Volume 119, Issue 4, 22 July 2003
 ARTICLES

 Theoretical Methods and Algorithms

Equationofmotion coupled cluster method with full inclusion of connected triple excitations for electronattached states: EAEOMCCSDT
View Description Hide DescriptionWe extend the full triples equationofmotion (EOM) coupled cluster (CC) method to electron attached states. Proper factorization of the three and fourbody parts of the effective Hamiltonian makes it possible to achieve for the EAEOM part a scaling no higher than The method is calibrated by the evaluation of the valence vertical electron affinities for the and molecules for several basis sets up to 160 basis functions. For EAEOMCCSDT gives 3.24 eV at the extrapolated basis limit, while the experimental adiabatic EA is equal to For the agreement is ∼1.9 eV compared to an adiabatic value of 2.1 eV.

Dose dependence of surface plasmon resonance of a nanoparticle composite
View Description Hide DescriptionThe linear optical absorptionproperties of a titaniumnanoparticle composite formed by implantation of low energy into single crystal are reported. Evolution of a surface plasmon resonance (SPR) induced by the formation of a metal nanoparticle composite is studied as a function of ion dose. At an implantation temperature of 25 °C, the threshold dose for the appearance of the SPR is indicating that spontaneous nucleation and clustering of titaniumnanoparticles in occurs at peak concentration of implants. The average particle size and volume fraction are dependent on the ion dose. The correlation of the optical response, i.e., the intensity and frequency of the observed SPR, with the nucleation/clustering of the metal nanoparticles is discussed on the basis of Mie scattering theory and Maxwell Garnett theory.

Integral expressions satisfied by the gradient of density functional potentials
View Description Hide DescriptionIt is shown that the product of the gradient of the density functionalcorrelation potential and the charge density and the product of the gradient of the sum of the exchange and Hartree potentials and the charge density integrate to zero. The integral of the product of the charge density and the gradient of exchange potential is equal to an integral expression of a product of occupied Kohn–Sham orbitals and the gradient of the mutual Coulomb interaction. It is also shown that the expectation value of the gradient of the mutual Coulomb interaction is zero for the ground state of a manyelectron system.

On the perturbation of multiconfiguration wave functions
View Description Hide DescriptionA simple variant of perturbation theory is used to correct reference states of a general multiconfigurational character. The full solution of an active space is not required, and no iterative procedure is applied to construct the resolvent operator. The perturbed wave function is expanded in a complete set of determinants from which the reference function is projected out, and the overlap between projected determinants is handled by an explicit, analytic inversion of the overlap matrix.

Generalization of the Nosé–Hoover approach
View Description Hide DescriptionA simple extension of the Nosé–Hoover canonical dynamics based on a more general form of the thermostat kinetic energy term in the Nosé Hamiltonian is considered. With this extension considerable enhancement of chaotic behavior is achieved, particularly for small and stiff systems. The considered deterministic thermostat exhibits most of the desirable properties of a good canonical thermostatting mechanism.

Physical interpretation and evaluation of the Kohn–Sham and Dyson components of the relations between the Kohn–Sham orbital energies and the ionization potentials
View Description Hide DescriptionTheoretical and numerical insight is gained into the relations between the Kohn–Sham orbital energies and relaxed vertical ionization potentials (VIPs) which provide an analog of Koopmans’ theorem for density functional theory. The Kohn–Sham orbital energy has as leading term where is the primary VIP for ionization with spectroscopic factor (proportional to the intensity in the photoelectron spectrum) close to 1, and the set contains the VIPs that are satellites to the ionization, with small but nonnegligible In addition to this “average spectroscopic structure” of the there is an electronshell step structure in from the contribution of the response potential Accurate KS calculations for prototype second and thirdrow closedshell molecules yield valence orbital energies which correspond closely to the experimental VIPs, with an average deviation of 0.08 eV. The theoretical relations are numerically investigated in calculations of the components of the relations for the molecule, and for the molecules CO, HF, HCN. The derivation of the relations employs the Dyson orbitals (the are their norms). A connection is made between the KS and Dyson orbital theories, allowing the spinunrestricted KS xc potential to be expressed with a statistical average of individual xc potentials for the Dyson spin–orbitals as leading term. Additional terms are the correction due to the correlation kinetic effect, and the “response” related to the correction to the energy of electrons due to the correlation with the reference electron.

