Ab Initio Self-consistent Field Calculations Research Articles

Environmental modifications of atomic properties: The ground and 1s2p excited states of compressed helium.

Atoms remaining as recognizably distinct constituents of bulk condensed phases can have properties modified from those of the isolated species. Dense helium bubbles at high pressures are a common form of radiation damage degrading the mechanical and electrical properties of host materials. Detailed knowledge is critical for predicting their long term performance. Modifications of the ground and first singlet excited states of confined compressed helium are investigated using an entirely non-empirical theory based on the results of ab initio self-consistent field calculations with corrections for the effects of electron correlation. For finite sized portions representing bulk condensed fcc and bcc phases of helium atoms, Hartree-Fock wavefunctions, energies, and charge distributions were computed as a function of different atomic densities using two models. The first model for the first excited state localizes the excitation on the central atom; in the second model, this is partially delocalized over the closest atomic neighbors. Total energies for the finite size portions are derived by adding the inter-atomic dispersive attractions and a density functional description of the short-range inter-atomic correlation energy. The experimental energy of the first allowed electronic transition increases with density being larger than in an isolated atom. The intra-atomic correlation energy does not contribute to this energy shift. The calculated energy shifts agree well with experiment for both bulk solid and liquid helium. The 2p orbital is increasingly compressed by density enhancement, thus generating the energy shifts. Consequently, calculations of the inelastic electron scattering cross sections are substantially incorrect if the compression of the final 1s2p state is not included. The character of the excitations is examined, and it is argued that these are of Frenkel rather than the Wannier type.

Electronic Structure of Ytterbium(III) Solvates-a Combined Spectroscopic and Theoretical Study.

The wide range of optical and magnetic properties of lanthanide(III) ions is associated with their intricate electronic structures which, in contrast to lighter elements, is characterized by strong relativistic effects and spin-orbit coupling. Nevertheless, computational methods are now capable of describing the ladder of electronic energy levels of the simpler trivalent lanthanide ions, as well as the lowest energy term of most of the series. The electronic energy levels result from electron configurations that are first split by spin-orbit coupling into groups of energy levels denoted by the corresponding Russell-Saunders terms. Each of these groups are then split by the ligand field into the actual electronic energy levels known as microstates or sometimes mJ levels. The ligand-field splitting directly informs on the coordination geometry and is a valuable tool for determining the structure and thus correlating the structure and properties of metal complexes in solution. The issue with lanthanide complexes is that the determination of complex structures from ligand-field splitting remains a very challenging task. In this paper, the optical spectra-absorption, luminescence excitation, and luminescence emission-of ytterbium(III) solvates were recorded in water, methanol, dimethyl sulfoxide (DMSO), and N,N-dimethylformamide (DMF). The electronic energy levels, that is, the microstates, were resolved experimentally. Subsequently, density functional theory calculations were used to model the structures of the solvates, and ab initio relativistic complete active space self-consistent field calculations (CASSCF) were employed to obtain the microstates of the possible structures of each solvate. By comparing the experimental and theoretical data, it was possible to determine both the coordination number and solution structure of each solvate. In water, methanol, and N,N-dimethylformamide, the solvates were found to be eight-coordinated and have a square antiprismatic coordination geometry. In DMSO, the speciation was found to be more complicated. The robust methodology developed for comparing experimental spectra and computational results allows the solution structures of homoleptic lanthanide complexes to be determined.

Relay-Like Exchange Mechanism through a Spin Radical between TbPc2 Molecules and Graphene/Ni(111) Substrates.

We investigate the electronic and magnetic properties of TbPc2 single ion magnets adsorbed on a graphene/Ni(111) substrate, by density functional theory (DFT), ab initio complete active space self-consistent field calculations, and X-ray magnetic circular dichroism (XMCD) experiments. Despite the presence of the graphene decoupling layer, a sizable antiferromagnetic coupling between Tb and Ni is observed in the XMCD experiments. The molecule-surface interaction is rationalized by the DFT analysis and is found to follow a relay-like communication pathway, where the radical spin on the organic Pc ligands mediates the interaction between Tb ion and Ni substrate spins. A model Hamiltonian which explicitly takes into account the presence of the spin radical is then developed, and the different magnetic interactions at play are assessed by first-principle calculations and by comparing the calculated magnetization curves with XMCD data. The relay-like mechanism is at the heart of the process through which the spin information contained in the Tb ion is sensed and exploited in carbon-based molecular spintronics devices.

