Results 1 - 10 of 3049
Results 1 - 10 of 3049. Search took: 0.028 seconds
|Sort by: date | relevance|
[en] The generalized Koopmans theorem gives an estimate of the lowest ionization energy of an N-electron system. The approximate lowest ionization energy is obtained as the lowest eigenvalue of a generalized eigenvalue problem. It has been shown that the generalized Koopmans theorem predicts the exact lowest ionization energy for Coulomb systems. However, it has also been shown that in general no eigenvalue of the generalized Koopmans procedure corresponds to an exact ionization energy. This paradox is resolved by demonstrating that the lowest eigenvalue of the generalized Koopmans procedure does, in general, not exist for Coulomb systems. Nevertheless, it will also be shown that the generalized Koopmans approach gives ionization energies which are arbitrary close to the exact lowest ionization energy. The eigenvalues of the extended Koopmans theorem have an accumulation point given by the exact lowest ionization energy
[en] Ionization potentials of chemical elements with the serial numbers 71-86 and 81-102 for some electrons of 0-level and P-level, respectively, are determined. The diagram of separation energy of K-, L-, M-, N-, O-, P-, Q- electrons of outer levels for all the elements of the D.I. Mendeleev periodic system is plotted
[en] The first ionization potential (IP_1) of element 103, lawrencium (Lr), has been successfully determined for the first time by using a newly developed method based on a surface ionization process. The measured IP_1 value is 4.963"0"."0"8_0_._0_7 eV. This value is the smallest among those of actinide elements and is in excellent agreement with the value of 4.963(15) eV predicted by state-of-the-art relativistic calculations also performed in this work. Our results strongly support that the Lr atom has an electronic configuration of [Rn]7s"25f"1"47p"1_1_/_2, which is influenced by strong relativistic effects. The present work provides a reliable benchmark for theoretical calculations and also opens the way for studies on atomic properties of heavy elements with atomic number Z > 100. Moreover, the present achievement has triggered a controversy on the position of lutetium (Lu) and Lr in the Periodic Table of Elements.
[en] We report the first adiabatic ionization energy (I1) of benzene, and the ortho, meta and para isomers of fluoro-, chloro- and bromobenzonitrile, extracted from gas-phase photoionization spectra measured up to the LiF cut-off. In every case, I1 for the disubstituted benzene species is blue-shifted from I1 in benzene. Moreover, I1 decreases along the series F, Cl, Br. We explain these trends qualitatively by invoking destabilization/stabilization of the topmost filled πMO of benzene by donor/acceptor substituents. (orig.)
[en] A computational approach is presented for prediction and interpretation of core-level spectra of complex molecules. Applications are presented for several isolated organic molecules, sampling a range of chemical bonding and structural motifs. Comparison with gas phase measurements indicate that spectral lineshapes are accurately reproduced both above and below the ionization potential, without resort to ad hoc broadening. Agreement with experiment is significantly improved upon inclusion of vibrations via molecular dynamics sampling. We isolate and characterize spectral features due to particular electronic transitions enabled by vibrations, noting that even zero-point motion is sufficient in some cases
[en] The use of back-transformed pair natural orbitals in the calculation of excited state energies, ionization potentials, and electron affinities is investigated within the framework of equation of motion coupled cluster theory and its similarity transformed variant. Possible approaches to a more optimal use of pair natural orbitals in these methods are indicated.
[en] Using beryllium as a model system it is suggested that cluster simulations of core-electron ionization potentials in metals should be carried out with the same number of electrons in the initial and final states
[en] Recently observed magic number data for NaN clusters in the size range from 100 to 900 atoms cannot be fully explained by density functional calculations using a homogeneous, spherical positive charge background. However, a centrally compressed spherical background yields steps in the ionization potential at just those magic numbers observed experimentally. (orig.)
[en] A set of 146 well-established ionization potentials and electron affinities is presented. This set, referred to as the G2 ion test set, includes the 63 atoms and molecules whose ionization potentials and electron affinities were used to test Gaussian-2 (G2) theory [J. Chem. Phys. 94, 7221 (1991)] and 83 new atoms and molecules. It is hoped that this new test set combined with the recently published test set of enthalpies of neutral molecules [J. Chem. Phys. 106, 1063 (1997)] will provide a means for assessing and improving theoretical models. From an assessment of G2 and density functional theories on this test set, it is found that G2 theory is the most reliable method. It has an average absolute deviation of 0.06 eV for both ionization potentials and electron affinities. The two modified versions of G2 theory, G2(MP2,SVP) and G2(MP2) theory, have average absolute deviations of 0.08 endash 0.09 eV for both ionization potentials and electron affinities. The hybrid B3LYP density functional method has the smallest average absolute deviation (0.18 eV) of the seven density functional methods tested for ionization potentials. The largest deviation for the density functional methods is for the ionization potential of CN (>1thinspeV). The BLYP density functional method has the smallest average absolute deviation (0.11 eV) of the seven density functional methods tested for electron affinities, while the BPW91, B3LYP, and B3PW91 methods also do quite well. copyright 1998 American Institute of Physics