Configuration Interaction Technique Research Articles

Spin transitions induced by a magnetic field in quantum dot molecules

We present a theoretical study of magnetic field driven spin transitions of electrons in coupled lateral quantum dot molecules. A detailed numerical study of spin phases of artificial molecules composed of two laterally coupled quantum dots with N=8 electrons is presented as a function of magnetic field, Zeeman energy, and the detuning using real space Hartree-Fock Configuration Interaction (HF-CI) technique. A microscopic picture of quantum Hall ferromagnetic phases corresponding to zero and full spin polarization at filling factors $\nu=2$ and $\nu=1$, and ferrimagnetic phases resulting from coupling of the two dots, is presented.

Decay of Hybridized Electronic States of a Na Cluster on Cu(001)

The electronic properties of a Na9+ cluster are markedly changed when deposited on a Cu(001) surface. Particularly, the lifetime of the (hybridized) single-particle electronic states are drastically enhanced indicating a change in electronic correlations upon absorption. To capture this effect, we developed a Green's function approach based on the configuration interaction technique. The calculated lifetimes (16.5 to 33 fs) of excited electronic states are in line with experimental observations. Our new method demonstrates the feasibility of accounting accurately for electronic correlation in large, nonperiodic systems.

Relativistic corrections to transition frequencies ofFe Iand search for variation of the fine-structure constant

Relativistic energy shifts of the low-energy levels of Fe have been calculated using the Dirac-Hartree-Fock and configuration interaction techniques. The results are to be used in the search for the space-time variation of the fine-structure constant in quasar absorption spectra. The values of the shifts are the largest among those used in the analysis so far. This makes Fe a good candidate for the inclusion into the analysis.

The electronic and optical properties of oligo(trans‐1,2‐di(2‐thienyl)‐1,3‐butadiene): A theoretical study

AbstractIn the present work we investigated the theoretical electronic structure of poly(trans‐1,4‐di(2‐thienyl)‐1,3‐butadiene) (PTB) and determined the optical properties of its neutral and doped oligomers. Geometrical optimizations were at the semiempirical level by using the Austin method 1 (AM1). The band structure of π electrons regarding to the neutral PTB polymer was obtained by using a tight‐binding Hamiltonian. The densities of electronic states (DOS) for neutral and doped copolymers were calculated by using the negative factor counting technique. The spatial charge distribution of the oligomeric chain was also analyzed. The energy of the electronic transitions and their associated oscillator strength values were calculated for the neutral, double, and single charged oligomers to determine the UV–vis absorption spectra. The calculations were performed using the intermediate neglect of differential overlap Hamiltonian in combination with the single configuration‐interaction technique in order to include correlation effects. The band gap obtained in the PTB was about 2.101 eV for the optics absorption and 1.73 eV for the DOS. The bipolaron states appear in the gap, about 0.57 eV and 0.48 eV below and above the conduction and valence bands, respectively. When the dopants concentration is increased the DOS showed that the energy gap tends to vanish, which may lead to semiconductor–metal transition. © 2008 Wiley Periodicals, Inc. Int J Quantum Chem, 2008

A relativistic 4-component general-order multi-reference coupled cluster method: initial implementation and application to HBr

We present the initial implementation of a determinant-based general-order coupled cluster method which fully accounts for relativistic effects within the four-component framework. The method opens the way for the treatment of multi-reference problems through a state-selective expansion of the model space. The evaluation of the coupled cluster vector function is carried out via relativistic configuration interaction expansions. The implementation is based on a large-scale configuration interaction technique, which may efficiently treat long determinant expansions of more than 108 terms. We demonstrate the capabilities of the new method in calculations of complete potential energy curves of the HBr molecule. The inclusion of spin–orbit interaction and higher excitations than coupled cluster double excitations, either by multi-reference model spaces or the inclusion of full iterative triple excitations, lead to highly accurate results for spectral constants of HBr.

On the ground and some low-lying excited states of ScB: A multiconfigurational study

