Ab Initio Multiconfiguration Self-consistent Field Research Articles

Ab initio MCSCF study on several azide molecules: energy component analysis of the pseudo-Jahn–Teller effect

An exploration of the most stable geometrical structures for the azide molecules R–N3 (R = H, BeH, BH2, CH3, C6H5) has been carried out by using the ab initio multi-configuration self-consistent field (MCSCF) method together with the aug-cc-pVTZ basis sets. The unknown hydridoberyllium azide (R = BeH) is predicted to suffer no pseudo-Jahn–Teller (JT) distortion and, hence, to assume a linear structure of C∞v symmetry. In contrast, each of the others is found to adopt a trans bent Cs structure with regard to an interior NN bond. To elucidate the nature of the pseudo-JT distortions, energy component analyses have been carried out at all of the stationary points for the relevant molecules. The results show that pseudo-JT stabilizations are classified into two groups, one in which the stability of the bent structure about the azido group arises from lowering of the interelectronic and internuclear repulsion energies and the other in which the stability results from lowering of the electron-nuclear attraction energy. These energy behaviors are accounted for in terms of an elongation or a contraction of the heavy-atom skeleton and a charge migration from some part of the molecule to the other ones. Moreover, it is shown that the theoretical structural characteristics for the known molecules are in good agreement with available experimental facts.

Bound states of He(2 3S)+He(2 3P)

We report the results of coupled channel calculations of the bound states of the He(2 3S1)+He(2 3P0,1,2) system using the recently available ab initio multi-configuration self-consistent field short range 1,3,5Σ+g,u and 1,3,5Πg,u potentials computed by Deguilhem et al. (J. Phys. B: At. Mol. Opt. Phys. 42 (2009) 015102) and present an analysis of the applicability of the single channel calculations of these authors.

The electronic structure of singlet cyclopentadienylidene

We have carried out ab initio multiconfigurational self-consistent field calculations on the lowest singlet state of the carbene C5H4. We find that there are, along a restricted geometry deformation coordinate, two local energy minima corresponding to localized and delocalized π bonds having 4π and 6π electrons, respectively. The role of 4n + 2 resonance in determining the nature of the electronic structure and reactivity of this carbene is analyzed and compared with that of other carbenes.

One-Dimensional Zigzag Chains of Cs-: The Structures and Properties of Li+(Cryptand[2.1.1])Cs- and Cs+(Cryptand[2.2.2])Cs-

The crystal structure and properties of lithium (cryptand[2.1.1]) ceside, Li+ (C211)Cs-, are reported. Li+ (C211)Cs- is the second ceside and third alkalide with a one-dimensional (1D) zigzag chain of alkali metal anions. The distance between adjacent Cs- anions, 6 A, is shorter than the sum of the van der Waals radii, 7 A. Optical, magic angle spinning NMR, two-probe alternating and direct current conductivity, and electron paramagnetic resonance measurements reveal unique physical properties that result from the overlap of adjacent Cs- wave functions in the chain structure. The properties of cesium (cryptand[2.2.2]) ceside, Cs+ (C222)Cs-, were also studied to compare the effects of the subtle geometric changes between the two 1D zigzag chain structures. Li+ (C211)Cs- and Cs+ (C222)Cs- are both low-band-gap semiconductors with anisotropic reflectivities and large paramagnetic 133Cs NMR chemical shifts relative to Cs- (g). An electronic structure model consistent with the experimental data has sp2-hybridized Cs- within the chain and sp-hybridized chain ends. Ab initio multiconfiguration self-consistent field calculations on the ceside trimer, Cs3(3-), support this model and indicate a net bonding interaction between nearest neighbors. The buildup of electron density between adjacent Cs- anions is visualized through an electron density difference map constructed by subtracting the density of three cesium atoms from the short Cs3(3-) fragment.

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.

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.

