Ab Initio Self-consistent Field Research Articles

Reaction Coordinate Analysis of the S2−S1 Internal Conversion of Phenylacetylene

The reaction coordinate of the S(2)-S(1) internal conversion (IC) of phenylacetylene (PA) was analyzed using the ab initio complete active space self-consistent field (CASSCF) method. In the first process after electronic excitation into S(2), the aromatic benzene ring is transformed into a nonaromatic quinoid structure. The ethynyl part (-C[triple bond]CH) takes an incomplete allenoid structure in which the CC bond elongates to an intermediate value between typical C[triple bond]C triple and C=C double bonds, but the bend angle of -CCH is 180 degrees . In the second process, PA takes a complete allenoid structure with an out-of-plane location of the beta-H atom (i.e., the H atom of the ethynyl part) and a further elongation of the CC bond so that PA is most stable in S(2) (S(2)-bent). The conical intersection between S(2) and S(1) (S(2)/S(1)-CIX) is located near the S(2)-bent geometry and is slightly unstable energetically. After transition at S(2)/S(1)-CIX, PA quickly loses both quinoid and allenoid structures and recovers the aromaticity of the benzene ring in S(1). Analysis of the dipole moment along the reaction coordinate shows that the weak electron-withdrawing group of the ethynyl part in S(0) suddenly changes into an electron-donating group in S(2) after the main transition of S(0)-S(2). The photoinduced change of the dipole moment is a driving force to the formation of a quinoid structure in S(2). Regarding the benefit of the reaction coordinate analysis of the multidimensional potential energy surfaces of PA, the present picture of the IC process is much more elaborate than our previous representation (Amatatsu, Y.; Hasebe, Y. J. Phys. Chem. A 2003 107, 11169-11173). Vibrational analyses along the reaction coordinate were also performed to support a time-resolved spectroscopic experiment on the S(2)-S(1) IC process of PA.

Ab Initio multireference configuration-interaction study of hydrogen molecule activation by Cs-promoted Pt clusters

The adsorption of the H2 molecule on CsnPt(5-n) bcc (111) clusters for Cs/Pt rates of 20%, 40%, and 80% is studied using ab initio multiconfigurational self-consistent field plus multireference configuration-interaction variational and perturbative calculations. The H2 interaction with the clusters is studied in ground and excited states with geometry optimization, where the hydrogen adsorption takes place by a Pt atom. These calculations are compared with those of H2 adsorption on Pt4. The most stable configurations of Cs/Pt4 and Cs2Pt3 clusters (Cs/Pt rates of 20% and 40%) are a doublet and a closed-shell singlet, respectively. Both clusters capture and activate the hydrogen molecule and their behaviors resemble Pt4. The H2 capture distances are, respectively, similar and smaller than Pt4 capture distances, while the H-H bond dissociation distances are similar and bigger than those of Pt4; however, none of them presents activation barriers. The most stable Cs4Pt cluster (Cs/Pt rate of 80%) is also a closed-shell singlet; it also captures and activates the hydrogen molecule and shows a different behavior as compared with Cs/Pt4, Cs2Pt3, and Pt4 clusters. The capture distance is quite smaller and is obtained after surmounting an activation barrier. For all clusters studied here, no hydrogen absorption was observed, only the adsorption of H2.

ASFMS: A program package for ab initio calculation of absorption spectra by full multiple scattering

The program package ASFMS for ab initio self-consistent field (SCF) all-electron full multiple scattering (MS) computations of electron structure and absorption spectra of large systems was developed. ASFMS can compute the X-ray absorption fine structure (XAFS) and near-edge structure (XANES) spectra. Unlike other programs ASFMS can be applied to calculations of pre-edge structure and transition intensity in ultraviolet region. Another advantage of ASFMS consists in using additional cluster boundary conditions for modeling of ionic and covalent solids. Effective algorithms are implemented to reduce the time of the computation and storage.