The electronic structure of a series of low-lying excited triplet and quintet states of scandium boride (ScB) was examined using multireference configuration interaction (including Davidson's correction for quadruple excitations) and single-reference coupled cluster (CC) methods with averaged natural orbital (ANO) basis sets. The CC approach was used only for the lowest quintet state. The authors have analyzed eight low-lying triplets 3Sigma-(2), 3Sigma+, 3Pi(3), and 3Delta(2) dissociating to Sc(2D)/B(2P) atoms and eight low-lying quintet states 5Sigma-, 5Sigma+, 5Pi(2), 5Phi, and 5Delta(3) dissociating to Sc(4F)/B(2P) atoms. They report the potential energy curves and spectroscopic parameters of ScB obtained with the multireference configuration interaction (MRCI) technique including all singly and doubly excited configurations obtained with the ANO-S basis set. For the two lowest states they obtained also improved ANO-L spectroscopic constants, dipole and quadrupole moments as well as scalar relativistic effects based on the Douglas-Kroll-Hess Hamiltonian. They provide the analysis of the bonding based on Mulliken populations and occupation numbers. Since the two lowest states, 3Sigma- and 5Sigma-, lie energetically very close, their principal goal was to resolve the nature of the ground state of ScB. Their nonrelativistic MRCI(Q) (including Davidson correction) results indicate that the quintet is more stable than the triplet by about 800 cm(-1). Inclusion of scalar relativistic effects reduces this difference to about 240 cm(-1). The dissociation energies for 5Sigma- ScB range from 3.20 to 3.30 eV while those for the 3Sigma- range from 1.70 to 1.80 eV.

Large-scale correlated calculations of linear optical absorption and low-lying excited states of polyacenes: Pariser-Parr-Pople Hamiltonian

We present large-scale correlated calculations of the linear optical absorption spectrum of oligo-acenes containing up to seven benzene rings. For the calculations we used the Pariser-Parr-Pople (PPP) Hamiltonian, along with the configuration interaction (CI) technique at various levels such as the full CI, the quadruple CI, and multireference singles-doubles CI. The role of Coulomb parameters used in the PPP Hamiltonian was examined by considering standard Ohno parameters, as well as a screened set of parameters. A detailed analysis of the many-body character of the important excited states contributing to the linear absorption has also been performed. The results of our calculations have been compared extensively with the theoretical work of other authors, as well as with experiments.

Oscillator strengths and radiative lifetimes for C2: Swan, Ballik-Ramsay, Phillips, and dΠg3←cΣu+3 systems

High level ab initio calculations, using multireference configuration interaction (MRCI) techniques, have been carried out to investigate the spectroscopic properties of the singlet A 1Piu<--X 1Sigmag+ Phillips, the triplet d 3Pig<--a 3Sigmau Swan, the b 3Sigmag-<--a 3Piu Ballik-Ramsay, and the d 3Pig<--c 3Sigmau+ transitions of C2. The MRCI expansions are based on full-valence complete active space self-consistent-field reference states and utilize the aug-cc-pV6Z basis set to resolve valence electron correlation. Core and core-valence correlations and scalar relativistic energy corrections were also incorporated in the computed potential energy surfaces. Nonadiabatic and spin-orbit effects were explored and found to be of negligible importance in the calculations. Harmonic frequencies and rotational constants are typically within 0.1% of experiment. The calculated radiative lifetimes compare very well with the available experimental data. Oscillator strengths are reported for all systems: fv'v", where 0<or=v<or=5.

Electron impact excitation of molecules: Calculation of the cross section using the similarity function method and ab initio data for electronic structure

Electron impact excitation cross sections of dipole-allowed transitions in diatomic molecules have been evaluated using the similarity function method allowing one to express cross sections through the oscillator strength of the transition and some universal function of the reduced incident electron energy. The necessary characteristics of the molecular terms involved and transition dipoles were determined by ab initio electronic structure calculations using the multireference configuration interaction technique or relativistic multipartitioning many-body perturbation theory. The results of specific calculations performed for CO and NO in a reasonable agreement with the available experimental data and the results of comprehensive quantum-mechanical calculations.

Ab initio prediction of the infrared absorption spectrum of the C2Br radical

The first three electronic states of the C2Br radical, correlating at linear geometries with 2Σ+ and 2Π states, have been studied ab initio, using Multi Reference Configuration Interaction techniques. The electronic ground state is found to have a bent equilibrium geometry, RCC=1.2621Å, R CBr=1.7967Å, ∠ CCBr=156.1°, with a very low barrier to linearity. Similarly to the valence isoelectronic radicals C2F and C2Cl, this anomalous behaviour is attributed to a strong three-state non-adiabatic electronic interaction. The Σ ,Π1/2,Π3/2 vibronic energy levels and their absolute infrared absorption intensities at a temperature of 5 K have been calculated for the 12 C12 C79Br isotopomer, to an upper limit of 2000 cm−1, using ab initio diabatic potential energy and dipole moment surfaces and a recently developed variational method.

