@article{Engels1994,
  author    = {Engels, Bernd},
  title     = {Detailed study of the configuration selected multi-reference configuration interaction method combined with perturbation theory to correct the wave function},
  url       = {http://nbn-resolving.de/urn:nbn:de:bvb:20-opus-59019},
  year      = {1994},
  abstract  = {A reliable prediction of the isotropic hyperfine coupling constant A\(_{iso}\) is still a difficult task for ab initio calculations. In previous studies, the configuration selected multireference configuration interaction method in combination with perturbation theory to correct the wave function (MRCI/ B\(_K\)) yielded accurate isotropic hyperfine coupling constants very economically. The present study gives a detailed analysis of the MRCI/ B\(_K\) method based on the X\(^2 \pi\) state of CH as a test case. Furthermore, a comparison to various other methods such as Maller-Ptesset perturbation theory and the coupled cluster approach is made. The success of the MRCI/ B\(_K\) method in predicting isotropic hyperfine coupling constants is explained in terms of the inßuence of higher than double excitations.},
  subject      = {Organische Chemie},
  language  = {en}
}
@article{StaikovaEngelsPeric1994,
  author    = {Staikova, M. and Engels, Bernd and Peric, M.},
  title     = {Ab initio investigation of the hyperfine structure in the 1\(^2\)Π\(_u\)(X\(^2\)A\(_1\), A\(^2\)B\(_1\) system of BH\(_2\))},
  url       = {http://nbn-resolving.de/urn:nbn:de:bvb:20-opus-59000},
  year      = {1994},
  abstract  = {No abstract available},
  subject      = {Organische Chemie},
  language  = {en}
}
@article{StaikovaEngelsPericetal.1993,
  author    = {Staikova, M. and Engels, Bernd and Peric, M. and Peyerimhoff, S. D.},
  title     = {Ab initio calculation of the vibronically averaged hyperfine coupling constants in the two lowest electronic states of H\(_2\)O\(^+\)},
  url       = {http://nbn-resolving.de/urn:nbn:de:bvb:20-opus-58998},
  year      = {1993},
  abstract  = {No abstract available},
  subject      = {Organische Chemie},
  language  = {en}
}
@article{GittinsHarrisFieldetal.1993,
  author    = {Gittins, C. M. and Harris, N. A. and Field, R. W. and Verges, J. and Ernst, W. E. and B{\"u}ndgen, P. and Engels, Bernd},
  title     = {Analysis and Depertubation of the C\(^2\)Π and D\(^2\)Σ\(^+\) states of CaF},
  url       = {http://nbn-resolving.de/urn:nbn:de:bvb:20-opus-58980},
  year      = {1993},
  abstract  = {No abstract available},
  subject      = {Organische Chemie},
  language  = {en}
}
@article{Engels1993,
  author    = {Engels, Bernd},
  title     = {Study of influences of various excitation classes on ab initio calculated isotropic hyperfine coupling constants},
  url       = {http://nbn-resolving.de/urn:nbn:de:bvb:20-opus-58973},
  year      = {1993},
  abstract  = {Reliable prediction of the isotropic hyperfine coupling constant A\(_{iso}\) is still a difficult task for ab initio calculations. Strang dependence on the method employed for its ca1culation has been found. Within a CI ansatz A\(_{iso}\) is considerably affected by the excitation classes taken into account within the CI calculation. In the present work the influence of various excitation classes on A\(_{iso}\) is examined. Calculations including all single, double, triple and a large part of the quadruple excitations are performed and the individual effects of the excitation classes are studied. It is found that the surprisingly good agreement found for S-CI treatments is due to large error cancellations. The importance of higher than double excitations arises from their indirect influence on the single excitations.},
  subject      = {Organische Chemie},
  language  = {en}
}
@article{MetropoulosEngelsPeyerimhoff1993,
  author    = {Metropoulos, B. and Engels, Bernd and Peyerimhoff, S.D.},
  title     = {On the chemi-ionization reaction O + CH ----> HCO\(^+\)+ e\(^-\). Coollinear O-CH Approach},
  url       = {http://nbn-resolving.de/urn:nbn:de:bvb:20-opus-58963},
  year      = {1993},