Automated calculation of fundamental frequencies: Application to using the coupledcluster singlesanddoubles with perturbative triples method
View Description Hide DescriptionAn automated scheme for calculating numerical derivatives of functions is presented and applied to the Taylor expansion of potential energy surfaces. The computational cost is reduced by invoking the symmetry properties of noncubic groups. The scheme is applied to the quartic force field of isotopomers of by numerical differentiation of the CCSD(T) energy, using the ccpCVQZ basis for the harmonic part of the potential and the ccpCVTZ basis for the anharmonic part. From this force field, zeroorder vibrational corrections to the geometry and the fundamental frequencies are calculated by secondorder perturbation theory. The results are compared with experiment and previous calculations.
 Gas Phase Dynamics and Structure: Spectroscopy, Molecular Interactions, Scattering, and Photochemistry

Ninedimensional quantum molecular dynamics simulation of intramolecular vibrational energy redistribution in the molecule with the help of coupled coherent states
View Description Hide DescriptionA previously developed method of coupled coherent states (CCS) is applied to the simulation of intramolecular vibrational energy redistribution in the molecule. All nine modes are taken into account within a fully quantum approach. Emphasis is placed on convergence with respect to the number of coherent states in relation to the desired propagation time, which was taken to be sufficient to resolve Fermi resonance splitting of ∼100 cm^{−1} at an excitation energy of ∼16 000 cm^{−1}. Fermiresonance beatings of energy between C–H stretch and two C–H bends as well as slow energy flow to the rest of the molecule are reproduced. Due to the use of Monte Carlo grids the CCS technique scales extremely well with the number of modes and allows fully quantum molecular dynamics simulations of polyatomic systems.

The structures of fluorene– determined by rotational coherence spectroscopy
View Description Hide DescriptionRotational coherencespectroscopy (RCS), via timecorrelated single photon counting, and twocolor resonant twophotonionization (R2PI) timeofflight mass spectrometry, have been used to characterize fluorene– van der Waals clusters generated in supersonic jets. Rotational coherence traces have been obtained at excitation energies corresponding to several resonant features in the R2PI spectra of RCS simulations and diagonalization of the moment of inertia tensor have been used to obtain excited state rotational constants and structures of that are consistent with the experimental rotational coherence traces. The RCS results indicate that: (i) the water molecule in resides above the central five member ring and interacts with both aromatic sites; (ii) the water molecules in form a water dimer that is most likely oriented along the long axis of fluorene and is hydrogenbonded to both aromatic sites. The R2PI spectra of and FL–HDO have also been obtained. The transition is a doublet in the R2PI spectra of and a singlet in the R2PI spectrum of FL–HDO. The presence of this doublet in the spectra, and the absence of such a splitting in the FL–HDO spectrum, is an indication of internal rotation of the water molecule on a potential energy surface that changes upon electronic excitation. Lastly, the use of RCS and timeresolved fluorescence as a tool for assigning features in R2PI spectra that are of ambiguous origin due to fragmentation of higher mass clusters into lower mass channels is demonstrated.

Experimental and theoretical investigation of the production of cations containing C–N bonds in the reaction of benzene with atomic nitrogen ions
View Description Hide DescriptionIn the last few years, astronomical spectra have revealed the presence of aromatic and polyaromatic molecules in extraterrestrial environments, near carbon stars, in molecular clouds and meteorites. Moreover, the recent observation of benzene in interstellar space has noticeably increased the interest in the entire class of molecules and in their chemical behavior. In this work, we have investigated the reaction between the benzene molecule and the atomic nitrogen cation and, in particular, the mechanisms by which the reactants are converted into cationic products containing at least one C–N bond, according to the general scheme We have measured the energy dependence of the cross section in a guided ion beam tandem mass spectrometer. Relevant stationary points of the potential energy surface have been studied by using the density functional theory hybrid functional B3LYP with the 631G^{*} basis set. Thermochemical calculations, and the comparison with experimental results, allow us to distinguish between exoergic and endoergic processes and to obtain a detailed description of the reaction mechanisms. We show that aromatic hydrocarbons may be converted into organicnitrogen compounds via the insertion of into the benzene ring and the formation of C–N bonds from C–C ring reactants.

