Effects of intermolecular hydrogen bonding between glycine and one water molecule on the vibrational spectrum are investigated, using ab initio (at the level of second order Moller-Plesset perturbation theory), empirical (OPLS-AA), and mixed ab initio/empirical quantum mechanics/molecular mechanics (QM/MM) potentials. Vibrational spectroscopy is calculated using the correlation corrected vibrational self-consistent field method that accounts for anharmonicities and couplings between different vibrational normal modes. The intermolecular hydrogen bonding interactions are found to be very strong and to affect vibrational frequencies and infrared intensities of both the glycine and the water molecule to a very large extent. The predicted ab initio anharmonic spectra can be used to identify amino acids in complexes with water in experimental studies. The OPLS-AA potential is found to describe hydrogen bonding between glycine and water incorrectly, and to predict erroneous vibrational spectra. Hybrid (QM/MM) techniques can, however, be used to calculate more reliable vibrational spectra, in agreement with full ab initio treatment of the whole system, provided that the regions that contain hydrogen bonds are described by ab initio potentials. (C) 2001 American Institute of Physics.

A new method for the treatment of correlation effects between modes in vibrational self-consistent-field (VSCF) calculations is introduced. It is based upon using a partially separable form for the wave function. As a result, some of the modes are treated as mutually fully correlated, while the rest are separable. The modes which are explicitly coupled together in the calculation are chosen on physical grounds. Trial calculations are performed upon H2O, H3O+, and CH3NH2 and indicate that the method performs well. The agreement with experiment for the explicitly coupled modes is improved when compared to both the vibrational self-consistent-field method and its correlation-corrected extension. When interfaced with an electronic structure code this method opens the way for the accurate first-principles prediction of vibrational frequencies of strongly coupled modes. If only a few modes are mutually strongly coupled, the method has a very favorable scaling with system size, as does VSCF itself. (C) 2001 American Institute of Physics.

Recent work by Rasanen and coworkers showed that photolysis of hydrides in rare-gas matrices results in part in formation of novel, rare-gas-containing molecules. Thus, photolysis of HCl in Xe and of H2O in Xe result respectively in formation of HXeCl and HXeOH in the Xe matrices. Ab initio calculations show that the compounds HRgY so formed are stable in isolation, and that by the strength and nature of the bonding these are molecules, very different from the corresponding weakly bound clusters Rg . . . HY. This paper presents a study of the formation mechanism of HRgY following the photolysis of HY in clusters Rg(n)(HY). Calculations are described for HXeCl, as a representative example. Potential energy surfaces that govern the formation of HXeCl in the photolysis of HCl in xenon clusters are obtained, and the dynamics on these surfaces is analyzed, partly with insight from trajectories of molecular dynamics simulations. The potential surfaces are obtained by a new variant of the DIM (diatomics in molecules) and DIIS (diatomics in ionic systems) models. Non-adiabatic couplings are also obtained. The main results are : (1) Properties of HXeCl predicted by the DIM-DIIS model are in reasonable accord with results of ab initio calculations. (2) The potential along the isomerization path HXeCl –> Xe . . . HCl predicted by DIM is in semiquantitative accord with the ab initio results. (3) Surface-hopping molecular dynamics simulations of the process in clusters, with ‘‘on the fly’’ calculations of the DIM-DIIS potentials and non-adiabatic couplings are computationally feasible. (4) Formation of HXeCl, following photolysis of HCl in Xe-54(HCl), requires cage-exit of the H atom as a precondition. The H atom and the Cl can then attack the same Xe atom on opposite sides, leading to charge transfer and production of the ionic HXeCl. (5) Non-adiabatic processes play an important role, both in the reagent configurations, and at the charge-transfer stage. The results open the way to predictions of the formation of new HRgY species.

HHeF, a first predicted chemically-bound helium compound, is a metastable species that disintegrates by tunneling through energy barriers into He+HF and H+He+F. The reaction paths for these decomposition processes are calculated with single-configurational Moller-Plesset (MP2) and multiconfigurational quasidegenerate MCQDPT2/MCSCF(10,6) electronic structure methods. The lifetime of HHeF, estimated using a one-dimensional model along the minimum energy path and the semiclassical WKB approximation, is more than 120 ps, that of DHeF is 14 ns. The relatively long lifetimes are encouraging for the preparation prospects of this helium compound. (C) 2001 American Institute of Physics.