Structures and Electronic Properties of Peanut-Shaped Dimers and Carbon Nanotubes

The structures and electronic properties of peanut-shaped dimers and carbon nanotubes constructed from C60 molecules are investigated using ab initio self-consistent field molecular and crystal orbital methods based on the density-functional theory. The calculations show that the formation of peanut-shaped structures without octagonal carbon rings is energetically favorable. The obtained band structures indicate that the peanut-shaped nanotube can be a semiconductor or a metal.

Calculation of bridging oxygen [formula omitted] quadrupolar coupling parameters in alkali silicates: A combined ab initio investigation

Ab initio band-structure calculations based on the density functional theory have been performed for several crystalline Li, Na, and K-silicates to obtain electric-field gradients (efg) for oxygen atoms. The efg for bridging oxygen environments in these compounds were also investigated by performing ab initio self-consistent field Hartree–Fock molecular orbital calculations on silicate clusters, and there is good agreement between these two approaches. By performing additional ab initio quantum chemistry calculations on model silicate clusters the factors influencing the O 17 quadrupole coupling parameters for bridging oxygen environments in alkali silicates have been examined. The quadrupolar asymmetry parameter was found to be dependent on the Si–O–Si angle and the nature of the modifier cation, in agreement with previous studies. In contrast, the quadrupolar coupling constant was found to have a strong dependence on Si–O distance, as well as Si–O–Si angle and the nature of the modifier cation. Analytical expressions describing these dependencies are proposed, which should assist in describing the local environments of bridging oxygen in crystalline and amorphous materials.

A study of the anharmonic effects on the vibrational spectra of a realistic retinal chromophore model

Ab initio and vibrational self-consistent field (VSCF) computations are used to investigate the vibrational normal coordinates of the protonated Schiff base (PSB) 4-cis-γ,η-dimethyl-C9H9 NH2+. The ground and the first excited states are investigated. Both harmonic and anharmonic frequencies for the first three overtones of the ground and first excited states are reported. Special attention is payed to the discussion of the normal coordinates modes that involve the central C= C bond which plays a significant role in the isomerization process.

Theoretical study of the H(2) reaction with a Pt(4) (111) cluster.

The C(s) symmetry reaction of the H(2) molecule on a Pt(4) (111) clusters, has been studied using ab initio multiconfiguration self-consistent field plus extensive multireference configuration interaction variational and perturbative calculations. The H(2) interaction by the vertex and by the base of a tetrahedral Pt(4) cluster were studied in ground and excited triplet and singlet states (closed and open shells), where the reaction curves are obtained through many avoided crossings. The Pt(4) cluster captures and activates the hydrogen molecule; it shows a similar behavior compared with other Pt(n) (n=1,2,3) systems. The Pt(4) cluster in their lowest five open and closed shell electronic states: (3)B(2), (1)B(2), (1)A(1) (3)A(1), (1)A(1), respectively, may capture and dissociate the H(2) molecule without activation barriers for the hydrogen molecule vertex approach. For the threefolded site reaction, i.e., by the base, the situation is different, the hydrogen adsorption presents some barriers. The potential energy minima occur outside and inside the cluster, with strong activation of the H-H bond. In all cases studied, the Pt(4) cluster does not absorb the hydrogen molecule.

Theoretical study of the reaction of H2 with a Cu2Pt2 cluster

The study of the interaction of a pyramidal tetramer of Cu2Pt2 with the H2 is reported here through ab initio multiconfigurational self-consistent field (MC-SCF) calculations, plus extensive multireference configuration interaction (MR-CI), variational and perturbative calculations. The lowest three electronic states X 1A′, a 3A′ and a 1A′ of the bare cluster were considered in order to study this interaction. For the H2 Cs approaching a Pt vertex, results show that the Cu2Pt2 pyramid cluster in its X 1A′ and a 1A′ states can spontaneously capture and dissociate the H2. For the H2 Cs approaching a Cu vertex, where H2 is located in the Cs reflecting plane, the Cu2Pt2 cluster in its X 1A′ electronic state shows capture of the hydrogen molecule after surmounting an energy barrier; moreover, in this approach the Cu2Pt2 cluster in its a 1A′ electronic state shows spontaneous capture of the hydrogen molecule. For the H2 approaching a Cu vertex, where the Cs reflecting plane bisects the H2 molecule, the Cu2Pt2 cluster in its three lowest-lying states is able to capture the hydrogen molecule after surmounting a small barrier. The Cu2Pt2+H2 Cs face-on interactions show a lower H2 activation than that which was obtained in the equivalent Pt4+H2 interactions.

