Single Excitation Configuration Research Articles

Accuracy of the MRSD-CI technique in computing the X 2∑ + ScO and TiN hyperfine tensors

The hyperfine tensors of X 2∑ + ScO and TiN are computed using the multi-reference single and double excitation configuration interaction (MRSD-CI) method. The accuracy of this method is studied as the number of double excitations and reference configurations in the CI wave function is increased. The computed b F ( 45 Sc) and b F ( 47 Ti) parameters are within 92 and 93% of the experimental values respectively while those of c( 45 Sc) and c( 47 Ti) are within 97 to 99%. Thus the MRSD-CI technique seems to be a feasible tool for predicting the hyperfine parameters in this class of diatomics.

Protonated High Energy Density Materials: N4 Tetrahedron and N8 Octahedron

Previous theoretical studies have demonstrated that tetrahedral N4 should be an extraordinarily effective high energy density material. But this species has thus far resisted laboratory efforts directed to its synthesis. Ab initio electronic structure methods have been used to examine the Td N4 and Oh N8 nitrogen clusters, including their protonated forms. We have optimized geometries using DZP, TZ2P, and TZ2P(f,d) basis sets with the Hartree−Fock self-consistent-field (SCF) method, second-order Moller−Plesset perturbation theory (MP2), single and double excitation configuration interaction (CISD), coupled-cluster (CCSD and CCSD(T)) methods, and three DFT/Hartree−Fock hybrid (B3LYP, B3P86, BHLYP) methods. Harmonic vibrational frequencies and infrared intensities have been obtained at the SCF, MP2, B3LYP, B3P86, and BHLYP levels of theory. The vertex protonated Oh N8 and Td N4 structures are found to represent minima on their respective potential energy surfaces. The bond protonated Td N4 molecule was dete...

An ab Initio and Semiempirical Study of the First- and Third-Order Polarizabilities in Benzene and Thiophene Derivatives: Electron Correlation Effects

The static first- and third-order polarizabilities of several benzene and thiophene derivatives are evaluated at the ab initio level via an efficacious general finite field approach. The impact of electron correlation is explored by calculating the molecular polarizabilities at the Moller−Plesset second-order perturbation (MP2) theory level using an extended basis set. Further, we examine the influence of molecular architecture on the nonlinear optical response, in particular the switch from an arylethenyl type of structure to a quinoid structure. Finally, we apply the correction vector method combined with the intermediate neglect of differential overlap−single and double excitation configuration interaction (INDO-SDCI) technique to evaluate the optical nonlinearities at the semiempirical level. The reliability of this procedure is established by comparison with the corresponding high-level ab initio polarizability values. The results clearly show the sensitive dependence of the nonlinear optical propert...

The 3A2, 1A2, 3B2, and 1B2 electronic states of CH2: Small bond angle states

Molecular structures with very small bond angles are a curiosity in chemistry. The two triplet (3A2 and 3B2) and two singlet (1A2 and 1B2) excited states of CH2 have been investigated systematically using ab initio electronic structure theory. For these four states total energies and physical properties including geometries, dipole moments, harmonic vibrational frequencies, and associated infrared intensities were determined with the single and double excitation configuration interaction (CISD) method using four different basis sets. It is confirmed in this study that the four states of CH2 all have bent structures with longer CH bond lengths and smaller bond angles than the four lower-lying (X̃, ã, b̃, and c̃) states of CH2. At the CISD optimized geometries single point energies were determined with complete active space self-consistent-field (CASSCF) and CASSCF second-order configuration interaction (SOCI) levels of theory. For the triplet excited states single point energies were also determined employing coupled cluster with single and double excitations (CCSD) and CCSD with perturbative triple excitations methods. At the CISD level with the largest basis set, the triple zeta plus triple polarizations with two sets of higher angular momentum and two sets of diffuse functions basis set [TZ3P(2 f,2d)+2diff], the bond angles were predicted to be 40.6° (3A2), 46.1° (1A2), 76.3° (3B2), and 81.3° (1B2), while the dipole moments were determined to be 2.35 (3A2), 2.26 (1A2), 1.69 (3B2), and 1.60 debye (1B2), respectively. With the most accurate method in this study, the CASSCF-SOCI level with the TZ3P(2 f,2d)+2diff basis set, the energy separations (Te value) between the ground state (X̃ 3B1) and the four excited states were predicted to be 73.7 kcal/mol (3.20 eV, 25 800 cm−1) for the 3A2 state, 96.8 kcal/mol (4.20 eV, 33 800 cm−1) for the 1A2 state, 151.0 kcal/mol (6.55 eV, 52 800 cm−1) for the 3B2 state, and 182.5 kcal/mol (7.91 eV, 63 800 cm−1) for the 1B2 state, respectively.

