Showing papers in "Journal of Chemical Physics in 1993"

Density‐functional thermochemistry. III. The role of exact exchange

Abstract: Despite the remarkable thermochemical accuracy of Kohn–Sham density‐functional theories with gradient corrections for exchange‐correlation [see, for example, A. D. Becke, J. Chem. Phys. 96, 2155 (1992)], we believe that further improvements are unlikely unless exact‐exchange information is considered. Arguments to support this view are presented, and a semiempirical exchange‐correlation functional containing local‐spin‐density, gradient, and exact‐exchange terms is tested on 56 atomization energies, 42 ionization potentials, 8 proton affinities, and 10 total atomic energies of first‐ and second‐row systems. This functional performs significantly better than previous functionals with gradient corrections only, and fits experimental atomization energies with an impressively small average absolute deviation of 2.4 kcal/mol.

Particle mesh Ewald: An N⋅log(N) method for Ewald sums in large systems

Abstract: An N⋅log(N) method for evaluating electrostatic energies and forces of large periodic systems is presented. The method is based on interpolation of the reciprocal space Ewald sums and evaluation of the resulting convolutions using fast Fourier transforms. Timings and accuracies are presented for three large crystalline ionic systems.

A New Mixing of Hartree-Fock and Local Density-Functional Theories

Abstract: Previous attempts to combine Hartree–Fock theory with local density‐functional theory have been unsuccessful in applications to molecular bonding. We derive a new coupling of these two theories that maintains their simplicity and computational efficiency, and yet greatly improves their predictive power. Very encouraging results of tests on atomization energies, ionization potentials, and proton affinities are reported, and the potential for future development is discussed.

Relativistic regular two‐component Hamiltonians

Abstract: In this paper, potential‐dependent transformations are used to transform the four‐component Dirac Hamiltonian to effective two‐component regular Hamiltonians. To zeroth order, the expansions give second order differential equations (just like the Schrodinger equation), which already contain the most important relativistic effects, including spin–orbit coupling. One of the zero order Hamiltonians is identical to the one obtained earlier by Chang, Pelissier, and Durand [Phys. Scr. 34, 394 (1986)]. Self‐consistent all‐electron and frozen‐core calculations are performed as well as first order perturbation calculations for the case of the uranium atom using these Hamiltonians. They give very accurate results, especially for the one‐electron energies and densities of the valence orbitals.

The equation of motion coupled‐cluster method. A systematic biorthogonal approach to molecular excitation energies, transition probabilities, and excited state properties

Abstract: A comprehensive overview of the equation of motion coupled‐cluster (EOM‐CC) method and its application to molecular systems is presented. By exploiting the biorthogonal nature of the theory, it is shown that excited state properties and transition strengths can be evaluated via a generalized expectation value approach that incorporates both the bra and ket state wave functions. Reduced density matrices defined by this procedure are given by closed form expressions. For the root of the EOM‐CC effective Hamiltonian that corresponds to the ground state, the resulting equations are equivalent to the usual expressions for normal single‐reference CC density matrices. Thus, the method described in this paper provides a universal definition of coupled‐cluster density matrices, providing a link between EOM‐CC and traditional ground state CC theory.Excitation energy,oscillator strength, and property calculations are illustrated by means of several numerical examples, including comparisons with full configuration interaction calculations and a detailed study of the ten lowest electronically excited states of the cyclic isomer of C4.

Gaussian basis sets for use in correlated molecular calculations. IX. The atoms gallium through krypton

Abstract: Valence correlation consistent and augmented correlation consistent basis sets have been determined for the third row, main group atoms gallium through krypton. The methodology, originally developed for the first row atoms, was first applied to the selenium atom, resulting in the expected natural groupings of correlation functions (although higher angular momentum functions tend to be relatively more important for the third row atoms as they were for the second row atoms). After testing the generality of the conclusions for the gallium atom, the procedure was used to generate correlation consistent basis sets for all of the atoms gallium through krypton. The correlation consistent basis sets for the third row main group atoms are as follows: cc-pVDZ: (14s11p6d)/[5s4p2d]; cc-pVTZ: (20s13p9d1f )/[6s5p3d1f]; cc-pVQZ: (21s16p12d2 f1g)/[7s6p4d2 f1g]; cc-pV5Z: (26s17p13d3f2g1h)/[8s7p5d3f2g1h]. Augmented sets were obtained by adding diffuse functions to the above sets (one for each angular momentum present in the set), with the exponents of the additional functions optimized in calculations on the atomic anions. Test calculations on the atoms as well as selected molecules with the new basis sets show good convergence to an apparent complete basis set limit.

