The history and evolution of configuration interaction

GRASP92: a package for large-scale relativistic atomic structure calculations

Semiempirical HAM/3 molecular orbital configuration interaction (CI) calculations have been employed to interpret the gas-phase He(I) photoelectron (PE) spectrum of 1,9-dimethylguanine (1,9-DMG). Previous interpretation of the PE spectrum of this molecule, which has a tautomeric structure similar to that of guanine in DNA and RNA, has relied on SCF calculations. Test calculations were performed on pyrimidine. Because HAM/3 calculations without CI were parameterized to describe PE data, ionization potentials (IPS) obtained at the SCF level agree better with experiment than IPS obtained with CI calculations. However, when pyrimidine results from the HAM/3 CI calculations are compared to previously reported results obtained using the outer-valence Green function (OVGF) method or the two-particle-hole Tamm–Dabcoff Green function approximation (2ph-TDA) method, the agreement is satisfactory. The average difference between corresponding values for the first four IPS of pyrimidine obtained from HAM/3 CI, OVGF, and the 2ph-TDA calculations is less than 0.5 eV. Furthermore, for the seven lowest-energy cation states of pyrimidine, the three computational methods yield normalized coefficients associated with the dominant Koopmans configurations that have squares that differ by less than 0.14. For 1,9-DMG, the results of HAM/3 CI calculations indicate that five of the seven lowest-energy ionization events, previously assigned to the first-to-third and to the sixth- and seventh-highest occupied orbitals, are well described within the framework of Koopmans theorem. On the other hand, the fourth and fifth ionization events in 1,9-DMG result in cation states in which there is significant hole-mixing of Koopmans configurations associated with removal of electrons from the second- and third-highest occupied π orbitals. © 1992 John Wiley & Sons, Inc.

A nonstandard computational scheme has been applied to calculate Fe(CO)(5) with the aim to illustrate the operation of the Dewar-Chatt-Duncanson model by computation. A full configuration interaction (CI) calculation in an active space has been performed. The active space is built from naturally localized molecular orbitals (NLMOs) localized in bond regions or forming lone pairs. For selecting this active space, Weinhold's perturbation theory formulated in the natural bond orbital (NBO) space has been applied. Bonding, lone pair, and antibond NBOs exhibiting large interaction energies serve to define the active space. The actually applied active space, however, comprises NLMOs that are close in shape to the NBOs indicated by perturbation theory. Thus, a CI calculation with localized orbitals has been performed meeting the classical reasoning of chemists that is often based on local bonding concepts. The computational scheme yields the Lewis structure for Fe(CO)(5) whose energy is identical to the Hartree-Fock energy. The Lewis energy comprises CO → Fe σ-electron transfer (ET) and CO ← Fe electron back donation (BD). This Lewis energy gets lowered by localized correlation energy contributions caused by ET processes where electrons are back donated from the Fe d-lone pairs into the CO ligands. Thus, electron correlation within the selected active space is dominated by electron BD. Energies and electron populations of the NBOs support the notion that electrons are preferentially back donated into the equatorial CO ligands. Weights of local Slater determinants, determining the correlation energy, also point to a predominant BD into the equatorial CO ligands. Correlation energy increments resulting from electron BD into single antibond orbitals of the CO ligands have been calculated. These energy quantities also demonstrate that BD into the equatorial CO ligands is more energy lowering than BD into the axial CO ligands.

X-ray absorption near edge structure (XANES) at L2,3-edge of 3d transition elements is dominated by strong correlation effects among 2p core hole and 3d electrons. In the present study, we have performed systematic configuration interaction (CI) calculations in order to reproduce and interpret Fe-L2,3 XANES of FeO, LaFeO3 and SrFeO3. Relativistic four components wave functions were obtained by solving Dirac equations with density functional theory. CI calculations were made using the relativistic molecular orbitals instead of atomic orbitals, which enables inclusion of the O-2p orbital contributions through covalency. The oscillator strength of the electric dipole transition was then computed. Experimental XANES spectra of three compounds were satisfactorily reproduced by the theoretical spectra obtained for (FeO6)m− clusters in octahedral symmetry. Chemical shifts between compounds were quantitatively reproduced as well. Component analysis of CI was systematically made in order to analyze the origin of differences in spectral shapes.