Theoretical Study on the Thermal and Photochemical Isomerization Reactions of Dicyanoacetylene Complex of Platinum Pt(PH3)2(C4N2)

The electronic mechanism of the photoisomerization of the dicyanoacetylene complex of platinum Pt(PH3)2(C4N2) to the acetylide complex Pt(PH3)2(CN)(C⋮CCN) has been investigated theoretically. The geometries of the ground and excited states are optimized by the Hartree−Fock (HF) and single excitation configuration interaction (SE-CI) methods, respectively. In the thermal process, the decomposition reaction of Pt(PH3)2(C4N2) into Pt(PH3)2 and C4N2 occurs preferentially but the association leads to the acetylene complex rather than to the acetylide complex. The reactant, transition state, and product all have planar structures. On the other hand, in the photochemical process, the dicyanoacetylene complex is isomerized smoothly into the acetylide complex. Neither the decomposition of Pt(PH3)2(C4N2) into the neutral separated system Pt(PH3)2 + C4N2 nor that into the ionic separated systems Pt(PH3)2+ + C4N2- and Pt(PH3)2(C4N2)+ + CN- occurs. The intramolecular photoisomerization occurs through a bisect complex ...

Valence ionization potentials of anionic phosphate esters: An ab initio quantum mechanical study

In earlier investigations N. S. Kim, P. R. LeBreton, J. Am. Chem. Soc. 118, 3694 (1996), and references therein, ultraviolet (UV) photoelectron data and ab initio molecular orbital calculations yielded information about nucleotide electronic structure by providing valence ionization potentials (IPs) of nucleotide bases and sugar model compounds. Here, model phosphate group ionization potentials have been evaluated by employing multireference, single and double excitation configuration interaction calculations with a complete active space self-consistent field (CASSCF) wave function. The five lowest energy IPs of H2PO4− and the four lowest energy IPs of (CH3)HPO4− and (CH3)2PO4− were evaluated. Calculations were performed using a (12, 9, 1)/[6, 4, 1] double-zeta basis set on phosphorus with a d-polarization function; (10, 7)/[4, 3] and (10, 6)/[4, 2] basis sets on oxygen and carbon, respectively; and a (6)/[3] basis set on hydrogen. Two types of CASSCF calculations were carried out. In one, denoted 8e8a/7e8a, the anions and radicals had 8 and 7 electrons, respectively, in 8 active orbitals. In the second, denoted 10e10a/9e10a, there were 9 and 10 electrons in 10 active orbitals. Ionization potentials of H2PO4−, (CH3)HPO4−, and (CH3)2PO4− were also obtained from second-order perturbation theory (CASPT2) calculations with the CASSCF reference functions. All of the ionization events examined are associated with removal of electrons from oxygen atom lone-pair orbitals on the closed-shell anions. For H2PO4− and (CH3)HPO4−, the lowest energy CASPT 2 ionization potentials obtained (4.29–4.36 eV and 4.12–4.27 eV, respectively) are smaller than corresponding IPs (4.89 and 4.69 eV) previously reported from results of 6-31+G* second-order Møller-Plesset (MP2) calculations. For the second through fifth IPs of H2PO4−, and the second through fourth IPs of (CH3)HPO4−, the values obtained from CASPT2 calculations are 0.42 to 1.95 eV smaller than values reported from the combined use of MP2 and configuration interaction singles calculations with the 6-31+G* basis set. A combination of results from MP2 and CASPT2 calculations yields values of 4.89, 5.42, 6.30, 6.64, and 7.41 eV for the five lowest IPs of H2PO4−; and values of 4.69, 5.43, 6.08, and 6.55 eV for the four lowest IPs of (CH3)HPO4−. © 1997 John Wiley & Sons, Inc. Int J Quant Chem 65: 1095–1106, 1997