Time-independent coupled cluster theory of the polarization propagator. Implementation and application of the singles and doubles model to dynamic polarizabilities and van der Waals constants†

Recently proposed time-independent coupled cluster theory of the polarization propagator [R. Moszynski, P. S. Żuchowski, and B. Jeziorski, Collect. Czech. Chem. Commun. 70, 1109, 2005] has been implemented at the single and double excitations (CCSD) level. The performance of the new approach was investigated by carrying out calculations of static and dynamic electric dipole polarizabilities for various molecules and by making a comparison with values obtained from other ab initio methods, including the full configuration interaction (FCI) technique. Our results show that the polarizabilities computed with the new approach are in a good agreement with the time-dependent CCSD and (when available) FCI values. The isotropic C 6 dispersion coefficients for several benchmark van der Waals complexes, e.g. dimers of helium, argon, water, and benzene, are also reported. They compare very well with existing experimental and best theoretical data. The new propagator, implemented already in the MOLPRO package, is computationally somewhat less demanding than the propagator of the time-dependent coupled cluster theory, and can be considered as an alternative to the latter in applications to large molecules and in studies of their interactions.

Transient absorption spectroscopy and quantum-chemical studies of matrix-isolated perylene derivatives

We present a comprehensive experimental and theoretical study of the optical properties of matrix-isolated molecules of the two perylene derivatives $N,{N}^{\ensuremath{'}}$-dimethylperylene-3,4,9,10-dicarboximide (MePTCDI) and 3,4,9,10-perylenetetracarboxylic dianhydride (PTCDA). A solid solution of the dyes in an $\mathrm{Si}{\mathrm{O}}_{2}$ matrix exhibits monomer-like behavior. Transient absorption pump-probe spectroscopy in the range $1.2--2.6\phantom{\rule{0.3em}{0ex}}\mathrm{eV}$ has been performed on an ultrafast time scale. The differential transmittance reveals contributions from ground-state bleaching, stimulated emission, and excited-state absorption. Both systems exhibit broad excited-state absorption features below $2.0\phantom{\rule{0.3em}{0ex}}\mathrm{eV}$ with a clear peak around $1.8\phantom{\rule{0.3em}{0ex}}\mathrm{eV}$. The spectra can be consistently explained by the results of quantum-chemical calculations. We have applied both the coupled cluster singles and doubles (CCSD) model and the multireference-determinant single and double configuration-interaction (MRD-CI) technique on the basis of the intermediate neglect of differential overlap (INDO) Hamiltonian. The results are insensitive to whether the geometry is optimized for the electronic ground state or first excited state. The experimental polarization anisotropies for the two major transitions are in agreement with the calculated polarizations.

Vortex rings in two-dimensional harmonic traps

We use the configuration interaction technique to study vortex formation in rotating systems of interacting spinless fermions and bosons trapped in a two-dimensional harmonic potential. In the fermionic case, the vortices appear as holes in the Fermi sea and localize in rings. The yrast spectrum is dominated by rigid rotation of the vortex ring, showing periodic oscillations. The Bose system shows a similar yrast spectrum and vortex formation. This can be explained by a one-to-one correspondence of the fermion and boson many-particle configurations. A simple mean-field model can reproduce the oscillations in the yrast spectrum, but fails to explain the localization of vortices.

Theoretical conformational study of poly(trans‐1, 2‐di(2‐thienyl) ethylene): Effects on the electronic structure and optical properties

AbstractIn the present study, we investigate the monomer ethylene‐bridged‐bithiophene (TET) and the dimer ethylene‐bridged‐bithiophene (TET)2 to find the more probable conformations of the oligomer and their electronic properties. Geometrical optimizations were carried out at semiempirical level using the Austin method one (AM1) and parametric method 3 (PM3). The electronic transition energies and their associated oscillator strength values are calculated for neutral oligomers. The calculations are conducted using the intermediate neglect of differential overlap Hamiltonian (INDO) in combination with a single configuration–interaction technique in order to include correlation effects. We also employ the negative factor counting (NFC) technique to obtain the electronic density of states (DOS). © 2006 Wiley Periodicals, Inc. Int J Quantum Chem, 2006

Bounds for the scattering length of spin-polarized helium from high-accuracy electronic structure calculations

We developed a series of correlation-consistent, polarized multiple zeta basis sets optimized specifically for the energy of the 2 3S state of helium atom. These basis sets were subsequently augmented with diffuse functions optimized for the van der Waals constants C6 through C14 which determine the asymptotic behavior of the second-order dispersion interaction between 2 3S helium atoms at large interatomic separation R. The resulting bases were applied to compute the Born-Oppenheimer (BO) potential for the lowest 5Sigmag+ state of the helium dimer. The coupled cluster and the full configuration-interaction techniques were employed to account for the electron correlation effects. The cardinal number extrapolation technique was used to obtain the complete-basis-set limit V(R) for the interaction potential and to find its lower VL(R) and upper VU(R) bounds. The resulting potentials were fitted to an analytical function containing accurate van der Waals constants C6 through C12 (including C11). We found that the complete-basis-set BO potential has a well depth De=1048.24+/-0.36 cm-1. The highest rotationless vibrational level is bound by D14=90.2+/-4.7 MHz, much stronger than the previous most accurate estimation of 15.2 MHz. The error bounds for De and D14 were obtained using the VL(R) and VU(R) potentials. The S-wave scattering length computed using the VL(R), V(R), and VU(R) potentials (assuming atomic masses) is aL=7.41 nm, a=7.54 nm, and aU=7.69 nm, respectively. We also computed the adiabatic, relativistic, and quantum electrodynamics (QED) corrections to the BO potential. When these corrections are taken into account the values of D14 and of a (both computed assuming nuclear masses) are 87.4+/-6.7 MHz and 7.64+/-0.20 nm; the error bounds reflect now also the uncertainty of the included adiabatic, relativistic, and QED corrections. The value of the scattering length resulting from our investigation lies outside the error bounds of all experimental determinations based on the properties of Bose-Einstein condensate of spin-polarized helium atoms.