Complex formation of Sn(II) with l-cysteine: An IR, DTA/TGA and DFT Investigation

The novel complex of Sn(II) with l-cysteine (L-H2Cys) has been synthesized and characterized by elemental analysis, TGA and IR spectroscopy. Vibrational assignment and DFT/PBE0/def2-TZVP ab initio simulation give evidence of cysteine molecule being coordinated to Sn(II) as three-dentate chelating N,O,S-donor ligand. The four Perdew density functionals TPSS, PBE0, PBE, TPSSh have been tested to provide consistency of simulated and experimental IR spectra, the best result is provided by unweighted Hartree-Fock density functionals (PBE, TPSS). On the contrary, the Hartree-Fock weighted functionals (PBE0, TPPSh) provide the most accurate geometry optimization. Unharmonic frequencies are obtained via ab initio vibrational self-consistent field (PT2-VSCF) calculations at DFT/TPSS/Def2-TZVP level, the vibrational assignment of IR spectra has been carried out.

Colour centres and nanostructures on the surface of laser crystals

This paper presents a study of structural and radiationinduced colour centres in the bulk and ordered nanostructures on the surface of doped laser crystals: sapphire, yttrium aluminium garnet and strontium titanate. The influence of thermal annealing, ionising radiation and plasma exposure on the spectroscopic properties of high-purity materials and crystals containing Ti, V and Cr impurities is examined. Colour centres resulting from changes in the electronic state of impurities and plasma-induced surface modification of the crystals are studied by optical, EPR and X-ray spectroscopies,scanning electron microscopy and atomic force microscopy. X-ray line valence shift measurements are used to assess changes in the electronic state of some impurity and host ions in the bulk and on the surface of oxide crystals. Conditions are examined for the formation of one- and two-level arrays of ordered crystallites 10-10 to 10-7 m in size on the surface of crystals doped with irongroup and lanthanoid ions. The spectroscopic properties of the crystals are analysed using ab initio self-consistent field calculations for Men+ : [O2-]k clusters.

Monosaccharide-Water Complexes: Vibrational Spectroscopy and Anharmonic Potentials

Ab initio vibrational self-consistent field (VSCF) calculations are used to predict the vibrational spectra of an extended series of monosaccharide·D(2)O complexes, including glucose, galactose, mannose, xylose, and fucose in their α and β anomeric forms, and compared with recently published experimental data for their (phenyl-tagged) complexes. Anharmonic VSCF-PT2 frequencies are calculated directly, using ab initio hybrid HF/MP2 potentials, to assess their accuracy in reproducing the vibrational anharmonicities and provide a more rigorous basis for vibrational and structural assignments. The average discrepancies between the calculated and experimental frequencies are ~1.0-1.5%, and the first-principles spectroscopic calculations, free of any empirical scaling, yield results of high accuracy. They encourage confidence in their future application to the assignment of other carbohydrate systems, both free and complexed, and an improved understanding of their intra- and intermolecular carbohydrate interactions.

Transition probabilities for the Au (S2, D2, and P2) with SiH4 reaction

Transition probabilities on the interaction of the ground and the lowest excited states of gold Au ((2)S:5d(10)6s(1), (2)D:5d(9)6s(2), and (2)P:5d(10)6p(1)) with silane (SiH(4)) are studied through ab initio Hartree-Fock self-consistent field calculations, where the atom's core is represented by relativistic effective core potentials. These calculations are followed by a multiconfigurational self-consistent field study. The correlation energy is accounted for through extensive variational and perturbative second order multireference Moller-Plesset configuration interaction analysis of selected perturbations obtained by iterative process calculations using the CIPSI program package. It is found that the Au atom in the ((2)P:5d(10)6p(1)) state inserts in the Si-H bond. In this interaction its corresponding D (2)A(') potential energy surface is initially attractive and only becomes repulsive after encountering an avoided crossing with the initially repulsive C (2)A(') surface linked to the Au((2)D:5d(9)6s(2))-SiH(4) fragments. The A, B, and C (2)A(') curves derived from the Au((2)D:5d(9)6s(2)) atom interaction with silane are initially repulsive, each one of them showing two avoided crossings, while the A (2)A(') curve goes sharply downwards until it meets the X (2)A(') curve interacting adiabatically, which is linked with the Au((2)S:5d(10)6s(1))-SiH(4) moieties. The A (2)A(') curve becomes repulsive after the avoided crossing with the X (2)A('), curve. The lowest-lying X (2)A(') potential leads to the HAuSiH(3) X (2)A(') intermediate molecule. This intermediate molecule, diabatically correlated with the Au((2)P:5d(10)6p(1))+SiH(4) system which lies 3.34 kcal/mol above the ground state reactants, has been carefully characterized as have the dissociation channels leading to the AuH+SiH(3) and H+AuSiH(3) products. These products are reached from the HAuSiH(3) intermediate without any activation barrier. The Au-SiH(4) calculation results are successfully compared to experiment. Landau-Zener theory of avoided crossings is applied to these interactions considering the angle theta instead of the distance r as the reaction coordinate.