The electronic excitation induced by ultrashort laser pulses and the subsequent photodissociation dynamics of molecular fluorine in an argon matrix are studied. The interactions of photofragments and host atoms are modeled using a diatomics-in-molecule Hamiltonian. Two types of methods are compared: (1) quantum-classical simulations where the nuclei are treated classically, with surface-hopping algorithms to describe either radiative or nonradiative transitions between different electronic states, and (2) fully quantum-mechanical simulations, but for a model system of reduced dimensionality, in which the two most essential degrees of freedom are considered. Some of the main results follow: (1) The sequential energy transfer events from the photoexcited Fz into the lattice modes are such that the ‘‘reduced dimensionality’’ model is valid for the first 200 fs. This, in turn, allows us to use the quantum results to investigate the details of the excitation process with short laser pulses. Thus, it also serves as a reference for the quantum-classical ‘‘surface hopping’’ model of the excitation process. Moreover, it supports the validity of a laser pulse control strategy developed on the basis of the ‘‘reduced dimensionality’’ model. (2) In both the quantum and quantum-classical simulations, the separation of the F atoms following photodissociation does not exceed 20 bohr. The cage exit mechanisms appear qualitatively similar in the two sets of simulations, but quantum effects an quantitatively important. (3) Nonlinear effects are important in determining the photoexcitation yield. In summary, this paper demonstrates that quantum-classical simulations combined with reduced dimensionality quantum calculations can be a powerful approach to the analysis and control of the dynamics of complex systems.

The vibrational self-consistent field method is used to analyze the inhomogeneous spectral distribution of transitions caused by vacancies and thermally populated phonons, specializing to molecular iodine isolated in an Ar matrix. At experimentally relevant temperatures, for a vacancy concentration of 1.4%, both defect-induced and phonon-induced spectral shifts contribute to the spectral distribution. Both contributions scale linearly with vibrational overtone number. The predicted widths are consistent with reported resonant Raman spectra. In time-resolved coherent anti-Stokes Raman scattering (TRCARS) measurements, spectral indistinguishability implies that all members of the inhomogeneous ensemble contribute coherently to the detectable homodyne signal. The connection between spectral distribution and the observable in TRCARS is derived. The predicted polarization beats and free induction decay due to the inhomogeneous ensemble are in qualitative agreement with experiments. (C) 2001 American Institute of Physics.

Anharmonic vibrational frequencies and intensities are calculated for 1:1 and 2:2 (HCl)(n)(NH(3))(n) and (HCl)(n-)(H(2)O)(n) complexes, employing the correlation-corrected vibrational self-consistent field method with ab initio potential surfaces at the MP2/TZP computational level. In this method, the anharmonic coupling between all vibrational modes is included, which is found to be important for the systems studied. For the 4:4 (HCL)(n)- (H(2)O)(n) complex, the vibrational spectra are calculated at the harmonic level, and anharmonic effects are estimated. Just as the (HCl)(n)(NH(3))(n) Structure switches from hydrogen-bonded to ionic for n = 2, the (HCl)(n)-(H(2)O)(n) switches to ionic structure for n = 4. For (HCl)(2)(H(2)O)2, the lowest energy structure corresponds to the hydrogen-bonded form. However, configurations of the ionic form are separated from this minimum by a barrier of less than an O-H stretching quantum. This suggests the possibility of experiments on ionization dynamics using infrared excitation of the hydrogen-bonded form. The strong cooperative effects on the hydrogen bonding, and concomitant transition to ionic bonding, makes an accurate estimate of the large anharmonicity crucial for understanding the infrared spectra of these systems. The anharmonicity is typically of the order of several hundred wavenumbers for the proton stretching motions involved in hydrogen or ionic bonding, and can also be quite large for the intramolecular modes. In addition, the large cooperative effects in the 2:2 and higher order (HCl)(n)(H(2)O)(n) Complexes may have interesting implications for solvation of hydrogen halides at ice surfaces.

An extension of the vibrational self-consistent field (VSCF) method is developed for quantitative calculations of molecular vibrational spectroscopy in a crystalline solid environment. The approach is applicable to fields such as matrix-isolation spectroscopy and spectroscopy of molecular crystals. Advantages of the method are that extended solid vibrations and their coupling to intramolecular modes are incorporated, and that the treatment includes anharmonic effects, both due to the intrinsic property of individual modes and due to coupling between modes. Suitable boundary conditions are adopted in treating the solid environment. In applications, e.g., molecules in rare-gas crystals, hundreds of coupled molecular and matrix modes can be handled computationally. The method is applied to the vibrational matrix-shift of iodine in an argon matrix, and the calculated overtone frequencies are compared to experimental values obtained from both time-domain coherent Raman and frequency-domain Resonance Raman measurements. The physical origin of the shifts is interpreted in detail, and the properties of the iodine-argon interactions essential to obtain the correct sign and magnitude of the shift are elucidated. An I-2-Ar potential, based on anisotropic atom-atom interactions and fitted to ab initio calculations, gives the best agreement with experiment. The results show that the VSCF solid-state approach is a powerful tool for matrix spectroscopy. (C) 2001 American Institute of Physics.