Does a Sodium Atom Bind to C60?

A Multi-Reference Configuration-Interaction study of the NaC60 system is presented. It is shown that the experimentally measured dipole moment of this system can be explained by the existence of a charge-transfer state of Na(+)C60(-) nature. Moreover, the present work shows that Configuration-Interaction techniques based on local orbitals permit a Multi-Reference treatment of systems containing several tens of atoms.

Fine-structure energy levels, oscillator strengths and lifetimes of chlorine-like chromium

We have done relativistic calculations for the evaluation of energy levels, oscillator strengths, transition probabilities and lifetimes for Cr VIII ion. Use has been made of configuration interaction technique by including Briet-Pauli approximation. The energies of various levels from the ground state to excited levels of 3s3p6, 3s23p43d, 3s23p44s, 3s23p44d of Cr VIII are given in LSJ coupling scheme after fine-tuning and are compared with the experimental results compiled in the NIST Data Base. Many new lines have been predicted which have not appeared so far in the NIST Data.

Ab initio prediction of the infrared-absorption spectrum of the C2Cl radical

The three lowest (1(2)A', 2(2)A', and 1(2)A") potential-energy surfaces of the C2Cl radical, correlating at linear geometries with 2Sigma+ and 2Pi states, have been studied ab initio using a large basis set and multireference configuration-interaction techniques. The electronic ground state is confirmed to be bent with a very low barrier to linearity, due to the strong nonadiabatic electronic interactions taking place in this system. The rovibronic energy levels of the 12C12C35Cl isotopomer and the absolute absorption intensities at a temperature of 5 K have been calculated, to an upper limit of 2000 cm(-1), using diabatic potential-energy and dipole moment surfaces and a recently developed variational method. The resulting vibronic states arise from a strong mixture of all the three electronic components and their assignments are intrinsically ambiguous.

Calculation of the energy levels of Ge, Sn, Pb, and their ions in theVN−4approximation

Energy levels of germanium, tin and lead together with their single, double and triple ionized positive ions have been calculated using the $V^{N-M}$ approximation suggested in the previous work (Dzuba, physics/0501032) (M=4 - number of valence electrons). Initial Hartree-Fock calculations are done for the quadruply ionized ions with all valence electrons removed. The core-valence correlations are included beyond the second-order of the many-body perturbation theory. Interaction between valence electrons is treated by means of the configuration interaction technique. It is demonstrated that accurate treatment of the core-valence correlations lead to systematic improvement of the accuracy of calculations for all ions and neutral atoms.

Properties of a New Fluorescent Cytosine Analogue, Pyrrolocytosine

Pyrrolocytosine is a novel, environment sensitive, fluorescent base that can be used in place of cytosine as a fluorescent marker in nucleic acids. In this work the results of a detailed computational investigation into the hybridization and photochemical properties of the base are reported. The interaction energy of the base pair formed between pyrrolocytosine and guanine, calculated at the MP2/6-31G(d,0.25)//HF/6-31G(d,p) level, was found to be -27.2 kcal mol(-1), comparing very favorably with the value calculated for the cytosine and guanine base pair, -25.8 kcal mol(-1). The wavelengths for the vertical transitions of pyrrolocytosine and cytosine were determined using both the configuration interaction technique, with singly excited configurations (CIS) and time-dependent density functional theory using the B3LYP functional (TDB3LYP). It was found that the spacing between the first pipi state and the first npi state was significantly larger in the case of pyrrolocytosine than cytosine, providing a rationale for the higher fluorescence quantum yield of the former. Hydrogen bonding of pyrrolocytosine to guanine did not affect the predicted fluorescence properties of pyrrolocytosine whereas stacking guanine above pyrrolocytosine, in a manner appropriate to B-form DNA, significantly reduced the predicted fluorescence. Calculations on the two base systems using the TDB3LYP method produced low-lying charge-transfer states which are not predicted when the CIS method is used and are not thought to be physically meaningful.