  abstract  = {We have investigated theoretically the importance of the O(\(^3\)P)+CH(a\(^4\sum^-\)) and the O(\(^3\)P)+CH(X\(^2\Pi\)) channels in the collinear chemi-ionization reaction O+CH->HCO\(^+\) +e\(^-\). We have found that both channels may lead to chemi-ionization via favorable Franck-Condon overlaps with the states ofthe ionic species.},
  subject      = {Organische Chemie},
  language  = {en}
}
@article{PericEngels1992,
  author    = {Peric, M. and Engels, Bernd},
  title     = {Ab initio calculation of the vibronically averaged hyperfine coupling constants in the 1\(^2\)Π\(_u\) electronic state of CH\(_2 ^+\)},
  url       = {http://nbn-resolving.de/urn:nbn:de:bvb:20-opus-58951},
  year      = {1992},
  abstract  = {The results of pure ab initio calculations of the hyperfine coupling constants for the 1 \(^2 \pi _u\) electronic state for various isotopomers of CHi in the energy range between 0 and 20 000 cm\(^{-1}\) are presented. Effects of vibronic and spin-orbit coupling are discussed.},
  subject      = {Organische Chemie},
  language  = {en}
}
@article{PericEngels1992,
  author    = {Peric, M. and Engels, Bernd},
  title     = {Ab initio calculations of the vibronically averaged values for the hyperfine coupling constants in NH\(_2\), NHD and ND\(_2\)},
  url       = {http://nbn-resolving.de/urn:nbn:de:bvb:20-opus-58941},
  year      = {1992},
  abstract  = {Vibronically averaged values for K =0 and K = 1 bending levels in the energy range between 0 and 25 000 cm\(^{-1}\) are computed for the \(^{14}\)N, H, and D atoms in NH\(_2\), NHD, and ND\(_2\) The pure ab initio electronic potentials, as well as those derived by fitting of experimentally observed band positions are employed. Effects of vibronic coupling and local perturbations of close-lying levels belanging to different electronic states are discussed.},
  subject      = {Organische Chemie},
  language  = {en}
}
@article{EngelsPericReuteretal.1992,
  author    = {Engels, Bernd and Peric, M. and Reuter, W. and Peyerimhoff, S.D. and Grein, F.},
  title     = {Study of the hyperfine coupling constants \(^{14}\)N and \(^1\)H) of the NH\(_2\) molecule in the X\(^2\)B\(_1\) ground state and the A\(^2\)A\(_1\) excited state},
  url       = {http://nbn-resolving.de/urn:nbn:de:bvb:20-opus-58937},
  year      = {1992},
  abstract  = {The hyperfine coupling constants (hfcc) A\(_{iso}\) and A\(_{ij}\) are calculated for the atoms of NH\(_2\) in its, two lowest-lying electronk states at various molecular geometries by means of the ab initio multireference configuration interaction .method. The vibronically averaged values of the hfccs for the K = 0 and 1 levels in \(^{14}\)N \(^1\)H\(_2\) in the energy range up to 20 000 cm\(^{-1}\) are computed. Polarization elfects which determine A\(_{iso}\) as well as a simple model to describe the dipolar hfccs are discussed. All resrilts are in excellent agreement with experimental data.},
  subject      = {Organische Chemie},
  language  = {en}
}
@article{ReuterEngelsPeyerimhoff1992,
  author    = {Reuter, W. and Engels, Bernd and Peyerimhoff, S.D.},
  title     = {The Reaction of Singlet and Triplet Methylen with Ethene - A Multi-Reference CI Study},
  url       = {http://nbn-resolving.de/urn:nbn:de:bvb:20-opus-58923},
  year      = {1992},
  abstract  = {Large-scale multireference configuration interaction (MRD-CI) calculations in a flexible atomic orbital (AO) basis are employed to study the reaction of C\(_2\)H\(_4\) with CH\(_2\) in its firSt triplet and singlet state. The minimum energy path (MEP) of both reactions is calculated, and different mechanisms are discussed. To examine the possible participation of the singlet state in the overall reaction starting from the triplet channel and terminating in the singlet-state c-C\(_3\)H\(_6\), various cuts through both hypersurfaces are calculated. lt is found that favorable interconversion from the trip1et to the singlet surface can only occur at !arge separations of the two fragments of CH2 and C\(_2\)H\(_4\). Experimental data considering the vibrational motion of CH\(_2\) in connection with the relative position of both surfaces are used to obtain an estimate for the overall barrier of the reaction. The height of the barrier is about 6 kcal/mol, while the barrier of the pure triplet reaction is calculated to be 7-9 kcal/mol.},