Theoretical Study on the Aromaticity of the Pyramidal MB6 (M: Be, Mg, Ca, and Sr) Clusters.

A series of metal−polyboron clusters with the general formula MB6 (M = Be, Mg, Ca, and Sr) were investigated using ab initio self-consistent field and density functional theory (DFT) methods. Calculation results show that the two clusters of BeB6 and MgB6 have C6v pyramidal structure with M2+ cation interacting with a planar hexagonal B62- dianion. Whereas the other two CaB6 (C1) and SrB6 (CS) clusters possess quasi-pyramidal structure with M2+ cation interacting with a chairlike B62- dianion. Molecular orbital (MO) analysis and nucleus-independent chemical shifts (NICS) further reveal that the four MB6 species all have three delocalized π MOs and two delocalized σ MOs and therefore exhibit the multiple-fold aromaticity.

Ab Initio Study of Aromatic Side Chains of Amino Acids in Gas Phase and Solution

The vertical transition properties have been calculated for the valence electronic excited ππ* singlet states 1 L b , 1 L a , 1 B b , and 1 B a of the chromophores benzene, phenol, and indole in bulk solvent. The polarizablecontinuum model was employed in the framework of a complete active space self-consistent reaction field with use of an atomic natural orbital basis set. Dynamical electron correlation was accounted for through the use of second-order multiconfigurational perturbation theory. The predicted solvatochromic shifts are compared with previous experimental and computational results. We find general agreement, in particular for the 1 L b state of each species, where a red-shift is observed in bulk solvent. To our knowledge this is the first ab initio self-consistent reaction field study of the high-lying 1 B b and 1 B a states of benzene, phenol, and indole in bulk solvent. At the correlated level, the 1 B b and 1 B a states undergo red-shifts for benzene, shifts to the red and to the blue respectively for phenol, and shifts to the blue for indole.

A multi-configurational study of the one-dimensional dissociation of azidopentazole (N 8) and derived N 7CH isomers

One-dimensional dissociation curves in N 8 and derived N 7CH were calculated at the ab initio complete active space self-consistent field and multi-configuration quasidegenerate perturbation theory levels using the 6-311G(d) basis set. A limited active space of 12 electrons distributed among 11 molcular orbitals was used. The calculated barrier heights, either intrinsic or reduced by a triplet state crossing, are deemed to be too small to support either N 8 or N 7H as a high energy density material. The limited size of the active space and basis set used here caused discontinuities and different asymptotic fragmentations in some of the dissociation curves.

Theoretical studies on one-dimensional polymers constructed from D 2d C 36 molecules

Stabilities and electronic properties for one-dimensional C 36 model polymers constructed from C 36 molecules with D 2d symmetry are calculated using the ab initio self-consistent field crystal orbital method based on density functional theory. The stabilities of these polymers are determined by the connection patterns and positions of the atoms forming intermolecular bonding between the neighbor C 36 cages. All the model structures are semiconductors. The band structures of corresponding anionic polymers are also calculated. It is found that the rigid-band theory is not valid for the anionic polymers. The possibility of superconduction is discussed based on the electron–phonon coupling mechanism.