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.

First-principles study of magnetic ordering of an Al infinite single-row atomicwire

In this paper we present a detailed analysis of the atomic and spin-electronic structure ofan Al infinite single-row atomic wire (Al-ISAW). Our work is based on ab initioself-consistent field calculations within the local density approximation, and we predictstructural transformations during elongation using the norm-conserving (NC) and projectoraugmented-wave (PAW) pseudopotentials. The results obtained by the NC pseudopotentialare in good agreement with those obtained by the PAW pseudopotential. We confirm thatthe Al-ISAW shows a metal–insulator transition and fractures when elongated beyondthe equilibrium length. Then, a wire with antiferromagnetic ordering is foundto be the lowest energetically. We find that the magnitude of spin polarizationin the vicinity of nuclei is marginal and does not play an important role in thePeierls instability. The present results show that the NC pseudopotential can givean accurate physical picture of the atomic and spin-electronic structures of theAl-ISAW.

Electronic phase transition and amorphization inAuIn2at high pressure

High-resolution angular-dispersive x-ray diffraction shows that the intermetallic compound AuIn 2 retains its initial CaF 2 structure to pressures of 23 GPa. We observe continuous broadening of diffraction lines with pressure, leading to amorphization at 24 GPa. The pressure-volume equation of state exhibits an anomaly around 2.7 GPa, in agreement with previous findings and not attributable to nonhydrostaticity. Instead, ab initio plane-wave self-consistent field calculations indicate that the observed anomaly is associated with topological changes in the electronic band structure under pressure.

Vibrational Spectroscopy of Protonated Imidazole and its Complexes with Water Molecules: Ab Initio Anharmonic Calculations and Experiments

The results of anharmonic frequency calculations on neutral imidazole (C3N2H4, Im), protonated imidazole (ImH+), and its complexes with water (ImH+)(H2O)n, are presented and compared to gas phase infrared photodissociation spectroscopy (IRPD) data. Anharmonic frequencies are obtained via ab initio vibrational self-consistent field (VSCF) calculations taking into account pairwise interactions between the normal modes. The key results are: (1) Prediction of anharmonic vibrational frequencies on an MP2 ab initio potential energy surface show excellent agreement with experiment and outstanding improvement over the harmonic frequencies. For example, the ab initio calculated anharmonic frequency for (ImH+)(H2O)N2 exhibits an overall average percentage error of 0.6% from experiment. (2) Anharmonic vibrational frequencies calculated on a semiempirical potential energy surface fitted to ab initio harmonic data represents spectroscopy well, particularly for water complexes. As an example, anharmonic frequencies for (ImH+)H2O and (ImH+)(H2O)2 show an overall average deviation of 1.02% and 1.05% from experiment, respectively. This agreement between theory and experiment also supports the validity and use of the pairwise approximation used in the calculations. (3) Anharmonic coupling due to hydration effects is found to significantly reduce the vibrational frequencies for the NH stretch modes. The frequency of the NH stretch is observed to increase with the removal of a water molecule or replacement of water with N2. This result also indicates the ability of the VSCF method to predict accurate frequencies in a matrix environment. The calculation provides insights into the nature of anharmonic effects in the potential surface. Analysis of percentage anharmoncity in neutral Im and ImH+ shows a higher percentage anharmonicity in the NH and CH stretch modes of neutral Im. Also, we observe that anharmonicity in the NH stretch modes of ImH+ have some contribution from coupling effects, while that of neutral Im has no contribution whatsoever from mode-mode coupling. It is concluded that the incorporation of anharmonic effects in the calculation brings theory and experiment into much closer agreement for these systems.