Relativistic Spin−Orbit Coupling Effects on Secondary Isotope Shifts of13C Nuclear Shielding in CX2(X = O, S, Se, Te)

Rovibrational corrections, temperature dependence, and secondary isotope shifts of the (13)C nuclear shielding in CX(2) (X = O, S, Se, Te) are calculated taking into account the relativistic spin-orbit (SO) interaction. The SO effect is considered for the first time for the secondary isotope shifts. The nuclear shielding hypersurface in terms of nuclear displacements is calculated by using a density-functional theory method. Ab initio multiconfiguration self-consistent field calculations are done at the equilibrium geometry for comparison. (13)C NMR measurements are carried out for CS(2). The calculated results are compared with both present and earlier experimental data on CO(2), CS(2), and CSe(2). The heavy-atom SO effects on the rovibrational corrections of (13)C shielding are shown to be significant. For CSe(2) and CTe(2), reliable prediction of secondary isotope effects and their temperature dependence requires the inclusion of the SO corrections. In particular, earlier discrepancies of theory and experiment for CSe(2) are fully resolved by taking the SO interactions into account.

Electronic structure of Ge 2 and Ge 2− and thermodynamic properties of Ge 2 from all electron ab initio investigations and Knudsen effusion mass spectroscopic measurements

The low-lying states of the molecule Ge 2 and of the ion Ge 2 − have been investigated by all electron ab initio multi-configuration self-consistent field (CASSCF) and multi-reference configuration interaction (MRCI) calculations. The relativistic corrections for the Darwin contact term and for the mass–velocity term have been determined in perturbation calculations. Treatment of the spin-orbit coupling has been included. The ground state of Ge 2 is derived as 0 g +( 3Σ g −) with equilibrium distance 2.422 Å, and vibrational frequency 270 cm −1 . The spectroscopic constants of 15 excited states are presented. Thermal functions based on the theoretically determined molecular parameters were used to derive the thermodynamic properties of the Ge 2 molecule from new mass spectrometric equilibrium data. The literature value for the dissociation energy of Ge 2 has been re-evaluated. The recommended dissociation energy, D ∘ 0(Ge 2), and enthalpy of formation, Δ f H ∘ 298.15(Ge 2), in kJ mol −1 are 260.7±6.8 and 484.8±6.8, respectively.

Valence Orbital Electron Momentum Spectroscopy For N2O

Valence orbitals of the ground electronic state of nitrous oxide (X1Σ+) have been studied in configuration space using an ab initio direct multiconfiguration self-consistent field (MCSCF) method an...

MCSCF response calculations of the excited states properties of the O2 molecule and a part of its spectrum

A number of transitions including the triplet–singlet band f′1Σu+←X3Σg−, have been studied by ab initio multi-configurational self-consistent field (MCSCF) response methods. Potential energy curves for eight excited states obtained by linear response calculation are in a good agreement with recent experimental data and exhibit some new findings. Quadrupole moments for the 12 lowest states have been calculated which can be used for intermolecular interaction analysis and solvent shift estimations. The nuclear quadrupole coupling constant, magnetizability tensor, nuclear spin–rotation coupling constant and rotational g-factor are also presented. For the three lowest singlet states these parameters are analyzed in detail.

Pt-Re small cluster interaction with H2

The interaction of small Pt-Re clusters with H2 is reported here through ab initio multicon-figuration self-consistent field (MC-SCF) calculations, plus extensive multireference configuration interaction (MR-CI), variational and perturbative calculations. These calculations provide a cluster model for the activation of hydrogen by Pt-Re bimetallic catalysts. It was found that the 6S(5d56s2) Re atom ground state needs an important activation to induce very weak capture of separated hydrogen atoms, whereas in the lowest excited states the activation energies are small or zero, with a very reasonable depth of well. The four lowest states of Pt-Re were found to be 4 Σ+, 6Πyz, Σ + and 6Πxz. Pt-Re interaction with H2 has been studied from both metal ‘sides’. It was established that Pt-Re with the platinum side in the ground electronic 4Σ+ state and in the lowest 6Π+ excited states is able to capture H2 molecules without activation, whereas in the 6Πyz and 6Πxz excited states there is no capture. The rhenium side of Pt-Re in its four lowest states considered cannot capture the H2 molecule. The interaction of Pt2-Re with H2 was studied also. For the ground 2B2 electronic state and the low lying 2A1 electronic state the platinum moiety can spontaneously capture and break H2. The rhenium side of Pt2-Re(B2), however, can capture H2 only after surmounting a small barrier, and the excited Pt2-Re(2A1) can spontaneously capture H2. For Pt2-Re in its low lying 4A1 electronic state both metal sides capture and break H2 after surmounting a small barrier.