Helium atoms are scattered from a beam of water clusters with mean size (n) over bar = 10 in an angular and velocity resolved collision experiment. The measured peaks are identified as elastic scattering, rotationally inelastic scattering of monomers, and vibrational excitation of the clusters. To interpret the latter processes quantum calculations are performed for He+(H2O)(11) collisions using the TDSCF approximation which includes the anharmonic force field of the water clusters and energy transfer between the modes. By comparison of the calculated and experimental results, the most probable excitations correspond to energy transfer for around 7 meV and, with smaller intensities, up to 20 meV. The excitations correspond to shearing modes of the outer rings and the middle ring of the highly nonrigid cluster against each other. (C) 1999 American Institute of Physics. [S0021-9606(99)01546-9].

An approximate quantum mechanical method is proposed for the calculation of inelastic scattering of an atom from a large anharmonic cluster or molecule. The method is based on: (a) computing the vibrational states of the cluster (or molecule) in the vibrational self-consistent field approximation; (b) treating the scattering of the atom to a first approximation as taking place from a vibrationally frozen cluster; (c) obtaining inelastic transitions by a distorted wave approximation, where the coupling is the vibrationally dependent part of the atom/cluster potential. Computationally convenient expressions are worked out. The method is applied to He scattering from Ar-13 and the results are compared to experimental data for size-dispersed clusters. Good qualitative agreement is found. The merits of the proposed method compared with alternative approaches are discussed. (C) 1998 American Institute of Physics. [S0021-9606(98)00215-3].

An approach based on the Time-Dependent Self-Consistent Field (TDSCF) is used to carry out quantum calculations of inelastic atom scattering from large, highly anharmonic clusters. The computation is carried out for low-energy collisions of Ar with (H2O)(11), and all the vibrational modes of the cluster are included. The method treats the collider atom classically, but the dynamics of the interacting anharmonic modes of (H2O)(11) is handled quantum mechanically. The results provide insight into the collision physics of large systems having soft anharmonic modes, and into the role of quantum effects in such cases. The main findings are the following: (a) Large differences are found between quantum and classical results with regard to energy transfer into specific cluster modes. (b) Classical calculations wrongly predict efficient excitation of many stiff modes, including processes that are quantum-mechanically forbidden. (c) Single quantum excitations are the most important transitions at the collision energy used. (d) Atom-atom pair distribution functions of (H2O)(11) after the collision show insignificant differences from the corresponding precollision distribution functions. The results show that quantum calculations of collision dynamics of low-temperature anharmonic clusters are feasible, and also necessary in view of the prediction of significant quantum effects. (C) 1998 American Institute of Physics.

The structural and dynamical properties of a saturated chemisorbed hydrogen monolayer on a model bcc(110) metal surface were studied by molecular dynamics simulations over a range of temperatures from 100 to 300 K. The potential function used, including both the hydrogen/metal interaction and the interactions between the hydrogens, was calibrated in part from experimental data. At low temperatures, the monolayer has an ordered, broken symmetry state with regard to the underlying metal surface. As temperature is increased, an order-disorder transition takes place. We report studies of static and dynamical structure factors; of pertinent order parameters and, where applicable, of phonon dispersion in order to gain insight into the phases. The disordered phase exhibits anisotropy with uniaxial short-range order. We comment on the relation of the results to recent experimental studies of H/W(110) and H/Mo(110), and suggest future experiments to explore the high temperature phase. (C) 1998 Published by Elsevier Science B.V. All rights reserved.

An approach based on He scattering is used to develop an atomic-level structural model for an epitaxially grown disordered submonolayer of Ag on Pt(111) at 38 K. Quantum scattering calculations are used to fit structural models to the measured angular intensity distribution of He atoms scattered from this system. The structure obtained corresponds to narrowly size-dispersed compact clusters with a modest translational disorder, and not to fractals, which might be expected due to the low surface temperature. The clusters are up to two layers in height, the lower one having only a few defects. The relations between specific features of the angular scattering distribution, and properties such as the cluster sizes and shapes, the inter-cluster distance distribution, etc, are discussed. The results demonstrate the usefulness of He scattering as a tool for unraveling new complex surface phases. (C) 1998 Elsevier Science B.V. All rights reserved.