A study on the structure and vibrations of diphenylamine by resonance-enhanced multiphoton ionization spectroscopy and ab initio calculations

Laser-desorption jet-cooling has been applied in combination with mass-selective gas-phase spectroscopic techniques to study the structure and low-frequency vibrations of diphenylamine (DPA). Two-color (1+1′) resonance-enhanced multiphoton ionization has been used to measure the vibrationally resolved excitation spectrum of the S1←S0 transition in the 305–309 nm region. Ion-dip measurements have been performed to determine the vibrational structure in the electronic ground state. The electronic spectra of DPA are dominated by long progressions in low-frequency vibrations involving the motion of the phenyl rings as a whole. For the interpretation of the experimental data ab initio calculations have been performed at the Hartree–Fock level for the S0-state and using single-excitation configuration interaction for the S1-state. The DPA molecule is found to change from a pyramidal geometry around the N-atom with unequal torsional angles of the phenyl groups in the S0-state to a planar geometry with equal torsional angles in the S1-state. The two most prominent vibrational motions are the in-phase wagging and the in-phase torsion of the phenyl rings. In addition, the resonance-enhanced multiphoton ionization spectra of the S1←S0 transition in the DPA-Ar, DPA-Kr, and DPA-Xe van der Waals complexes have been measured. From these spectra it is inferred that there is a coupling between the van der Waals modes and the low-frequency intra-molecular modes of DPA.

Charge-transfer excitons in optical-absorption spectra of C60 dimers and polymers.

Charge-transfer (CT) exciton effects are investigated for the optical absorption spectra of crosslinked C60 systems by using the intermediate exciton theory. We consider the C60-dimers, and the two (and three) molecule systems of the C60-polymers. We use a tight-binding model with long-range Coulomb interactions among electrons, and the model is treated by the Hartree-Fock approximation followed by the single-excitation configuration interaction method. We discuss the variations in the optical spectra by changing the conjugation parameter between molecules. We find that the total CT-component increases in smaller conjugations, and saturates at the intermediate conjugations. It decreases in the large conjugations. We also find that the CT-components of the doped systems are smaller than those of the neutral systems, indicating that the electron-hole distance becomes shorter in the doped C60-polymers.

Experimental and theoretical characterization of the S1–S transition of benzo[a]pyrene

We use fluorescence excitation and two-color resonance-enhanced multiphoton ionization (REMPI) spectroscopy to characterize the S1–S0 transition of benzo[a]pyrene, B[a]P. In addition, two-color REMPI with a tunable ionization laser provides an improved measurement of the molecular ionization potential, 7.103±0.004 eV. Ab initio calculations are performed to determine the vibrational frequencies for the 90 vibrational modes in the S1 and S0 electronic states. Our results indicate that the single-excitation configuration interaction (CIS) level of theory provides an excellent representation of excited-state frequencies for this large molecule. Virtually all of the vibronic bands between ∼380 nm and 397 nm, as well as numerous bands between 373 nm and 380 nm, are assigned with the aid of the calculated vibrational frequencies. In total, 27 of the 61 a′ symmetry vibrational modes and 5 of the 29 a″ vibrational modes in the S1 state have now been assigned.