Theoretical Study on the Aromaticity of the Pyramidal MB6 (M = Be, Mg, Ca, and Sr) Clusters

A series of metal−polyboron clusters with the general formula MB6 (M = Be, Mg, Ca, and Sr) were investigated using ab initio self-consistent field and density functional theory (DFT) methods. Calculation results show that the two clusters of BeB6 and MgB6 have C6v pyramidal structure with M2+ cation interacting with a planar hexagonal B62- dianion. Whereas the other two CaB6 (C1) and SrB6 (CS) clusters possess quasi-pyramidal structure with M2+ cation interacting with a chairlike B62- dianion. Molecular orbital (MO) analysis and nucleus-independent chemical shifts (NICS) further reveal that the four MB6 species all have three delocalized π MOs and two delocalized σ MOs and therefore exhibit the multiple-fold aromaticity.

Ammonia-chain clusters: Vibronic spectra of 7-hydroxyquinoline⋅(NH3)2

Mass- and isomer-selected S1←S0 resonant two-photon ionization and S1→S0 fluorescence spectra were measured for the 7-hydroxyquinoline⋅(NH3)2 [7HQ⋅(NH3)2] and d2-7-hydroxyquinoline⋅(ND3)2 clusters cooled in supersonic expansions. UV/UV hole burning measurements prove that a single cluster isomer is formed. Ab initio self-consistent field and density functional calculations predict that the most stable cluster form has an “ammonia wire” hydrogen bonded to the –OH and N groups of the cis-7HQ rotamer. The experimental S0 and S1 frequencies are in very good agreement with the calculated normal mode frequencies for both the normal and deuterated ammonia-wire clusters. S1←S0 excitation leads to contractions of the –O–H⋯N and NH3⋯NH3 hydrogen bonds, as well as smaller displacements for the NH3⋯N(quinoline) stretch and the in plane rotation (or bend) of the ammonia dimer relative to 7HQ. The coupling of these modes to the S1←S0 electronic excitation indicates that hydrogen bond contractions in the excited state are important and may be prerequisite for the S1 state proton transfer processes that occur in the larger 7HQ⋅(NH3)n (n⩾4) clusters. The calculated electron density differences upon S1←S0 excitation show large π-electron flows on the 7HQ moiety. However, the σ-electronic rearrangements that directly drive the hydrogen bond rearrangements are one to two orders of magnitude smaller.

The features of the electron density in XeF 5XF 6 (X=P, As, Sb, Bi, V, Nb, Ta) molecules

The equilibrium structures of the molecules XeF 5 +XF 6 − (X=P, As, Sb, Bi, V, Nb, Ta) have been established using ab initio self-consistent field calculations. Topological analyses of electron density (ED) and electron localization function have revealed the active reaction center, the presence of which is a reason for the molecules to form the polymolecular structures. The strength of all the bonds in molecules has been estimated by analysis of the ED values at the bond critical points. Estimates of stability, reaction activity and solubility of these compounds are based on comparative analyses of the Xe–F–X bridging fragments.

Structure and bonding of the (C6H6)+ 2 radical

To elucidate the relative stability of various structures of the benzene dimer cation radical, (C6H6)+ 2 in its ground and low-lying excited states, ab initio complete active space self-consistent field (CASSCF), multi-reference singly and doubly excited configuration interaction (MRSDCI), and multi-reference coupled pair approximation (MRCPA) calculations were performed. Full optimization was performed at the CASSCF level for various structures of the dimer cation, followed by MRSDCI and MRCPA calculations. It was found that the global minimum of the cation is at a slipped C2h sandwich structure but there are some other sandwich structures with almost the same stability, being within about kcal mol−1. T-shape structures are less stable than the sandwich structures, by more than 5 kcal mol−1 by MRCPA calculations. Low lying electronic excited states in various structures are also discussed.