The combinatorial growth of configuration interaction (CI) has long limited this formally exact quantum chemistry method to only the smallest molecules. Here, we report a numerically exact CI calculation exceeding one quadrillion (1015) determinants, made possible by a lossless categorical compression strategy within the small-tensor-product distributed active space (STP-DAS) framework. This approach overcomes the traditional memory bottlenecks of CI by a numerically exact compression of the wavefunction representation and reformulating the most computationally demanding matrix–vector operations. Using this method, we performed a fully relativistic CI calculation of the ground state of HBrTe with over 1015 complex-valued determinants in just 34.5 h on 1000 computing nodes—the largest CI calculation ever reported. We further achieved fast computation for systems with hundreds of billions of determinants on only a few compute nodes. Extensive benchmarks confirm that the method retains full numerical exactness while cutting memory and computational cost by orders of magnitude. Compared to previous state-of-the-art CI calculations, this work achieves a 1000 times increase in CI space, a 106-fold increase in floating-point operations performed, and a 106-fold improvement in computational speed.

Configuration Interaction (CI) calculations are carried out for bond-breaking processes by employing localized molecular orbitals (LMO). It is found that the advantage of this choice relative to the use of canonical orbitals is two-fold: First, the convergence of the truncated CI is speeded up by employing LMO; secondly, since the LMO are localized in certain regions of the molecular system one can divide the LMO in two sets, one which is important for the description of the process under consideration and the other which describes the molecular part not directly involved in the process (“spectator group”). It is found to be sufficient to correlate only the “local active region” of the molecule, and this procedure leads to a much shorter CI expansion without loss of accuracy in describing the chemical process. The CH3 group has been used as “spectator group” and the dissociation into radicals CH3X → CH3 +X and CH3CH2X → CH3CH2+X with X=H, F, NH3+ as well as the charge transfer reaction CH3-X+ → CH3++X and CH3CH2-X+ → CH3CH2++X with X=N2 and NH3 has been studied. For both type of radical reactions the CH3 bonds need not be included in the local active region, while in the charge-transfer process the CH3 can only be considered a spectator group if it is not connected directly to the separating group.

A computational strategy to analyze Cu-O(2) adducts based on the use of difference-dedicated configuration interaction (DDCI) calculations is presented. The electronic structure, vertical gaps and nature of the metal-O(2) interaction, and the extension of the charge transfer between both fragments have been investigated. Relative stabilities between isomers are determined from triplet states CCSD(T) calculations. The key point of the here proposed strategy rests on the use of a rationally designed active space, containing only those orbitals, which optimize the interaction pathways between LCu and O(2) fragments. The procedure has been tested on a broad set of model and synthetic biomimetic systems, the results compared with previous theoretical evaluations and/or available experimental data. Our study indicates that this strategy can be considered as an alternative approach to multireference second-order perturbation theory methods to deal with this type of systems with remarkable biradical nature.

Experimental photoelectron spectrum of Si2C2- was assigned by ab initio Monte Carlo simulation with the multicanonical/histogram reweighting technique. At first, two structures, linear and ring, were optimized at the MP2/6-31G* level. The multireference configuration interaction (MRCI) calculation reveals that the spectrum cannot be assigned by considering only a single isomer. Also, MRCI calculations cannot explain different widths of the peaks in the spectrum. Multicanonical Monte Carlo simulation with configuration interaction calculation at each step of the simulation was performed for both ring and linear isomers to include the effects of large nuclear motion at finite temperature on the spectrum. It was found that a mixture of two isomers at the experimental condition is necessary to assign the spectrum.