Nonexponential relaxations in strong and fragile glass formers

Abstract: Deviations from thermally activated and from exponential response are typical features of the vitrification phenomenon and previously have been studied using viscoelastic, dielectric, calorimetric, optical, and other techniques. Linear response data from literature on about 70 covalent glass formers, ionic melts, supercooled liquids, amorphous polymers, and glassy crystals are surveyed. Except for orientational glasses and monohydric aliphatic alcohols a distinct but broad correlation of non‐Debye behavior with non‐Arrhenius relaxations is found. Within the broad trend several groups of materials, distinguished by their respective molecular complexity, can be identified and are shown to exhibit narrow correlations. At a given degree of deviation from Arrhenius behavior externally imposed stresses are relaxed with a departure from exponential behavior which is stronger the more the molecular or atomic subunits of the glassforming material are interconnected with each other.

Coupled cluster theory for high spin, open shell reference wave functions

Abstract: The coupled cluster method restricted to single and double excitations (CCSD) is considered for the case of a spin restricted Hartree–Fock open shell reference determinant. A spin–orbital based formulation, in which the cluster operator spans exactly the minimal first order interacting space, is presented, and computationally optimal working equations are given. In the limit of a large number of closed shell orbitals, the cost is identical to that of an optimum treatment of an equivalent closed shell problem, which is obtained as a special case of the formulation presented. The theory is applied to the calculation of a number of diatomic potential energy functions and compared with spin‐unrestricted theory.

Coupled‐cluster methods with noniterative triple excitations for restricted open‐shell Hartree–Fock and other general single determinant reference functions. Energies and analytical gradients

Abstract: A new, noniterative triples correction to the coupled‐cluster singles and doubles (CCSD), method, for general single determinant reference functions is proposed and investigated numerically for various cases, including non‐Hartree–Fock (non‐HF) reference functions. It is correct through fourth‐order of perturbation theory for non‐HF references, and unlike other such methods, retains the usual invariance properties common to CC methods, while requiring only a single N7 step. In the canonical Hartree–Fock case, the method is equivalent to the usual CCSD(T) method, but now permits the use of restricted open‐shell Hartree‐Fock (ROHF) and quasirestricted Hartree–Fock (QRHF) reference determinants, along with many others. Comparisons with full configuration interaction (FCI) results are presented for CH2, CH2+, CH3, NH2, and SiH2. The paper also reports the derivation and initial computational implementation of analytical gradients for the ROHF‐CCSD(T) method, which includes unrestricted Hartree–Fock (UHF) CCSD...

The performance of a family of density functional methods

Abstract: The results of a systematic study of molecular properties by density functional theory (DFT) are presented and discussed. Equilibrium geometries, dipole moments, harmonic vibrational frequencies, and atomization energies were calculated for a set of 32 small neutral molecules by six different local and gradient‐corrected DFT methods, and also by the ab initio methods Hartree–Fock, second‐order Mo/ller–Plesset, and quadratic configuration interaction with single and double substitutions (QCISD). The standard 6‐31G* basis set was used for orbital expansion, and self‐consistent Kohn–Sham orbitals were obtained by all DFT methods, without employing any auxiliary fitting techniques. Comparison with experimental results shows the density functional geometries and dipole moments to be generally no better than or inferior to those predicted by the conventional ab initio methods with this particular basis set. The density functional vibrational frequencies compare favorably with the ab initio results, while for at...