Theoretical study of interaction of small clusters of IrPt with H2

The study of the interaction of small clusters of IrPt with H2 is reported here through ab initio multiconfiguration self-consistent field (MC-SCF) calculations, plus extensive multireference configuration interaction (MR-CI), in its variational and perturbative modes. These calculations provide a cluster model for the activation of hydrogen by IrPt bimetallic catalysts. First, we studied the IrPt dimer interaction with H2. The five lowest states of the IrPt dimer are: 2Σ+, 2Δxy, 4Πyz, 4Σ+, and 4Δxy. For the IrPt dimer interaction with H2, we found that the IrPt dimer for both metal sides is able to capture the H2 molecule without any activation barriers, relaxing the H–H bond. The IrPt2 trimer interaction with H2 was also studied. The ground and the lowest states of the IrPt2 trimer are a 4A1 electronic state and a 4B2 electronic state, respectively. We found that for both metal sides, the IrPt2 cluster in its ground 4A1 and the low-lying 4B2 electronic states can spontaneously capture and break the hydrogen molecule. Large H–H relaxation is obtained and no activation barriers were found. © 2000 John Wiley & Sons, Inc. Int J Quant Chem 80: 657–663, 2000

Indirect Fluorine Coupling Anisotropies in p-Difluorobenzene: Implications to Orientation and Structure Determination of Fluorinated Liquid Crystals

The spin-spin coupling tensors, JXF (X ) H, C, F), in p-difluorobenzene (C6H4F2) were determined using ab initio multiconfiguration self-consistent field linear response calculations and NMR experiments performed in nematic liquid crystal phase. The theoretical spin-spin coupling constants are in fair agreement with experimental results. By supplying the experimental data analysis with some of the results of the theoretical calculations, the remaining anisotropy and asymmetry parameters pertinent to the n JCF (n ) 1, 2, 3, 4) and 5 JFF tensors were obtained in good agreement with the ab initio data. The results indicate that the tensorial properties of the fluorine couplings typically contribute a few percent to the corresponding experimental anisotropic couplings. In D2h or lower point group symmetries, the indirect coupling can even dominate the experimental dipolar coupling because of occasional cancellation of the direct part. Consequently, the contribution of JXF aniso must be taken into account when using anisotropic couplings in accurate determinations of the geometry or orientation of fluorinated liquid crystals or other molecules containing fluorine-substituted phenyl rings dissolved in mesophases.

Influence of ground-state geometry on carbon monoxide x-ray emission spectral profiles

Photon excitation x-ray spectator emission dynamics of the core excited carbon monoxide molecule has been studied using ab initio multiconfiguration self-consistent field calculations and a modified two-step absorption-followed-by-emission approach. The core excitations of the CO molecule and the core ionizations of the species have been considered as the precursor steps for producing the same decaying electronic states of CO. The purpose of this study is to illustrate the influence of the absorption process, using target species with different ground electronic state geometries, on the de-excitation spectra. Nuclear dynamical effects of the decaying processes of carbon and oxygen core excited states of CO to the singlet low-lying excited (final) states of and have been studied. Vibrational envelopes of the decay spectra of CO are simulated.