Fractional occupation numbers and spin density functional calculations of degenerate systems

AbstractWe implemented ab initio self‐consistent field (SCF) fractional occupation numbers (FON) calculation with Dunlap's interpolation scheme for the twisted ethylene, which is a prototype molecule of a σ–π biradical system. The calculational results are compared with those of complete‐active‐space (CAS) SCF and spin‐unrestricted Kohn–Sham (UKS) calculations on potential surfaces, occupation numbers of natural orbitals, and correlation entropies. It was found that the UKS methods gave similar results to CASSCF, while the FON solutions appeared in only the nearly complete degenerate region. © 2003 Wiley Periodicals, Inc. Int J Quantum Chem 93: 317–323, 2003

Anisotropic NMR interaction tensors in the decamethylaluminocenium cation.

Solid-state NMR experiments, analytical and numerical simulations of solid NMR powder patterns, ab initio self-consistent field and hybrid density functional theory calculations, and single-crystal X-ray diffraction are used to characterize the molecular structure and anisotropic NMR interaction tensors in the bis(pentamethylcyclopentadienyl)aluminum cation, [Cp(2)Al](+). This highly symmetric main group metallocene has a structure analogous to that of ferrocene and the cobaltocenium cation. The single-crystal X-ray diffraction structure is reported for [Cp(2)Al][AlCl(4)]. Solid-state (27)Al[(1)H] magic-angle spinning and static NMR experiments are used to study the aluminum chemical shielding and electric field gradient tensors, revealing axial symmetry in both cases with a large chemical shielding span of Omega = 83(3) ppm and a small nuclear quadrupole coupling constant, C(Q)((27)Al) = 0.86(10) MHz. Carbon-13 CPMAS NMR experiments in combination with ab initio calculations and simulations of the effects of chemical exchange on (13)C static powder patterns reveal dynamic rotation of rings and suggest a low internal rotational barrier for this process. Theoretical computations of interaction tensors using the Gaussian 98 and Amsterdam Density Functional software packages are in good agreement with experiment and lend insight into the molecular origin of these NMR interactions. Orientations of the NMR tensors determined from theory, the large chemical shielding span, and the very small value of C(Q)((27)Al) can all be rationalized in terms of the high molecular symmetry.

Prediction of the 1-octanol/H2O partition coefficient, log P, by ab initio MO calculations: hydrogen-bonding effect of organic solutes on log P.

To predict the 1-octanol/H2O partition coefficient, log P, based on molecular structures, we calculated the solvent accessible surface area and the solvation energy difference of 166 organic molecules between 1-octanol and water environments with the ab initio molecular orbital self-consistent reaction field method, and then analyzed the relationships among the measured log P values with these two structural quantities by multiple linear-regression analyses. Physicochemically meaningful correlations were obtained, suggesting that non-hydrogen bonding and hydrogen-acceptor molecules behave similarly to each other in partitioning, but that hydrogen-donor molecules behave differently from the former molecules. The results provide a new computational approach for predicting log P.

The lowest doublet and quartet potential energy surfaces involved in the N(4S)+O2 reaction. II. Ab initio study of the C2v-symmetry insertion mechanism

In the present work we have carried out ab initio complete active space self-consistent field (CASSCF) and second-order perturbation theory on CASSCF wave function (CASPT2) calculations and also some density functional theory calculations with the aug-cc-pVTZ Dunning’s basis set on the lowest A1, B1, A2, and B2 doublet and quartet potential energy surfaces (PES) that could be involved in the title reaction. Thus, several minima, transition states, and surface crossings have been found for the C2v-insertion reaction mechanism. The results agree very well with available experimental data [i.e., for NO2 (2A1), MIN2 (2B2), NO2 (2Πu)] and with other previous ab initio calculations. Six A′/A′- and four A′/A″-type surface crossings were located and classified for these PES’, whose only one (i.e., B22/2A1) has been previously reported in theoretical and experimental studies. High-energy barriers were found for the direct C2v-insertion mechanism (3.11 and 2.54 eV for the lowest doublet and quartet PES’ at the CASPT2/aug-cc-pVTZ level, respectively), clearly showing that this competitive mechanism is much less favorable than the direct Cs-abstraction or the indirect Cs-insertion reaction mechanisms reported in Paper I.