Gaussian-2 theory using reduced Moller--Plesset orders

Abstract: Two variations of Gaussian‐2 (G2) theory are presented. In the first, referred to as G2 (MP2) theory, the basis‐set‐extension energy corrections are obtained at the 2nd order Mo/ller–Plesset (MP2) level and in the second, referred to as G2(MP3) theory, they are obtained at the MP3 level. The methods are tested out on the set of 125 systems used for validation of G2 theory [J. Chem Phys. 94, 7221 (1991)]. The average absolute deviation of the G2(MP2) and G2(MP3) theories from experiment are 1.58 and 1.52 kcal/mol, respectively, compared to 1.21 kcal/mol for G2 theory. The new methods provide significant savings in computational time and disk storage.

Effective core potential methods for the lanthanides

Abstract: In this paper a complete set of effective core potentials (ECPs) and valence basis sets for the lanthanides (Ce to Lu) are derived. These ECPs are consistent not only within the lanthanide series, but also with the third‐row transition metals which bracket them. A 46‐electron core was chosen to provide the best compromise between computational savings and chemical accuracy. Thus, the 5s and 5p are included as ‘‘outer’’ core while all lower energy atomic orbitals (AOs) are replaced with the ECP. Generator states were chosen from the most chemically relevant +3 and +2 oxidation states. The results of atomic calculations indicate that the greatest error vs highly accurate numerical potential/large, even‐tempered basis set calculations results from replacement of the large, even‐tempered basis sets with more compact representations. However, the agreement among atomic calculations remains excellent with both basis set sizes, for a variety of spin and oxidation states, with a significant savings in time for the optimized valence basis set. It is expected that the compact representation of the ECPs and valence basis sets will eventually encourage their use by computational chemists to further explore the bonding and reactivity of lanthanide complexes.

Quasidegenerate perturbation theory with multiconfigurational self‐consistent‐field reference functions

Abstract: A quasidegenerate perturbation theory based on multiconfigurational self‐consistent‐field (MCSCF) reference functions is derived. The perturbation theory derived here is for multistate, where several MCSCF functions obtained by the state‐averaged MCSCF method are used as the reference and an effective Hamiltonian is constructed by perturbation calculation. The energies of states interested in are obtained simultaneously by diagonalization of the effective Hamiltonian. An explicit formula of the effective Hamiltonian through second order is derived as well as general formalism, and is applied to calculate potential curves of the system H2, Be–H2, CO, NO, BN, and LiF. The results agree well with those of full configuration interaction or multireference single and double excitation configuration interaction methods for both the ground and the excited states.

Structure and hydrogen bond dynamics of water–dimethyl sulfoxide mixtures by computer simulations

Abstract: We have used two different force field models to study concentrated dimethyl sulfoxide (DMSO)–water solutions by molecular dynamics. The results of these simulations are shown to compare well with recent neutron diffraction experiments using H/D isotope substitution [A. K. Soper and A. Luzar, J. Chem. Phys. 97, 1320 (1992)]. Even for the highly concentrated 1 DMSO : 2 H2O solution, the water hydrogen–hydrogen radial distribution function,g HH(r), exhibits the characteristic tetrahedral ordering of water–water hydrogen bonds. Structural information is further obtained from various partial atom–atom distribution functions, not accessible experimentally. The behavior of water radial distribution functions,g OO(r) and g OH(r) indicate that the nearest neighbor correlations among remaining water molecules in the mixture increase with increasing DMSO concentration. No preferential association of methyl groups on DMSO is detected. The pattern of hydrogen bonding and the distribution of hydrogen bond lifetimes in the simulated mixtures is further investigated. Molecular dynamics results show that DMSO typically forms two hydrogen bonds with water molecules. Hydrogen bonds between DMSO and water molecules are longer lived than water–water hydrogen bonds. The hydrogen bond lifetimes determined by reactive flux correlation function approach are about 5 and 3 ps for water–DMSO and water–water pairs, respectively, in 1 DMSO : 2 H2O mixture. In contrast, for pure water, the hydrogen bond lifetime is about 1 ps. We discuss these times in light of experimentally determined rotational relaxation times. The relative values of the hydrogen bond lifetimes are consistent with a statistical (i.e., transition state theory) interpretation.

