Volker Staemmler

PNO–CI (pair natural orbital configuration interaction) and CEPA–PNO (coupled electron pair approximation with pair natural orbitals) calculations of molecular systems. I. Outline of the method for closed‐shell states

The methods of configuration interaction with double substitutions to pair natural orbitals (PNO−CI) and of the coupled electron pair approximation (CEPA) proposed by W. Meyer are improved by combination with a new scheme of the calculation of the pair natural orbitals (PNO) and an efficient iterative scheme for the diagonalization of the CI matrix. The relevant matrix elements for the closed shell case are tabulated, the quantities that are pertinent for an analysis of the correlation energy are defined, and the organization of the computer programs is described.

PNO–CI (pair natural orbital configuration interaction) and CEPA–PNO (coupled electron pair approximation with pair natural orbitals) calculations of molecular systems. II. The molecules BeH2, BH, BH3, CH4, CH−3, NH3 (planar and pyramidal), H2O, OH+3, HF and the Ne atom

PNO–CI and CEPA–PNO calculations are performed for the molecules MgH2, AlH, AlH3, SiH4, PH3, H2S, HCl, and the Ar atom. Two types of Gaussian basis sets are used; both sets contain one p‐set on H. The ’’small’’ set includes one d‐set on the heavy atom, the ’’standard’’ basis two d‐sets and one f‐set. Both for MgH2 and Ar, a ’’large’’ and a ’’very large’’ basis are used as well, which contain additional polarization functions. The energy improvement due to the different polarization functions is analyzed. Hartree–Fock limits for the molecular energies are estimated. The computed valence shell correlation energies are analyzed in terms of quantities defined in part I, in particular in terms of the IEPA (independent electron pair) correlation energies eμIEPA and the error ΔEIEPA of the IEPA scheme. Both the valence shell interorbital pair correlation energies and the IEPA error are smaller in absolute value than those of the corresponding first row hydrides, provided that one uses the localized representatio...

A multi-configuration reference CEPA method based on pair natural orbitals

SummaryA multi-reference CI scheme is proposed which is aiming at a considerable reduction of the generally very large number of configurations of CI expansions in multi-configuration reference cases. This reduction is achieved by combining the idea of internal contraction, the concept of pair natural orbitals (PNOs) and CEPA (coupled electron pair) type approximations for the contributions of higher than double excitations. This latter estimate leads to size consistent results and also permits to employ reference wavefunctions that contain only the dominantly occupied configurations of the considered system. Applications to two test cases, the lowest states (3P,1D and1S) of the carbon atom and the symmetry forbiddenC2v insertion reaction of Be and H2, show that our method is able to truncate CI expansions to lengths of no more than 103–104 without losing more than 1–2% of the correlation energy. The calculated excitation energies and energy barriers agree with the full CI results in the respective basis within about 1 kcal/mol. Thus the MC-CEPA-PNO method presents a very efficient way to obtain “chemical accuracy” in CI-calculations for molecular systems.

Photodissociation dynamics of H2O and D2O in the first absorption band: A complete abinitio treatment

We report a detailed theortical study of the photodissociation of H2O and D2O in the first absorption band (λ∼165 nm). The calculations are three dimensional and purely quantum mechanical. They include an ab initio potential energy surface for the A state and a calculated SCF dipole moment function for the X→A transition. The dynamical calculations are performed within the infinite‐order‐sudden approximation for the rotational degree of freedom of OH and the LHL approximation for the masses. The resulting vibrational–translational motion is then treated exactly in two dimensions using hyperspherical coordinates. This study does not include any adjustable parameters. The thermally averaged total absorption spectra for H2O and D2O agree perfectly with the experimental spectra. Even finer details such as the progression of ‘‘vibrational’’ structures are well reproduced. They are not induced by any selective absorption but can be explained on the basis of the A state potential energy surface and details o...

CEPA calculations of potential energy surfaces for open-shell systems.: IV. Photodissociation of H2O in the A1B1 state

Abstract A three-dimensional potential energy surface for the photodissociation of H2O in its lowest excited singlet state A 1B1 in C2v or A 1A″ in C3 symmetry, respectively, has been calculated with quantum-chemical ab initio methods including electron correlation. The main features of the surface are discussed and qualitative explanations are given for the experimentally observed vibrational and rotational excitations of the product OH(2Π) radicals. The surface will be used in subsequent investigations of the dynamics of the H2O photodissociation process.