Microscopic twofluid theory of rotational constants of the complex in droplets
View Description Hide DescriptionWe present a microscopic quantum analysis for rotational constants of the complex in helium droplets using the local twofluid theory in conjunction with path integral Monte Carlo simulations.Rotational constants are derived from effective moments of inertia calculated assuming that motion of the molecule and the local nonsuperfluid helium density is rigidly coupled to the molecular rotation of OCS and employing path integral methods to sample the corresponding and helium densities. The rigid coupling assumption for is calibrated by comparison with exact calculations of the free complex. The presence of the molecule is found to induce a small local nonsuperfluid helium density in the second solvation shell which makes a nonnegligible contribution to the moment of inertia of the complex in helium. The resulting moments of inertia for the complex embedded in a cluster of 63 helium atoms are found to be in good agreement with experimentally measured values in large helium droplets. Implications for analysis of rotational constants of larger complexes of OCS with multiple molecules in helium are discussed.

Quantum/classical studies of collision dynamics
View Description Hide DescriptionThe dynamics of the and Ar–HCl reactions is investigated using a multiple configuration quantum/classical approach. In this work the dynamics of the hydrogen atom is propagated quantum mechanically in the three Cartesian coordinates of the atom, while the dynamics of the other atoms is propagated classically, in a centerofmass frame. It is found that the introduction of the argon atom affects the reaction probability through two mechanisms. For nearly collinear O+Ar–HCl collisions, the argon atom blocks the transition state for the O+HCl reaction and inhibits the reaction. On the other hand when the collision geometry is such that the oxygen atom does not collide with the argon atom, the reaction probability is increased. These results are analyzed in terms of the shape of the ground state Ar–HCl wave function. The energy transfer dynamics from the oxygen atom and to the argon atom is also investigated.

Theoretical ab initio study of the electronic states of KrH and Quantum defect and complex coordinate calculations on the Rydberg states of KrH
View Description Hide DescriptionPotential energy curves have been calculated for the ground and excited electronic states of KrH and the cation by ab initioconfiguration interaction calculations using effective core potentials for Kr. Quantum defect functions have been determined from the ab initio potentials of the lowlying Rydberg states of KrH and potential energy curves have been generated for higher Rydberg states. The resulting bound–bound transition energies are in excellent agreement with experimental data. The interaction of the state with the and and states and their predissociation by has been treated by multistate complex scaling calculations for both KrH and KrD. Much larger predissociation widths are obtained in KrH than in KrD, in agreement with experimental observations.

Spin–orbit density functional theory calculations for heavy metal monohydrides
View Description Hide DescriptionSpin–orbit density functional theory method implemented in the NWCHEM program package has been employed with the shapeconsistent relativistic effective core potentials to calculate spectroscopic constants (bond lengths, frequencies, and dissociation energies) and estimate spin–orbit effects for 6th(Tl–At) and 7th(113–117)row element monohydrides. Results calculated with local density approximation and gradientcorrected approximation of the exchangecorrelation functional are usually similar to those of other allelectron relativistic density functional approaches. The spin–orbit effects on the spectroscopic constants are in good agreement with previous twocomponent coupledcluster singles and doubles with perturbative triples results calculated with same relativistic effective core potentials and basis sets. Spin–orbit density functional theory calculations with extended basis sets and extensive set of functionals for TlH, (113)H, PbH, (114)H, and PbO molecules indicate that there could be substantial variations among functionals and that the hybrid functionals produce the results in excellent overall agreement with empirical measures.

Vibrational relaxation in and clusters excited by femtosecond stimulated emission pumping
View Description Hide DescriptionVibrational relaxation dynamics in and clusters are studied using femtosecondstimulated emission pumping (fsSEP) in conjunction with femtosecondphotoelectron spectroscopy. fsSEP generates coherently excited within the cluster; results are reported here for excitation energies of 0.57 and 0.75 eV. The timedependent PE spectra track relaxation of the clustered through coherent intensity oscillations observed at short times (<10 ps) and shifts of the photoelectron spectra that can be seen out to several hundred picoseconds. The relaxation rates depend on the cluster type and excitation energy: the overall time scale in clusters is relatively independent of both, but in clusters the time scale generally increases with cluster size and decreases with excitation energy. The observed dynamics for and several of the clusters directly probe the time scale for solvent evaporation.