Ab initio calculation of quadrupole moments of CO2 and N2 molecules as functions of nuclear geometry

Ab initio multireference configuration interaction calculations of the static dipole and quadrupole moments of CO2 and N2 as functions of the internuclear distances have been carried out using the multireference single and double excitation configuration interaction method (MRDCI). The results are compared with the outcome of Hartree-Fock calculations and help shed light on the effect of such moments on the low energy interaction of these molecules during scattering processes, and hence on their influence on the threshold behaviour of the inelastic cross-sections involving vibrational degrees of freedom.

A cluster model study for Li KVV transitions on a Cu (001) surface

To elucidate the assignment of the Li KVV Auger transitions on the adsorption of Li on a Cu surface, we performed cluster model calculations, which include multiconfiguration self-consistent field and multireference single and double excitation configuration interaction calculations. The calculations showed that the peak at 49 eV is assigned to doubly ionized states from diffuse free-electron-like orbitals and the peak at 46 eV is due to doubly ionized states from a diffuse free-electron-like orbital and a localized Cu 3d orbital.

Electron-impact excitation of low-lying preionization-edgen→σ* and Rydberg transitions of CHF2Cl and CHFCl2: Absolute generalized oscillator strength measurement

Angle-resolved electron energy loss spectroscopy has been used to determine the absolute generalized oscillator strengths (GOSs) of valence-shell electronic transitions of difluorochloromethane (CHF2Cl) and dichlorofluoromethane (CHFCl2) as functions of energy loss and momentum transfer at an impact energy of 2.5 keV. Absolute GOS profiles of the prominent low-lying preionization-edge energy loss features of CHF2Cl and CHFCl2 were determined and found to be consistent with the previous assignments of the underlying transitions made by VUV photoabsorption spectroscopy. In particular, the lowest-lying features at 8.0 eV in CHF2Cl and at 7.5 eV in CHFCl2 have been attributed predominantly to electronic excitations from the Cl 3p nonbonding (n) orbitals to the C–Cl σ* antibonding orbital, in good accord with single-excitation configuration interaction (CI) excited-state calculations. The corresponding GOS profiles of these n(Cl 3p)→σ*(C–Cl) (HOMO→LUMO) transitions revealed an interesting trend of increased dipole character with increasing Cl content, i.e., from an essentially quadrupole-dominated profile, characterized by a maximum at K2∼0.9 a.u., in CHF2Cl to a mixed dipole-quadrupole profile in CHFCl2 and CHCl3. The CI calculations further showed that some of the underlying n(Cl 3p)→σ*(C–Cl) transitions in CHF2Cl, CHFCl2, and CHCl3, like the other chlorofluorocarbons: CF3Cl, CF2Cl2, CFCl3, and CCl4, could lead to dissociation of the C–Cl bond. In addition, the GOS profiles of the remaining low-lying preionization-edge features at 9.8 and 11.2 eV in CHF2Cl and at 9.4, 10.7, and 11.6 eV in CHFCl2 were also determined. These features have been previously assigned as Rydberg transitions originated from the nonbonding HOMOs. In particular, these experimental GOS profiles were found to be dominated by a strong maximum at K=0, which is indicative of strong dipole interactions. The weak secondary maxima observed at K2∼2.8–3.5 a.u. could be interpreted qualitatively in terms of the spatial overlaps between the initial-state and final-state orbital wave functions. Together with our earlier work on CHF3 and CHCl3, the present work on the remaining members of the CHFmCl3−m (m=0–3) series, CHF2Cl and CHFCl2, provides further evidence for the empirical trends on the preionization-edge structures observed in the CFnCl4−n (n=0–4) series.