Strong relaxations at the Cr2O3(0001) surface as determined via low-energy electron diffraction and molecular dynamics simulations

Abstract The surface structure of Cr 2 O 3 (0001) was investigated by quantitative low-energy electron diffraction and molecular dynamic simulations. In qualitative agreement with each other, both methods indicate strong vertical relaxations at and near the surface. These relaxations are concomitant with a charge reduction and depolarization, which stabilize the surface, yielding energies close to those found for non-polar oxide surfaces with non-divergent surface potentials. The lateral arrangement of oxygen atoms is identical to that in the bulk, i.e. there are no lateral distortions to accomodate the strong interlayer relaxations. The latter extend extend deep into the surface, with the experimentally determined changes of the first four interlayer distances being −38%, −21%, −25% and +11% with respect to the unrelaxed bulk values.

An efficient first-order CASSCF method based on the renormalized Fock-operator technique

SummaryA new efficient first-order CASSCF method (multiconfiguration SCF (self consistent field) in a complete active space) is described. Its main characteristics are (i) use of the generalized Brillouin theorem (Fock-operator method), (ii) renormalization of single excitations, (iii) fast microiterations containing only two-index transformations, i.e. M3N2 steps. Convergence is generally reached in eight to twelve macroiterations. The method is applied to several examples (LiH, N2, AlO) and compared to other MCSCF (multiconfiguration SCF) methods.

Electronic surface state of NiO (100)

Abstract The electronic structure of the (100) surface of NiO has been studied using EELS (electron energy loss spectroscopy) and ab initio calculations. In addition to the previously documented bulk excitations of NiO, two new states at energies of 0.57 and 1.62 eV have been found. These states are attributed to d—d transitions of the nickel surface ions. As expected for surface states, they are affected by the interaction with an adsorbate, i.e. adsorption of NO leads to a shift to higher energy. Ab initio cluster calculations show that electronic structure of the surface is considerably different from that of the bulk which is a result of the lower symmetry of the crystal field at the surface (O 2 →C 4v . The nature of the observed surface states has been identified by a comparison of the experimental data with theoretical results.

Ab initio calculations for the adsorption of small molecules on metal oxide surfaces. I. Cluster calculations for carbon monoxide CO on nickel oxide NiO(100)

Quantum chemical ab initio calculations for the adsorption of CO on the NiO(100) surface have been performed at different levels of accuracy: self‐consistent field (SCF), complete active space self‐consistent field, and coupled electron pair approximation. Basis sets of double zeta and triple zeta + polarization (TZP) quality have been used. The NiO(100) surface is represented by a cluster containing one Ni2+ ion and the five adjacent O2− ions. The charge neutrality of the cluster and the saturation of the dangling bonds is achieved by adding eight protons, which gives the total composition Ni(H2O)3(OH)2. Alternatively, the Ni2+(O2−)5 unit is embedded in a lattice of point charges which correctly represent the half‐infinite ionic crystal. In the most favorable configuration, CO is adsorbed linearly in the on‐top position on the Ni2+ ion, with the C atom pointing toward the surface. The binding energies at the SCF level are rather small, only 0.08 eV and 0.03 eV for CO and OC bound to the cluster (TZP basi...

Theoretical investigation of weak hydrogen bonds to sulfur

The interaction energies of the dimethylsulfide–methanol (I) and dimethylthiocarbonyl–methanol (II) complexes are calculated as a function of the S⋯H–O distances at various levels of theory and compared to those of their oxygen analogs. At the coupled cluster level the binding energy of (I) is −5.46 kcal/mol, only slightly smaller than the hydrogen bond energy of −5.97 kcal/mol for the corresponding oxygen analog, i.e., the dimethylether–methanol complex. It is also considerably larger than for dimethylether–methylthiol, where S and O of the parent complex are interchanged. Density functional theory is unable to describe these weak interactions properly. Choosing second-order Moller–Plesset perturbation theory, the interaction potential surfaces of both complexes with respect to the three relevant intermolecular coordinates are compared. The interactions in the hydrogen bonds involving sulfur are classified by Morokuma, atoms-in-molecules, and natural bond orbital analyses.