Anharmonic Vibrational Calculations Modeling the Raman Spectra of Intermediates in the Photoactive Yellow Protein (PYP) Photocycle

The role of anharmonic effects in the vibrational spectroscopy of the dark state and two major chromophore intermediates of the photoactive yellow protein (PYP) photocycle is examined via ab initio vibrational self-consistent field (VSCF) calculations and time-resolved resonance Raman spectroscopy. For the first time, anharmonicity is considered explicitly in calculating the vibrational spectra of an ensemble consisting of the PYP chromophore surrounded by model compounds used as mimics of the important active-site residues. Predictions of vibrational frequencies on an ab initio corrected semiempirical potential energy surface show remarkable agreement with experimental frequencies for all three states, thus shedding light on the potential along the reaction path. For example, calculated frequencies for vibrational modes of the red-shifted intermediate, PYPL, exhibit an overall average error of 0.82% from experiment. Upon analysis of anharmonicity patterns in the PYP modes we observe a decrease in anharmonicity in the C8-C9 stretching mode nu29 (trans-cis isomerization marker mode) with the onset of the cis configuration in PYPL. This can be attributed to the loss of the hydrogen-bonding character of the adjacent C9-O2 to the methylamine (Cys69 backbone). For several of the modes, the anharmonicity is mostly due to mode-mode coupling, while for others it is mostly intrinsic. This study shows the importance of the inclusion of anharmonicity in theoretical spectroscopic calculations, and the sensitivity of experiments to anharmonicity. The characterization of protein active-site residues by small molecular mimics provides an acceptable chemical structural representation for biomolecular spectroscopy calculations.

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.

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.

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.

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.

Formation of anion fragments from gas-phase glycine by low energy (0–15 eV) electron impact

We have measured the formation of anion fragments in gas phase glycine (H2NCH2COOH) via dissociative electron attachment (DEA) reactions in the 0–15 eV electron energy range, using a monochromatic electron beam and mass spectrometric detection of the negative ions. By far the most intense product observed is the closed shell glycine anion (H2NCH2COO)− which appears from a low-energy resonance with a peak located at 1.4 eV and a cross section in the range 10−16 cm2. The corresponding precursor ion can be characterized by electron attachment into the empty π* orbital of the −COOH group as recently assigned from electron transmission experiments and ab initio self-consistent field calculations [Aflatooni, Hitt, Gallup, and Burrow, J. Chem. Phys. 115, 6489 (2001)]. This precursor state is also observed to decompose (with much lower intensity) yielding a negative ion fragment with 58 amu, which is attributed to anions of the stoichiometric composition H2C2O2− or H4C2NO−. A further prominent DEA peak is observed at 6 eV, which is likely associated with a core excited resonance, and leads to formation of at least six different negative ion fragment species with the following mass numbers: 16 amu (O−/NH2−), 17 amu (OH−), 26 amu (CN−), 28 amu (H2CN−), 45 amu (HCO2−), 56 amu (H2C2NO−).

Evidence of a weak electronic density distortion in polymerized A 1C 60 (A=K and Rb) compared to pristine C 60

Compton profile measurements on K 1C 60 and Rb 1C 60 and C 60 powders have been carried out using inelastically scattered photons. We compare experimental Compton Profile Difference (CPD) of K 1C 60 and that of C 60 with the corresponding calculated results, obtained from ab initio self-consistent field calculations of the energy band structure. This permits us to isolate the contribution of the distortion of the C 60 orbitals for each compound and to compare them. In previous paper, we have shown that for A n C 60 compounds this approach leads to a good agreement between theory and experiment. In this paper, we show that it is not the case for K 1C 60. The distortion contribution is overestimated by calculations leading to a CPD narrower than the experimental one. Furthermore similar measurements performed on heavy ions intercalated compounds (Rb 1C 60) show clearly that CPD depends on the number of ions and not on their nature. We think that our results corroborate the conclusions of a very small distortion of C 60 molecules in the polymerized phase obtained by neutron diffraction experiments by Fox et al. (Chem. Phys. Lett. 249 (1996) 195).

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.