Ab initio studies of cyclic water clusters (H2O)n, n=1–6. I. Optimal structures and vibrational spectra

Abstract: The optimal structures and harmonic vibrational frequencies of cyclic water clusters, (H2O)n, have been determined at the Hartree–Fock (for n=2–6) and second order perturbation theory (for n=2–4) levels of theory with an augmented correlation consistent double zeta basis set At the MP2 level this basis set yields very accurate results for the structure, dipole moment, and polarizability of the water monomer as well as results of comparable accuracy for the structure, binding energy, and harmonic vibrational frequencies of the water dimer The optimal structure of (H2O)4 and the harmonic frequencies of (H2O)3,4 are the first ones reported at a correlated level for these species Analysis of the structural trends reveals that the separation between neighboring oxygen atoms decreases exponentially with increasing cluster size The predicted R0(O–O) for the ring hexamer is less than 002 A shorter than the interoxygen separation in ice Ih Furthermore, the trends in the harmonic vibrational frequencies sugge

Effects of electron correlation in the calculation of nuclear magnetic resonance chemical shifts

Abstract: Using second‐order many‐body perturbation theory [MBPT(2)] and the gauge‐including atomic orbital (GIAO) ansatz, electron correlation effects are investigated in the calculation of NMR chemical shieldings and shifts. A thorough discussion of the theory, aspects of the implementation as well as the computational requirements of the GIAO‐MBPT(2) method are presented. The performance of the GIAO‐MBPT(2) approach is tested in benchmark calculations of 13C, 15N, and 17O chemical shifts. Comparison with available experimental gas phase NMR data shows that GIAO‐MBPT(2) improves in all cases considered here over the GIAO results obtained at the Hartree–Fock self‐consistent‐field (HF‐SCF) level. Correlation effects turn out to be particularly important for molecules with multiple bonds, e.g., carbonyl or cyano compounds, and it seems that GIAO‐MBPT(2) slightly overestimates these effects for difficult cases having relatively large correlation contributions of 30 to 110 ppm. For CO, N2, N2O, additional calculations with large basis sets are presented to check the accuracy of the GIAO‐MBPT(2) method and the geometry dependence of the calculated chemical shieldings is analyzed.

The effect of long‐range electrostatic interactions in simulations of macromolecular crystals: A comparison of the Ewald and truncated list methods

Abstract: Simulations of the HIV‐1 protease unit cell using a 9 A cutoff, 9/18 A ‘‘twin‐range’’ cutoff, and full Ewald sums have been carried out to 300 ps. The results indicate that long‐range electrostatic interactions are essential for proper representation of the HIV‐1 protease crystal structure. The 9 A simulation did not converge in 300 ps. Inclusion of a 9/18 A ‘‘twin‐range’’ cutoff showed significant improvement. Simulation using the Ewald summation convention gave the best overall agreement with x‐ray crystallographic data, and showed the least internal differences in the time average structures of the asymmetric units. The Ewald simulation represents an efficient implementation of the Particle Mesh Ewald method [Darden et al., J. Chem. Phys. 98, 10 089 (1993)], and illustrates the importance of including long‐range electrostatic forces in large macromolecular systems.

The use of systematic sequences of wave functions for estimating the complete basis set, full configuration interaction limit in water

Abstract: An assortment of 1‐ and 2‐electron water properties were extracted from a systematic sequence of wave functions. The regularity inherent in this sequence permitted simple exponential fits of the resulting energies and, in many cases, the properties. To the extent the exponential fit accurately reflects the asymptotic convergence of a specific property, it provides an estimate of the complete basis set, full configuration interaction (CI) limiting value at a reduced computational expense. As a consequence of the vast reduction in the number of configurations that must be treated variationally, the proposed scheme may make possible improved estimates of the complete basis set, full CI limit beyond what could be obtained from explicit computations. In order to judge the accuracy of the procedure, we have carried out the highest level ab initio calculations to date on water, recovering in excess of 96% of the estimated valence correlation energy.