Photodissociation dynamics of azulene
View Description Hide DescriptionPhotodissociation of azulene at 193 nm was studied in a molecular beam using multimass ion imaging techniques. Most of the azulene molecules excited by 193 nm photon quickly relax to the ground electronic state through internal conversion, then isomerize to naphthalene, and eventually dissociate through the H atom elimination channel with a rate of A small amount of azulene entering different isomerization channels was found. The effect of dissociation in the energy transfer experiments using azulene as a vibrationally highly excited molecule and the existence of azulene in an interstellar medium is discussed.

The electronic spectroscopy and molecular structure of the HPCl free radical: A potential III–V semiconductor growth intermediate
View Description Hide DescriptionThe electronic spectra of jetcooled HPCl and DPCl have been obtained for the first time using the pulsed electric discharge technique with a precursor mixture of and or From a combination of laserinduced fluorescence and wavelength resolved emission spectra, all of the vibrational frequencies in the ground and excited states of both isotopomers have been measured and vibrational force fields have been determined. Rotational analyses of the bands of both isotopomers showed small doublet splittings characteristic of an asymmetric top molecule with a single unpaired electron. From the rotational constants and the force fields, estimated equilibrium structures were derived with and and The experimental data firmly establish that the observed spectra and those previously obtained by chemiluminescence techniques [Bramwell et al., Chem. Phys. Lett. 331, 483 (2000)] are due to the HPCl free radical.

A study of the molecular structure and Renner–Teller effect in the electronic spectrum of jetcooled boron disulfide,
View Description Hide DescriptionThe electronic band system of jetcooled boron disulfide has been recorded in the 760–530 nm region by laserinduced fluorescence(LIF) and wavelength resolved emission techniques. The free radical was produced in a pulsed electric discharge jet using a precursor mixture of and in argon. The LIFspectrum consists of a long progression of strong bands involving the (B–S stretching) mode and combinations involving and as well as many hot bands built on the band. Detailed analysis of the spectrum shows that the ground state exhibits a substantial Renner–Teller effect with and a spin–orbit coupling constant of Angular momentum coupling is negligible in the excited state, which has a much smaller spin–orbit coupling constant of The vibrational frequencies of are and Rotational analysis of the band allowed a determination of the effective molecular structures as and Ab initio methods have been used to predict the spectroscopic parameters and good agreement with experiment was found. Our present understanding of the LIFspectrum is in accord with the pioneering analysis of the absorptionspectrum of matrix isolated published by Brom and Weltner [J. Mol. Spectrosc. 45, 82 (1973)] three decades ago.

A scaling rule for molecular electronic transition dipole moments: Application to asymptotically allowed and forbidden transitions
View Description Hide DescriptionGuided by the work of Woerdman and Monyakin, we propose rules that allow the electronic transitiondipole moment for a transition in one molecule to be determined from that of a similar one in an isovalent species. The rule can be applied to asymptotically allowed and forbidden transitions. We have tested it by applying it in two specific cases: the moments for the and transitions in are found from those in which are asymptotically allowed and the moments for the B→X transition in and which are asymptotically forbidden, are found from moment data for Transition moments calculated with this rule are within 15% of the available literature values and behavior as a function of internuclear separation is well described.

Twocolor photoassociation spectroscopy of the lowest triplet potential of
View Description Hide DescriptionWe have performed a type of Autler–Townes spectroscopy to locate a number of rovibrationalhyperfine levels of the potential, the lowest triplet potential of the dimer. The spectroscopy starts with the photoassociation of ultracold atoms in a magnetooptical trap. We have measured the binding energies of over 100 individual states spanning the vibrational levels of this potential (binding energies up to We obtain a typical accuracy of 15 MHz and a typical resolution of 20 MHz, improving on the 10 GHz accuracy and 30 GHz resolution previously available for the vibrational states Vibrational, rotational, and hyperfine structures are resolved. Additionally, we have been able to resolve the magnetic electron–electron spin–spin dipole splitting of a number of these hyperfine levels. The measured rotational and hyperfine structures show good agreement with theoretical calculations. An analysis of the remaining discrepancies indicates where possible refinements to the potentials can be made. We also observe evidence for the presence of secondorder spin–orbit coupling.