  subject      = {Organische Chemie},
  language  = {en}
}
@article{PericEngelsPeyerimhoff1991,
  author    = {Peric, M. and Engels, Bernd and Peyerimhoff, S.D.},
  title     = {Ab initio investigation of the vibronic structure of the C\(_2\)H spectrum Computation of the vibronically-averaged values for the Hyperfine Coupling Constants},
  url       = {http://nbn-resolving.de/urn:nbn:de:bvb:20-opus-58915},
  year      = {1991},
  abstract  = {The vibronically averaged values for tbe hyperfine coupling constants in the X\(^2 \sum\)-A\(^2 \Pi\) system of the ethynyl radical are computed by means of tbe ab initio metbod calculations. The results point at tbe importance of taking into account the coupling of a1l tbree electronic states in question ( I\(^2\)A', 2\(^2\)A', and 1\(^2\)A") for a reliable explanation of the available experimental findings. The mean values of the hfcc's for K = 0 and 1 levels in \(^{13}\)C\(_2\)H and \(^{13}\)C\(_2\)D in the energy range up to 6000 cm\(^{-1}\) are predicted.},
  subject      = {Organische Chemie},
  language  = {en}
}
@article{PericEngelsPeyerimhoff1991,
  author    = {Peric, M. and Engels, Bernd and Peyerimhoff, S.D.},
  title     = {Ab initio investigation of the vibronic structure of the C\(_2\)H spectrum Calculation of the hyperfine coupling constants for the three lowest lying electronic states},
  url       = {http://nbn-resolving.de/urn:nbn:de:bvb:20-opus-58901},
  year      = {1991},
  abstract  = {The hyperfine coupling constants (isotropic hfcc and four Cartesian components of the ani~ tropic tensor) are calculated for all three atoms of C\(_2\)H in its three lowest-lying electronic states at various molecu)ar geometries by means of the ab initio configuration interaction ( MRO.CI) method. The off-diagonal electronic matrix elements involving the two species ofthe A' symmetry are also computed. A diabatic transforrnation is perforrned Jeading to simple geometrical depen· dences of the hyperline coupling constants.},
  subject      = {Organische Chemie},
  language  = {en}
}
@article{Engels1991,
  author    = {Engels, Bernd},
  title     = {Estimation of the influence of the configurations neglected within truncated MR-CI wavefunctions on molecular properties},
  url       = {http://nbn-resolving.de/urn:nbn:de:bvb:20-opus-58890},
  year      = {1991},
  abstract  = {Reliable prediction of the isotropic hyperfine coupling constant, a\(_{iso}\), is still a difficult task for ab initio calculations. Strong dependence on the method used for its calculation is found. Within a truncated multi-referencc ansatz a\(_{iso}\) is strongly affected by the size ofthe reference space and the nurober of terms in the truncated Cl expansion. In the present paperdifferent effects of the neglected Cl space are discussed. Modified B\(_K\) and A\(_K\) methods are used to estimate the contributions ofthe neglected configurations. lt can be shown that a combination of both methods is able to recover about 90-9 S\% of the total error in a\(_{iso}\)· Furthermore, it was found that to obtain about 90\% of the B\(_K\) correction only about I 0-20\% ofthe configurations within H0 have to be corrected.},
  subject      = {Organische Chemie},
  language  = {en}
}
@article{BuendgenEngelsPeyerimhoff1991,
  author    = {B{\"u}ndgen, P. and Engels, Bernd and Peyerimhoff, S.D.},
  title     = {An MRD-CI study of low-lying electronic states in CaF},
  url       = {http://nbn-resolving.de/urn:nbn:de:bvb:20-opus-58880},
  year      = {1991},