A method is developed to optimize the separated-pair independent particle (SPIP) wave function; it is a special case of the separated-pair theory obtained by using two-term natural expansions of the geminals. The orbitals are optimized by a theory based on the generalized Brillouin theorem and iterative configuration interaction (CI) calculations in the space of the SPIP function and its single excitations. The geminal expansion coefficients are optimized by serial 2 x 2 CI calculations. Formulas are derived for the matrix elements. An algorithm to implement the method is presented, and the work needed to evaluate the molecular integrals is discussed.

Using a valence double-zeta polarization basis, full configuration–interaction (FCI) calculations are carried out on water at its equilibrium geometry and at geometries where the OH bond lengths are stretched until dissociation. At the same geometries and with the same basis set configuration interaction calculations at excitation levels up to hextuples, multireference singles doubles configuration interaction calculations, coupled cluster calculations at excitation levels up to quadruples, Mo/ller–Plesset perturbation theory calculations through order fifteen, and complete active space second-order perturbation theory calculations are also carried out. The static correlation contribution increase with increasing bond length. The calculations show that the coupled cluster approach has a remarkable ability to describe even relatively large static correlation contributions. The single reference perturbation expansion breaks down for larger OH bond length, while the multireference approach preserves the accuracy for the whole potential curve. At the equilibrium geometry, FCI calculations have also been carried out for the lowest state of 2A1, 2B1, and 2B2 symmetry of H2O+, and the results compared with state of the art correlation results for total energies and ionization potentials (IP’s). Differential energies (IP’s) are obtained more accurately than absolute (total) energies in the size extensive coupled cluster and perturbation approaches. For the nonsize extensive configuration interaction method errors are obtained of the same size for differential and absolute energies.

The role played in configuration interaction calculations by the particular form of the orbitals selected as basis is explored by a direct application on the electronic structure of the ground state of the OH molecule. Two configuration interaction studies have been carried out using different sets of one-particle functions: (1) the set natural to the isolated atoms (called AO's) and (2) the orbitals belonging to the molecule (MO's). In (1), using the Hartree-Fock 1s, 2s, and 2p functions for atomic oxygen and the 1s state for hydrogen, and keeping the 1s oxygen state doubly occupied, all Slater determinants of the appropriate ground state symmetry (2π) were found. The dissociation energy and the expansion coefficients of the determinants were determined by a variational procedure which minimized the energy of the system. The nonorthogonality of the basic AO's was treated without approximation by the method of Löwdin. The necessary integrals were calculated on Whirlwind I, the M.I.T. high-speed digital computer; all two-center integrals were computed by expanding the hydrogen orbital about the oxygen center (analogously to the methods of Barnett and Coulson, Coolidge, Löwdin, etc.). In (2), the molecular orbitals (MO's) were found as linear combinations of the above AO's, with coefficients determined by a modified self-consistent field (Roothaan) procedure, in which the energy of a single Slater determinant was made stationary at three values of the internuclear separation. A configuration interaction calculation was then made using these ``best'' MO's as the basic set of one-particle functions. Also included are a calculation of the dipole moment and Mulliken's electron distribution analysis. The results of these studies are discussed and comparison is made to experiment and other theoretical calculations.

We examine the behaviour of 2 electrons on a 2 x 2 square lattice sites. The Hubbard model was used to model the system and was diagonalized using the configuration interaction method which is a variational method. The variations of the ground state energy ground state double occupied sites D, and the ground state kinetic energy with the interaction strength (u 0) were examined. The functions and were found to be parabolic functions of. This suggests that superconductivity is driven by both electron and hole doping. Key words: Hubbard model, Superconductivity, Configuration interaction and Electron correlation.