‘‘Ab initio’’ liquid water

Abstract: An ab initio molecular dynamics simulation of liquid water has been performed using density functional theory in the Kohn–Sham formulation and a plane wave basis set to determine the electronic structure and the forces at each time step. For an accurate description of the hydrogen bonding in the liquid, it was necessary to extend the exchange functional with a term that depends on the gradient of the electron density. A further important technical detail is that supersoft pseudopotentials were used to treat the valence orbitals of the oxygen atoms in a plane wave expansion. The structural and dynamical properties of the liquid were found to be in good agreement with experiment. The ab initio molecular dynamics also yields information on the electronic structure. The electronic feature of special interest is the lowest unoccupied molecular orbital (LUMO) of the liquid which is the state occupied by a thermalized excess electron in the conductive state. The main result of calculating the liquid LUMO is that it is a delocalized state distributed over interstitial space between the molecules with a significant admixture of the σ* orbitals of the individual water molecules.

Direct evaluation of phase coexistence by molecular simulation via integration along the saturation line

Abstract: Thermodynamic integration along a path that coincides with the saturation line is proposed as an efficient means for evaluation of phase equilibria by molecular simulation. The technique allows coexistence to be determined by just one simulation, without ever attempting or performing particle insertions. Prior knowledge of one coexistence point is required to start the procedure. Integration then advances from this state according to the Clapeyron formula—a first‐order ordinary differential equation that prescribes how the pressure must change with temperature to maintain coexistence. The method is unusual in the context of thermodynamic integration in that the path is not known at the outset of the process; results from each simulation determine the course that the integration subsequently takes. Predictor–corrector methods among standard numerical techniques are shown to be particularly well suited for this type of integration. A typical integration step along the saturation line proceeds as follows: An increment in the temperature is chosen, and the saturation pressure at the new temperature is ‘‘predicted’’ from previous data (the initial coexistence datum and/or previous simulations). Simultaneous but independent NPT simulations of the coexisting phases are initiated at the said conditions. Averages taken throughout the simulations are repeatedly used to ‘‘correct’’ the estimate of the pressure to convergence. Thus strictly the pressure is not fixed during the simulation. Vapor–liquid coexistence of the van der Waals model is first used to study the numerical integration method without the complications of molecular simulation. In a second application the phase envelope of the Lennard‐Jones model fluid is computed, and many variations of the technique are examined. Overall, the results are remarkably consistent and in agreement with previous simulation studies. Difficulty is encountered upon approach of the critical point, but, by artificially coupling the simulation volumes, the method remains effective in this regime so long as a suitably small integration step is employed. Many extensions and improvements of the technique are discussed.

Two-dimensional femtosecond vibrational spectroscopy of liquids

Abstract: The nonlinear optical response of liquids subjected to a series of N femtosecond laser pulses is calculated using a multimode harmonic model for nuclear motions, with nonlinear coupling to the radiation field through the coordinate dependence of the electronic polarizability. Using electronically off‐resonant optical fields, this multidimensional spectroscopy is shown to provide direct information regarding the homogeneous or the inhomogeneous nature of the spectral density obtained from optical birefringence measurements. Complementary information can be obtained using infrared pulses where the multiple time correlation functions of the nuclear dipole moment (rather than the electronic polarizability) are being probed.

A diffusion Monte Carlo algorithm with very small time-step errors

Abstract: We propose modifications to the simple diffusion Monte Carlo algorithm that greatly reduce the time‐step error. The improved algorithm has a time‐step error smaller by a factor of 70 to 300 in the energy of Be, Li2 and Ne. For other observables the improvement is yet larger. The effective time step possible with the improved algorithm is typically a factor of a few hundred larger than the time step used in domain Green function Monte Carlo. We also present an optimized 109 parameter trial wave function for Be which, used in combination with our algorithm, yields an exceedingly accurate ground state energy. A simple solution to the population control bias in diffusion Monte Carlo is also discussed.