  abstract  = {Dipole moments and various spectroscopic constants of some low-lying electronic states of the CaF molecule have been calculated using the multireference single· and double-excitation configuration-interaction (MRD-CI) method. The electronic structure of the highly ionic molecule in various excited states can be explained in tenns of different polarisations of the mainly Cacentered valence electron in the field of the F\(^-\) anion. Plots of natural orbitals occupied by the valence electron in the different states give a qualitative picture of the charge distribution and provide a visualisation of the different polarisations of the valence electron in the various states. Comparisons with the electrostatic polarisation model ofT{\"o}rring, Ernstand K{\"a}ndler (TEK model) are made. The unknown A' \(^2 \Delta\) state is predicted to lie about 21200 cm\(^{-1}\) above the ground state.},
  subject      = {Organische Chemie},
  language  = {en}
}
@article{FunkenEngelsPeyerimhoffetal.1990,
  author    = {Funken, K. and Engels, Bernd and Peyerimhoff, S.D. and Grein, F.},
  title     = {Study of the hyperfine coupling constants of the moleculs NH<sub>2</sub>, NHD and ND<sub>2</sub>},
  url       = {http://nbn-resolving.de/urn:nbn:de:bvb:20-opus-58875},
  year      = {1990},
  abstract  = {In the present paper we c:alculate tbe magnetic hyperfine couplina constants (hfcc) ai.ID and A11 of the ground states of the isotopes NH2, NHD and ND2 using truncated MR..CI methods. Differences from other theoretical methocls and shortoominp of the truncated Cl approach in calculating tlj10 are studied. Polarization effects wbich detennirae ailo. as weU as a simple model to describe the dipolar hfcc's, are discussed. All results are in. excellent aareement with experimental data. lt is shown that ab initio methods are able to obtain reliable values for otf-diaaonal values of A41 which are difficult to measure experimentaDy.},
  subject      = {Organische Chemie},
  language  = {en}
}
@article{KarnaGreinEngelsetal.1990,
  author    = {Karna, S.P. and Grein, F. and Engels, Bernd and Peyerimhoff, S.D.},
  title     = {Ab initio configuration-interaction studies of the ground state potential energy and hyperfine coupling constants of \(^{35}\)Cl\(_2^-\)},
  url       = {http://nbn-resolving.de/urn:nbn:de:bvb:20-opus-58869},
  year      = {1990},
  abstract  = {Potential energy and spectroscopic constants for the X\(^2 \sum^+ _\mu\) ground state of a;, were calculated by configuration-interaction (Cl) methods, using large basis sets with polarization and diffuse functions. From these CI wavefunctions, the isotropic (a\(_{iso}\)) and dipolar (A\(_{dip}\)) components of the hyperfine coupling constant were obtained. The effects of various s, p basis sets, polarization and diffuse functions, as well as the influence of reference configurations and configuration selection thresholds were investigated. The best values obtained are 35·31 G for a\(_{iso}\) and 29·440 for A\(_{dip}\)• tobe compared with experimental values of 37 ± 1 G and 32 ± 1 G, respectively. It is shown that the contributions to a1so of the K and L shells are opposite in sign, differing by about 4 G. Upon vibrational averaging, both a\(_{iso}\) and A\(_{dip}\) move towards smaller values as v increases. An adiabatic electron affinity of 2·46eV was obtained for CL\(_2\) , and a vertical electron detachment energy of 3·71 eV for Cl;.},
  subject      = {Organische Chemie},
  language  = {en}
}
@article{EngelsPeyerimhoffSkell1990,
  author    = {Engels, Bernd and Peyerimhoff, S.D. and Skell, P.S.},
  title     = {Theoretical study of the potential energy surface governing the stereochemistry in ClC\(_2\)H\(_4\) reactions},
  url       = {http://nbn-resolving.de/urn:nbn:de:bvb:20-opus-58851},
  year      = {1990},
  abstract  = {Large-scale multireference configuration interaction calculations in a double·t·type AO basis including polarization functions are carried out for the potential surface of the ClC\(_2\)H\(_4\9 system. The charge distribution for various extreme points of the surface is discussed. The absolute minimum is found for an asymmetric ClC2H4 structure. The symmetrical bridged nuclear conformation is also found to be stable with respect to dissociation into Cl + C\(_2\)H\(_4\)• The activation energy for rotation about the C-C axis is calculated tobe around 18 kJ/mol, which is comparable tothat for the 1,2 migration {around 26 kJ/mol). The stereochemistry is governed by the fact that addition of CI to C\(_2\)H\(_4\) (or dissociation) is a two-step reaction proceeding through a symmetrica1 intermediate. The direct addition pathway possesses a small barrier of about 8 kJ jmol.},