The present study provides a wide range of calculations of atomic structure data for the tungsten ion W LXI, that belongs to the silicon isoelectronic sequence. The fully relativistic Multi-configuration Dira-Hartree–Fock (MCDHF) method including configuration interaction (CI) is applied to the current ion. Calculated results of energy levels, oscillator strengths, and radiative rates of W LXI are presented. A set of two configuration state lists including the configurations of 3s23p2, 3s3p3, 3s23p3d, and 3p4 at different angular momenta (J) and parity (P) are considered in the configuration interaction calculations of level energies and oscillator strengths. To increase the accuracy of the results, the electronic correlations from higher orbitals, i.e., 7l orbitals are included in the calculations. Quantum electrodynamics effects such as vacuum polarization and self-energy are recalled in the CI calculations. Many more transitions in the soft x-ray region (between 20 and 200 angstroms) have been identified. The flexible atomic code package has been used to compute the electron impact ionization cross-sections of 3s23p2 3P0,1, 1D2 levels in silicon-like tungsten. The cross-sections of the transitions from the ground level in W LXI to the first levels in the 61st ionization of tungsten have been computed using FAC code. The total ionization cross-sections of the 3s23p2 at electron energy range from 5–35 keV are evaluated as well.

Bakgrunn: \nIn vitro fertiliseringsklinikken ved Universitetssykehuset i Nord Norge har vært sterkt delaktig i utviklingen av et regime til bruk i klinisk in vitro fertilisering som går ut på å dyrke embryo i langtidskultur i fem dager til blastocyster. Dette for å selektere de mest levedyktige embryoene for tilbakeføring ved å observere hvilke embryoer som utvikler seg best og ekskludere de som tidlig stagnerer eller går til grunne. Formålet med denne studien er å se om resultatene fra IVF-behandlingen, som antall fødsler, aborter og andel dyrkede blastocyster står i sammenheng med utviklingen av nye medier for langtidskultur.\nMateriale og metode: \nDette er en retrospektiv studie som inkluderer alle par som gjennomgikk in vitro fertilisering ved IVF-klinikken i Tromsø i perioden 01.08.93-30.01.95 og året 2005.\nResultater og fortolkning: \nDe endelige resulatene ved IVF-klinikken i Tromsø var omtrent de samme i perioden 01.08.93-31.01.95 som i år 2005, med en fødselsrate av alle tilbakeføringer på 42% i første periode og 39% i andre periode. Pasientpopulasjonen er blitt eldre og indikasjonene for IVF behandling er tydelig endret. Særlig er andelen tubefaktor, som generelt gir høy suksessrate, langt lavere i 2005. Til tross for at pasientene er blitt eldre og vanskeligere å behandle har resultatene ikke blitt dårligere. Dette henger tett sammen med bedre kulturmedier. Forholdene for embryoene in vitro er betydelig bedret, og med de nye mediene sees en hurtigere embryovekst og bedre embryokvalitet.

The electronic structure of the lead monofluoride molecule is studied by means of ab initio configuration interaction (CI) calculations including the spin-orbit interaction. Potential-energy curves are generated for a large number of electronic states, of which only the X1 2Π1/2 ground and X2 2Π3/2 and A 2Σ+ excited states have been observed experimentally. Two different methods are compared for the inclusion of spin-orbit effects in the theoretical treatment, a contracted CI which employs a basis of large-scale Λ–S eigenfunctions to form a rather small matrix representation of the full relativistic Hamiltonian (two-step approach), and a more computationally laborious technique which involves solution of a secular equation of order 250 000 S2 eigenfunctions of different spin and spatial symmetry to achieve a potentially more evenly balanced description of both relativistic and electron correlation effects (one-step approach). In the present application, it is found that both methods achieve quite good agreement with measured spectroscopic constants for the X1, X2, and A states. The simpler of these methods is also employed to predict the radiative lifetimes of the latter two states. The key A 2Σ+–X 2Π transition moment in these calculations is found to vary strongly with internuclear distance and to vanish in the neighborhood of the respective equilibrium distances of both participating states. The computed lifetime for the A, v′=0 state of 16 μs overestimates the corresponding measured value by a factor of three, but those of higher vibrational states are found to decrease rather sharply with increasing v′, suggesting that only a slight displacement of the theoretical A–X transition moment curve is needed to explain the above discrepancy.