The rheological behavior of concentrated colloidal dispersions

Abstract: A simple model for the rheological behavior of concentrated colloidal dispersions is developed. For a suspension of Brownian hard spheres there are two contributions to the macroscopic stress: a hydrodynamic and a Brownian stress. For small departures from equilibrium, the hydrodynamic contribution is purely dissipative and gives the high‐frequency dynamic viscosity. The Brownian contribution has both dissipative and elastic parts and is responsible for the viscoelastic behavior of colloidal dispersions. An evolution equation for the pair‐distribution function is developed and from it a simple scaling relation is derived for the viscoelastic response. The Brownian stress is shown to be proportional to the equilibrium radial‐distribution function at contact, g(2;φ), divided by the short‐time self‐diffusivity, D0s(φ), both evaluated at the volume fraction φ of interest. This scaling predicts that the Brownian stress diverges at random close packing, φm, with an exponent of −2, that is, η’0 ∼ η(1 − φ/φm)−2, ...

Molecular dynamics simulation of a polymer chain in solution

Abstract: Results of a molecular dynamics simulation of a single polymer chain in a good solvent are presented. The latter is modeled explicitly as a bath of particles. This system provides a first‐principles microscopic test of the hydrodynamic Kirkwood–Zimm theory of the chain’s Brownian motion. A 30 monomer chain is studied in 4066 solvent particles as well as 40/4056 and 60/7940 systems. The density was chosen rather high, in order to come close to the ideal situation of incompressible flow, and to ensure that diffusive momentum transport is much faster than particle motions. In order to cope with the numerical instability of microcanonical algorithms, we generate starting states by a Langevin simulation that includes a coupling to a heat bath, which is switched off for the analysis of the dynamics. The long range of the hydrodynamic interaction induces a large effect of finite box size on the diffusive properties, which is observable for the diffusion constants of both the chain and the solvent particles. The ...

Atomic force microscopy and surface-enhanced Raman spectroscopy. I. Ag island films and Ag film over polymer nanosphere surfaces supported on glass

Abstract: The surface roughness and nanometer scale structure of Ag films used for surface‐enhanced Raman scattering (SERS) are characterized using atomic force microscopy (AFM). Two important types of thin film based SERS‐active surface have been examined in this study: (1) Ag island films (AgIF’s) on smooth, insulating substrates and (2) thick Ag films evaporated over both preroughened and smooth substrates. AFM is demonstrated to be capable of quantitatively defining the three‐dimensional (3D) structure of these roughened surfaces. The effects of mass thickness, dm, and thermal annealing on the nanostructure of AgIF’s are studied in detail. Particle size histograms are calculated from the AFM images for both ‘‘as‐deposited’’ and annealed IF’s with dm=1.8 and 3.5 nm. Quantitative measurements of the SERS enhancement factor (EF) are coupled with the AFM data and interpreted within the framework of the electromagnetic theory of SERS. AFM images for thick evaporated Ag films over a monolayer of polymer nanospheres (AgFON) shows the clear presence of ‘‘random substructure roughness’’ reducing their utility as controlled roughness surfaces. Similar roughness structures are observed for thick evaporated Ag films on smooth, insulating substrates. Nevertheless, AgFON surfaces are demonstrated to be among the most strongly enhancing thin film based surfaces ever studied with EF’s comparable to those found for electrochemically roughened surfaces. Applications of FON surfaces to ultrahigh sensitivity SERS, anti‐Stokes detected SERS, and surface‐enhanced hyper‐Raman spectroscopy (SEHRS) are reported.

Efficient molecular dynamics and hybrid Monte Carlo algorithms for path integrals

Abstract: New path integral molecular dynamics (PIMD) and path integral hybrid Monte Carlo (PIHMC) algorithms are developed. It is shown that the use of a simple noncanonical change of variables that naturally divides the quadratic part of the action into long and short wavelength modes and multiple time scale integration techniques results in very efficient algorithms. The PIMD method also employs a constant temperature MD technique that has been shown to give canonical averages even for stiff systems. The new methods are applied to the simple quantum mechanical harmonic oscillator and to electron solvation in fluid helium and xenon. Comparisons are made with PIMC and the more basic PIMD and PIHMC methods.