Theoretical characterization of H2 adsorption on AuPt clusters

Linear and triangular AuPtn cluster reactions with H2 are studied theoretically, using ab initio multiconfiguration self-consistent field (MC-SCF) calculations, followed by extensive multireference configuration interaction (MR-CI) variational and perturbative. Both the linear dimer and the triangular trimers capture the hydrogen molecule by the Pt cluster side and by the Au cluster side. Gold has an electronic effect on the Pt activity, more important than a geometrical one, poisoning the Pt activity to dissociate H2 and lowering the adsorption heats. This effect is stronger in the AuPt dimer, where the H2 capture occurs only at the molecular level, without showing hydrogen bond dissociation. The trimers look more active, relaxing the H(SINGLE BOND)H bond until breakage. No activation barriers are observed in all the cases considered. The hydrogen molecule is not able to cross the clusters due to the large barriers present. © 1998 John Wiley & Sons, Inc. Int J Quant Chem 67: 399–409, 1998

Theoretical characterization of H2 adsorption on AuPt clusters

Linear and triangular AuPtn cluster reactions with H2 are studied theoretically, using ab initio multiconfiguration self-consistent field (MC-SCF) calculations, followed by extensive multireference configuration interaction (MR-CI) variational and perturbative. Both the linear dimer and the triangular trimers capture the hydrogen molecule by the Pt cluster side and by the Au cluster side. Gold has an electronic effect on the Pt activity, more important than a geometrical one, poisoning the Pt activity to dissociate H2 and lowering the adsorption heats. This effect is stronger in the AuPt dimer, where the H2 capture occurs only at the molecular level, without showing hydrogen bond dissociation. The trimers look more active, relaxing the H(SINGLE BOND)H bond until breakage. No activation barriers are observed in all the cases considered. The hydrogen molecule is not able to cross the clusters due to the large barriers present. © 1998 John Wiley & Sons, Inc. Int J Quant Chem 67: 399–409, 1998

Relativistic potential energy surfaces of XH2 (X=C, Si, Ge, Sn, and Pb) molecules: Coupling of 1A1 and 3B1 states

Potential energy surfaces of the 1A1 and 3B1 states for XH2 molecules (X=C, Si, Ge, Sn, Pb) are investigated with ab initio full valence multiconfigurational self-consistent field wave functions, using effective core potentials. Spin–orbit coupling is also calculated to construct relativistic potential energy surfaces. The relativistic potential energy surfaces are compared with the adiabatic nonrelativistic potentials. Simple one dimensional Landau–Zener transition probabilities are calculated at the minimum energy crossing points of XH2 molecules to estimate the intersystem crossing probability.

Vibrational properties of CN − adsorbed on platinum and silver: ab initio MCSCF calculations compared with SFG experiments

The electronic and vibrational properties of CN − adsorbed on Ag electrodes have been studied through ab initio multiconfigurational self-consistent field (MCSCF) calculations of the molecular ions AgCN − and AgNC − and compared with PtCN − and PtNC −. Results of calculations are consistent with the experimental data obtained by in situ visible infrared sum frequency generation (SFG) spectroscopy of CN − adsorbed on polycrystalline Ag and Pt electrodes.

3D imaging of the collision-induced Coulomb fragmentation of water molecules.

The multiple ionization and fragmentation of ${\mathrm{H}}_{2}\mathrm{O}$ by fast ${\mathrm{H}}^{+}$ and ${\mathrm{He}}^{+}$ ions was studied using a position- and time-sensitive multiparticle detector. By the coincident measurement of correlated fragment momenta a complete three-dimensional image of the breakup process can be obtained for each individual event. Of special interest is the process ${\mathrm{H}}_{2}\mathrm{O}\ensuremath{\rightarrow}{\mathrm{H}}^{+}+{\mathrm{H}}^{+}+{\mathrm{O}}^{+}$ where a simple Coulomb explosion model is insufficient to explain the measured fragmentation energies and angular correlations. Better agreement is achieved with ab initio multiconfiguration self-consistent field calculations of the intermediate ${\mathrm{H}}_{2}{\mathrm{O}}^{3+}$ parent ion.