Open chain versus cyclic 14-electron triatomics: molecular structures and vibrational frequencies of P 2Si, P 2C, SiN 2 and Si 2S

Since the studies of Walsh in the 1950s, triatomic molecules have provided a field of investigation for the fundamentals of bonding. A general consensus has been that rings are higher in energy than open chains due to the strain associated with narrow bond angles. The 14-valence-electron triatomics P 2Si, P 2C, SiN 2, and Si 2S were investigated using ab initio molecular quantum mechanical methods. Double zeta plus polarization (DZP) and triple zeta plus double polarization (TZ2P) basis sets have been used in conjunction with self-consistent field (SCF), single and double excitation configuration interaction (CISD) and coupled cluster (CCSD) methods. The all third period P 2Si and Si 2S were found to have rings as their lowest-energy geometries. The ring geometry of P 2C is high in energy because it necessitates formation of a carbene. The ring geometry of SiN 2 is only about 5 kcal/mol above the linear SiNN isomer.

Ab initio explanation of the apparent violation of detailed balance for H2 adsorption/desorption from Si(100)

Abstract The experimentally observed apparent violation of detailed balance in the adsorption and desorption of H2 on Si(100)-2 × 1 is explained via a dihydride intermediate that can be accessed via two different transition states, as found by complete active space self-consistent field/multireference single and double excitation configuration interaction (CASSCF/MRSDCI) calculations. Our calculations explain the low sticking probability by predicting an early symmetric transition state to adsorption with a high barrier. We predict that desorption, on the other hand, occurs through an asymmetric transition state and find excellent agreement between our calculated barrier to desorption and experimental values. For the asymmetric transition state we calculate a small barrier to adsorption, however, we suggest that impinging H2 seldom sees this orientationally constrained late transition state to adsorption, explaining the apparent violation of detailed balance.

Formation ofC60dimers: A theoretical study of electronic structure and optical absorption

The polymerization of solid ${\mathrm{C}}_{60}$ is studied theoretically within a semi-empirical quantum chemical framework. Model systems consisting of two interacting ${\mathrm{C}}_{60}$ molecules are used in order to model polymerization of neutral ${\mathrm{C}}_{60}$, as well as of alkali metal doped ${\mathrm{C}}_{60}$. The geometries and electronic structures of the systems are obtained from semiempirical, AM1, calculations. It is found that the charged systems have a substantially lower energy barrier toward formation of ${\mathrm{C}}_{60}$ dimers, a result in agreement with experiment. This effect is explained in terms of the occupation of bonding intermolecular orbitals as extra charge is added. The reduction of the energy barrier in the case of photopolymerization is motivated in a similar way by promotion of electrons from antibonding, or to bonding, orbitals in the excited state. Furthermore, optical absorption spectra of the neutral and doubly charged ${\mathrm{C}}_{60}$ dimers are calculated from the spectroscopic parametrization of the semiempirical intermediate neglect of differential overlap (INDO) Hamiltonian combined with single excited configuration interaction. The absorption spectrum of the neutral dimer displays distinct peaks with energies and oscillator strengths in qualitative agreement with experiment. Charging of the ${\mathrm{C}}_{60}$ dimer leads to characteristic polarized transitions at low energies. \textcopyright{} 1996 The American Physical Society.

Electronic and optical properties of neutral and charged poly ( p-phenylene vinylene )

Electronic and optical properties of a conjugated polymer, poly ( p-phenylene vinylene) (PPV), are theoretically studied. Long-range interactions among π electrons as well as electron-lattice coupling are taken into account in both neutral and charged states of a PPV chain by applying the Pariser-Parr-Pople model with intermediate interaction strength. This model coupled with the single-excitation configuration interaction method gives an excitation energy diagram for neutral PPV, consistent with experiment. The optical absorption spectrum is calculated in a wide frequency range, and its polarization dependence and spectral features are analyzed in detail. A polaron state in doped PPV is investigated, and its spatial pattern and absorption spectrum calculated. Its spin density gives an electron-nuclear double resonance spectrum, which is in good agreement with experiment. The present model provides a unified description of the electronic properties of neutral and doped PPV.