  subject      = {Organische Chemie},
  language  = {en}
}
@article{EngelsPeyerimhoff1989,
  author    = {Engels, Bernd and Peyerimhoff, S.D.},
  title     = {Study of orbital transformations in configurational interaction calculation of hyperfine coupling in nitrogen and CH molecule},
  url       = {http://nbn-resolving.de/urn:nbn:de:bvb:20-opus-58847},
  year      = {1989},
  abstract  = {Multi-reference configuration interaction calculations employing various orbital transformations are undertaken to obtain the isotropic hyperfine coupling constant a\(_{iso\) in nitrogen and a\(_{iso\) (H) in the CH molecule. The natural orbital (NO) basis is found to be more effective than the simple RHF-MO basis; the most obvious is a basis of spin natural orbitals (SNO). It is found that a\(_{iso\) is approached from opposite sides in the NO and 2s shell SNO basis if the CI expansion is increased. Both results are within a few percent of the full CI Iimit for the nitrogen atorn (in the given AO basis) and the experimental value for Hin the CH radical. Various features ofthe SNO are discussed.},
  subject      = {Organische Chemie},
  language  = {en}
}
@article{EngelsPeyerimhoff1989,
  author    = {Engels, Bernd and Peyerimhoff, S.D.},
  title     = {The hyperfine coupling constants of the X\(^3\)Σ\(^-\) state of NH : Influence of polarization functions and configuration space on the description of spin polarization},
  url       = {http://nbn-resolving.de/urn:nbn:de:bvb:20-opus-58832},
  year      = {1989},
  abstract  = {The hyperfine coupling constants for the \(^3\)Σ\(-\) ground state of the NH molecule are determined by configuration interaction calculations whereby the infl.uence of polarization functions as weil as of the configuration space on the spin polarization mechanism is analysed. The dipolar part Au(N) and Au(H) can be obtained very reliably without much computational effort (A .. (N) == -45·3 MHz and A"(H) = -62·3 MHz). The value for the isotropic contribution a1.., in the best AO basis and MRD-CI treatment is - 64·5 MHz for H and 16·6 MHz for nitrogen compared to the corresponding experimental quantities of -66 MHz and 19 MHz respectively. Their determination depends on a subtle balance of the lu, 2u and 3u shell correlation description, whereby the dominant contribution to a1..,(H) results from the 2u shell. It is shown that the often good agreement of a110 values with experiment in a small basis singledouble configuration interaction treatment results from a cancellation of two errors.},
  subject      = {Organische Chemie},
  language  = {en}
}
@article{EngelsPeyerimhoff1989,
  author    = {Engels, Bernd and Peyerimhoff, S.D.},
  title     = {Theoretical study of FC\(_2\)H\(_4\)},
  url       = {http://nbn-resolving.de/urn:nbn:de:bvb:20-opus-58824},
  year      = {1989},
  abstract  = {Large-scale multireference configuration interaction (MRD-CI) calculations in a quite flexible AO basis are employed to study the energy hypersurface for the reaction intermediate FC\(_2\)H\(_4\) • The reaction F + C\(_2\)H\(_4\) -> FC\(_2\)H\(_4\) as weil as the 1,2 migration of the fluorine atom in FC\(_2\)H\(_4\) is investigated. In addition the rotation around the CC bond in the optimum conformation is studied. The absolute minimum in the potential energy is found for the asymmetric structure but the symmetric structure is also found to be stable with respect to the dissociation, so that a shuttling of the fluorine atom is in principle possible but highly unlikely because ( l) the activation energy is high ( II 5-130 kJ fmol) and the saddle point lies only 4(}-50 kJ jmol below the dissociation Iimit of F + C\(_2\)H\(_4\) and (2) the competitive motion, i.e., rotation around the CC axis, is nearly free (I 1-17 kJ/mol).},
  subject      = {Organische Chemie},
  language  = {en}
}