schemes have been tested for the difficult class of first row transition metal complexes. The systems investigated are the M+, MH+, MCH+3, and MCH+2 systems for the entire row and the Ni(CO)x systems with x=1–4. In general quite satisfactory results are obtained both at the PCI‐80 and hybrid density functional levels. In particular, for the MH+ and MCH+3 systems the PCI‐80 average deviation to experiments is of the same size as the uncertainty in the experiments. The MCH+2 systems are somewhat more difficult to describe and a rather large error is found for chromium at the PCI‐80 level, due to a large coupling of exchange energy loss and change of correlation energy resulting from the formation of two covalent d‐bonds. Scaling at the modified coupled pair functional (MCPF) level is also compared to scaling at the coupled cluster singles and doubles (CCSD) level. In most cases very similar results are obtained, but classes of systems can be identified where scaling works better at the...

First row benchmark tests of the parametrized configuration interaction with parameter X (PCI‐X) scheme

A recently suggested scheme termed parametrized configuration interaction with parameter X (PCI‐X) for scaling the correlation energy has been applied on a benchmark test consisting of 32 first row molecules. Several different methods like Mo/ller–Plesset second‐order perturbation theory (MP2), modified coupled pair functional method (MCPF), averaged coupled pair functional method (ACPF), coupled cluster singles and doubles (CCSD), and CCSD with a perturbational estimate of triple excitations [CCSD(T)] have been tested using systematically chosen basis sets ranging from double zeta (DZ) to very large atomic natural orbital (ANO) sets containing several sets of d and f functions. For each method and basis set the scaling parameter is optimized. The scaling does in all cases lead to large, sometimes dramatic, improvements of the results. Typically, using a single reference state method like MCPF the average absolute deviation compared to experiments for the benchmark goes from an unscaled value of about 20 ...

Experimental and theoretical study of oxidative addition reaction of nickel atom to O–H bond of water

The reaction of atomic nickel with water in the gas phase has been investigated by kinetic studies under static pressure conditions near room temperature, and by accurate quantum chemical calculations. Experimental and theoretical results are consistent with a reaction mechanism involving formation of a weakly bound nickel–water adduct, which may react further by oxidative addition of nickel to the O–H bond of water to form the insertion product HNiOH. Experimental estimates of reaction energetics have been made by using unimolecular reaction theory calculations to model rate coefficients obtained by fitting kinetic data to a simple rate equations model. These experimental estimates are in agreement with the theoretical results, and indicate that the insertion product is bound by at least 20–25 kcal/mol, relative to nickel plus water. There is also agreement that the barrier to oxidative addition is no greater than 1–2 kcal/mol, and may be smaller. This theoretical result was obtained only at the highest ...

Geometry optimization for second-row transition metal complexes

Abstract A systematic investigation is presented of the geometry optimization step in studies of the energetics of reactions involving second-row transition metal complexes. The systems studied are products of the oxidative addition between metal hydrides and methane from yttrium to palladium. One case of a transition state is also investigated for the reaction between palladium and water. Three different levels of geometry optimization are compared, the SCF level, the MP2 level and the QCISD level, using polarized and non-polarized basis sets. Comparisons are made to results obtained for first-row transition metal complexes. The results are put in perspective by a consideration of the overall accuracy obtainable in the final energy evaluation step at the optimized geometries.

The interaction of ammonia, carbonyl, ethylene and water with the copper and silver dimers

Abstract The bond strengths of ammonia, carbonyl, ethylene and water to the copper and silver dimers have been calculated using the recently suggested PCI- X scheme, and the results are compared to experiments. For most of these systems good agreement between theory and experiments is obtained. The simplest description of the bonding is one where the lone pair of the ligands interacts with the lowest empty orbital of the cluster. For the copper and silver dimers where the bonding is formed by sp hybrids, the lowest empty orbital will be the sp hybrids pointing out behind the bond. The present M 2 L systems will therefore form quasilinear structures. The use of effective one-electron ECPs was tested and was found to give reasonable results for silver but poor results for copper.

Selectivity and reactivity in asymmetric allylic alkylation

Hydroxyalkyl)-6-oxazolyl-and 2-(1-alkoxyalkyl)-6-oxazolylpyridines serve as versatile ligands in the palladium-catalyzed allylic substitution of rac-1,3-diphenyl-2-propenyl acetate with dimethyl malonate as nucleophile. The enantioselectivity of the reaction is dependent on the conformation of the ligands, as deduced by NMR, X-ray crystallography and DFT calculations of palladium(II) complexes of the ligands. The reactions are slow, requiring up to four days reaction time. However, with the use of microwave flash heating, reaction times are reduced to 2 min, with only minor loss in stereoselectivity.

CO bonding on tin modified Pt()-(1×2)

The influence of tin on the bonding of CO to Pt(l 10) has been studied using high-resolution electron energy loss spectroscopy, density functional theory and photoelectron spectroscopy. CO binds on-top to platinum at all investigated tin and CO coverages. Bonding of CO in Sn-Pt alloy rows is weakened compared to clean Pt(l 10). However, several different adsorption sites in Pt-only rows, inequivalent with regard to their coordination environment, show very similar characteristics with respect to CO adsorption.

Theoretical Models for Organometallic Reactions

Quantum chemical methods are applied to calculate potential energy surfaces for the reaction between transition metal systems and simple hydrocarbons like methane, ethane and ethene. Naked metal atoms are used to model metal complexes, making it possible to make clear cut comparisons between different metals and also between different classes of reactions. The use of simple models also allows for the application of sophisticated quantum chemical methods, including an accurate treatment of electron correlation effects, thus ensuring that reliable relative energies are obtained. it is shown that the variation in both binding energies and activation energies can be understood from the electronic spectra of the metal atoms. The effects of using more realistic model systems are also investigated. Finally, the results from a methodological investigation of calculated binding energies in an experimentally rather well known system, Ni(CO)4, are discussed. For this system, which belongs to a class of molecules that are unusually difficult to treat accurately by quantum chemical methods, the discrepancy between calculated and experimental binding energies is about 10 percent.