J‐aggregate formation of a water‐soluble porphyrin in acidic aqueous media

Abstract: J aggregate of a water‐soluble porphyrin, 5,10,15,20‐tetra(4‐sulfophenyl)porphyrin (H2TPPS44−), formed in acidified aqueous solutions, exhibits sharp and intense absorption bands at 491 and 707 nm. These characteristic transitions, J bands, are of linear oscillators polarized in the long axis of rodlike aggregate. The molecules in the aggregate stack so as to lift the degeneracy of the porphyrin planar oscillator excited states. Measurements of flow‐induced linear dichroism, circular dichroism, magnetic circular dichroism, as well as polarized fluorescence excitation spectra provide evidence not only of linear oscillator character of the intense J band at 491 nm, but also of presence of another diffuse absorption band around at 420 nm polarized in the short axis of the aggregate, which is the counterpart of the 491 nm band of porphyrin Soret origin. Extrinsic circular dichroism is induced upon addition of L‐tartaric acid or by mechanical swirling flow in the period of aggregate growth. Resonance Raman spe...

Molecular ordering of organosulfur compounds on Au(111) and Au(100): Adsorption from solution and in ultrahigh vacuum

Abstract: Low‐energy electron diffraction and reflection‐absorption infrared spectroscopy were used to study the monolayers formed by the adsorption of n‐alkane thiols [HS(CH2)mCH3] on both (111) and (100) single‐crystal gold substrates. Samples were prepared by dosing either from solution (m=15, 17, 18, and 21) or in ultrahigh vacuum (m=0–9). On Au(111), ordered surface structures are obtained which can be indexed as (n√3×√3)R30°, where n varies from 1 to 6. On Au(100), the adsorption of short chain thiols leads to the formation of a c(2×2) overlayer while the longer chain homologs show additional diffraction spot splittings. It is also found that chain length influences both the character of the diffraction seen and perturbs the reactive sticking probability of molecules dosed in UHV. Infrared studies reveal that the polymethylene chains of the monolayers formed on Au(100) are comprised of nearly all‐trans conformations and are less canted than the comparable structures formed on Au(111). A simple model is propos...

Factors affecting lifetimes and resolution of Rydberg states observed in zero-electron-kinetic-energy spectroscopy

Abstract: It is shown that under the usual conditions of zero‐electron‐kinetic‐energy, pulsed field ionization (ZEKE–PFI) spectroscopy the lifetimes of very high‐lying Rydberg states are increased by at least approximately the factor n (in addition to the expected factor of n3), the principal quantum number, due to strong l mixing by the Stark effect. Additional factors may increase lifetimes by still another factor of approximately n. Pulsed field ionization under typical conditions is shown as likely to be predominantly diabatic and the effect on resolution is assessed. Factors affecting rotational line intensities are also discussed.

Towards an accurate molecular orbital theory for excited states: Ethene, butadiene, and hexatriene

Abstract: A newly proposed quantum chemical approach for ab initio calculations of electronic spectra of molecular systems is applied to the molecules ethene, trans‐1,3‐butadiene, and trans‐trans‐1,3,5‐hexatriene. The method has the aim of being accurate to better than 0.5 eV for excitation energies and is expected to provide structural and physical data for the excited states with good reliability. The approach is based on the complete active space (CAS) SCF method, which gives a proper description of the major features in the electronic structure of the excited state, independent of its complexity, accounts for all near degeneracy effects, and includes full orbital relaxation. Remaining dynamic electron correlation effects are in a subsequent step added using second order perturbation theory with the CASSCF wave function as the reference state. The approach is here tested in a calculation of the valence and Rydberg excited singlet and triplet states of the title molecules, using extended atomic natural orbital (ANO) basis sets. The ethene calculations comprised the two valence states plus all singlet and triplet Rydberg states of 3s, 3p, and 3d character, with errors in computed excitation energies smaller than 0.13 eV in all cases except the V state, for which the vertical excitation energy was about 0.4 eV too large. The two lowest triplet states and nine singlet states were studied in butadiene. The largest error (0.37 eV) was found for the 2 1Bu state. The two lowest triplet and seven lowest singlet states in hexatriene had excitation energies in error with less than 0.17 eV.