Theoretical study on the photostimulated desorption of CO from a Pt surface

Photostimulated desorptions (PSD’s) of CO, CO+, and CO− from a Pt surface are studied theoretically using Pt2–CO model cluster including image force correction. Calculations are performed by the single excitation configuration interaction and the symmetry adapted cluster (SAC)/SAC-CI methods. The PSD’s of the ground state CO occur as the Menzel–Gomer–Redhead (MGR) process and those of CO+ (n cation) and excited (n→π*) CO* through the modified MGR process in which the upper repulsive potential curves are nonadiabatic; the process proceeds through a sequence of nonadiabatic transitions between the similar pertinent states embedded in the metal excited bands. The excited states as the desorption channels are characterized by the excitations from the Pt–CO bonding orbitals to the antibonding MO’s: metal-adsorbate chemical bond cleavage by photons which leads to a repulsive potential is essential for the PSD. The electrostatic image force interaction plays only a minor role and the present result does not support the Antoniewicz model. The calculated excitation-energy thresholds for the CO, CO+, and CO* desorptions are 1.6∼2.6, 11.3, and 11.3–12.7 eV, respectively, which explains the energy thresholds and the fluence dependencies of the incident laser in the PSD experiments. On the other hand, the PSD giving CO− would occur with the energy range of 6.2–8.2 eV, one to two photon energy of the 193 nm (6.4 eV) laser. Since the upper nonadiabatic potential curves have shallow minima, in this case, the lifetime of the CO− species would be larger than those of the CO+ and CO* species. The present study clarifies the electronic structures of the desorbed CO+, CO−, and CO* species, which have not been identified experimentally.

On the Nature of Electronic Transitions in Radicals: An Extended Single Excitation Configuration Interaction Method

We propose that the generalization of pure spin single reference excited state theories from molecules with closed shell ground states to radicals be based on a redefinition of the meaning of excitation levels. Open shell single substitutions are expanded to include spin-adapted double excitations which traverse the singly occupied orbitals. These excitations are deleted from the expanded open shell double substitutions, which include the corresponding spin-adapted triple substitutions (e.g., a pair of the excitations traverses the singly occupied orbitals), etc. Applied to single excitation configuration interaction (CIS), this approach defines a spin-adapted extended CIS (XCIS) theory for the excited states of radicals. XCIS is size consistent and variational and has computational complexity that scales in the same way as CIS with molecular size. More low-lying excited states of radicals are correctly described by XCIS than by the restricted open shell CIS method. This is demonstrated by calculations on...

Theoretical studies and spectroscopic investigations of ground and excited electronic states of thiophene oligomers

Abstract CNDO/S calculations including single and double excited configurations were performed on thiophene oligomers n T with 2–6 monomer units. In contrast to the results on linear polyenes only single excited configurations are necessary to explain the experimental results. The theoretical energies for the lowest 1 1 B states are in good agreement with the low energy absorption origins and the high energy fluorescence maxima and with the 1 1 B state energies observed at low temperature. The results show that the 2 1 A state lies above the 1 1 B state for 2T–6T. Transient absorption bands measured by picosecond spectroscopy could be assigned to transitions between 1 1 B and higher calculated 1 A states of all studied oligomers n T.

A perturbative correction to restricted open shell configuration interaction with single substitutions for excited states of radicals

A perturbative correction, termed RCIS(D), to restricted open shell single excitation configuration interaction (ROCIS) is formulated and implemented for excited states of radicals. RCIS(D) gives spin-pure energies (but not wavefunctions), is size-consistent, and has computational complexity scaling with the fifth power of molecular size. All these properties are analogous to second order restricted Møller-Plesset theory for the ground state. Comparison of calculated vertical excitation energies against experiment for a diverse series of small radicals, BeH, BeF, CH, CH 3, CN, CO +, NO and OH, suggests that RCIS(D) constitutes a useful improvement over ROCIS.