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The influence of the nearest neighbor and next-nearest neighbor water molecules on the strength of the hydrogen (H) bonds was examined for the polyhedral clusters of cubic (H2O)8, dodecahedral (H2O)20 and tetrakaidecahedral (H2O)24 cages. The relative stability and the characteristics of the H bond networks are also studied. The charge-transfer (CT) and dispersion interaction terms of every pair of H bonds are evaluated using perturbation theory based on the locally-projected molecular orbitals (LPMO PT). Every water molecule and every H-bonded pair in these polyhedral clusters are classified by the types of the neighbor molecules and H bonds. The relative binding energies among the polyhedral clusters are grouped by these classifications. The optimized OO distances, which are strongly correlated with the calculated pairwise CT terms, are dependent on the 49 sub-groups of the H bonds determined by the type of the neighbor molecules. The electronic origin of this dependence is analyzed using Mulliken's charge-transfer theory, and employing a few assumptions, the analytical formulas for the contribution of the CT terms to the H bond energy are derived.

The anion-water bonds and hydrogen bonds between water molecules in X(-)(H(2)O)(n) (X = F and Cl, n = 3-7) clusters are analyzed by evaluating the charge-transfer (CT) and dispersion terms for every pair of ions and molecules with the perturbation theory based on the locally projected molecular orbitals. In particular, the relative stabilities and the bond strengths in all 11 distinct cubic X(-)(H(2)O)(7) isomers are analyzed by classifying the ligand water (L) with the numbers of the donating (n) and accepting (m) OHs as LD(n)A(m). The number of LD(0)A(2) waters determines the relative stability. It is demonstrated that the strengths of the anion-ligand bonds are strongly influenced by two other hydrogen bonds of the water molecules adjacent to the ligand. When the model theory of Mulliken's charge-transfer interaction is applied to the anion-ligand and water-water hydrogen bonds, the dependence of the bond strengths on the chains of the hydrogen bonds is explained.

The ionic clusters of boron tetrahydride anions (boronhydrides) and aluminum tetrahydride anions (alanate) with Li and Na ions were studied using the ab initio molecular orbital theories. The structures of the clusters of [M(BH4)]n and [M(AlH4)]n (M = Li, Na; n = 1–6) were optimized using the second order Moller–Plesset perturbation theory (MP2) with the aug-cc-pVDZ basis set. The works were extended to the ionic clusters [Mm(XH4)n]+m−n. The stability of these clusters by oxidation (ionization) and by reduction (electron attachment) was also examined. These clusters are found as partial structures of the crystals of M(BH4) and M(AlH4) studied both theoretically and experimentally. Various unique chemical species produced after reduction and oxidation were found as molecular units in the clusters. For instance, Rydberg double anions of (BH4)2− and (AlH4)2− are stable without changing their geometries from the valence-saturated closed shell anions (XH4)−. The molecular units, H2B–H2 and H2Al–H2, are repeatedly found when they are ionized (oxidized) from the parent clusters. The geometric and electronic structures are sometimes drastically deformed from the parents and they might play a role in the decomposition process of the solid materials to produce hydrogen molecules. The anions, [H3XHXH3]− and [H3XH2XH3]−, are also found in the intermediates of the ionization processes.

The perturbation expansion based on the locally-projected molecular orbital (LPMO PT) was applied to the study of the hydrogen-bonded networks of water clusters with up to 16 molecules. Utilizing the local nature of the occupied and excited MOs on each monomer, the charge-transfer and dispersion terms are evaluated for every pair of molecules. The two terms are strongly correlated with each other for the hydrogen-bonded pairs. The strength of the hydrogen bonds in the clusters is further classified by the types of the hydrogen donor and acceptor water molecules. The relative energies evaluated with the LPMO PT among the isomers of (H2O)6, (H2O)11, and (H2O)16 agree very well with those obtained from CCSD(T) calculations with large basis sets. The binding energy of the LPMO PT is approximately free of the basis set superposition errors caused both by the orbital basis inconsistency and by the configuration basis inconsistency.

Paracyclophanes are simple idealized model molecules for the study of interacting π-stacking systems. In this study, the excited states of [2.2]paracyclophane ([2.2]PCP), [3.3]paracyclophane ([3.3]PCP), and siloxane-bridged paracyclophane (SiPCP) are systematically investigated using the multiconfiguration quasi-degenerated perturbation theory (MCQDPT) method. The excited states of the alkyl- and silyl-substituted benzene monomers and benzene dimer, which can be regarded as the building blocks of paracyclophanes, are also examined at the same level of theory for more detailed understanding. The accuracy of the time-dependent density functional theory (TD-DFT) method required for excited state geometry optimization of the paracyclophanes is confirmed from calculations of the benzene dimer. The equilibrium distances between the benzene rings of the paracyclophanes in the first excited states are shorter than those in the ground state, and the benzene rings at the excited state optimized geometries are in an almost eclipsed parallel configuration, which indicates excimer formation. The calculated transition energies and oscillator strengths are generally in good agreement with the corresponding experimental results. A clear correlation between the excited state properties and the molecular structures is systematically demonstrated based on the calculation results for the substituted benzene monomers and benzene dimer. The transition energies of SiPCP are close to the corresponding absorption and fluorescence energies of the experimentally studied phenylene-silica hybrids, which indicates that the electronic properties of organic-silica hybrids, which is a new class of material with potential in photofunctional applications, can be approximated by simple siloxane-bridged cyclophane derivatives.

The aromatic excimers of benzene, naphthalene, anthracene, pyrene, and perylene are systematically investigated using the multiconfiguration quasi-degenerate perturbation theory (MCQDPT) method, which is one of high-level ab initio quantum chemical methods. The reference configuration space for MCQDPT is carefully designed for an appropriate description of the target electronic state with a tractable computational cost. The dimers with eclipsed parallel arrangement are investigated. The basis set dependence of the selected spectroscopic parameters is examined for the benzene and naphthalene dimers, and that of the excimer binding energy is found to be significant. In contrast, the equilibrium intermolecular distance and excimer fluorescence energy are less sensitive to the size of the basis sets used, and they agree with the corresponding experimental values, even with a nonextensive basis set size. The calculated spectroscopic parameters for anthracene, pyrene, and perylene dimers are also in good agreement with the experimental results. The electronic properties of the excimers are discussed in relation to those of the corresponding monomers. The wave functions of the excimers are analyzed in detail to clarify the origin of the attractive nature between the two monomers.

The efficient code to evaluate the molecular interaction energy at the Hartree-Fock level of theory is developed. The method is the third-order single excitation perturbation theory based on the locally projected (LP) molecular orbitals (MO). It is shown that with a proper scaling of the molecular orbital integrals the basis set superposition error can be under controll even with a small size of basis sets. If the basis set contains augmented diffuse functions, the interaction energy, close to the counterpoise corrected energy, can be directly evaluated without the scaling, as fast as a single supermolecule Hartree-Fock calculation. The key of the method is to use the absolutely local occupied and excited MOs. With these MOs, the amount of the charge-transfer (CT) and the energy contribution of the CT terms are evaluated.

Small clusters of group 1 metal complexes with hydrogen fluoride molecules M(HF)n, M = Li, Na, and K, are studied with the ab initio molecular orbital method. The trimer M(HF)3 forms a C3v cluster, in which the metal atom is ionized and the ejected electron is trapped on the top of three equivalent HF molecules. The optimized geometric structure of Li(HF)3 is almost identical with that of the ion pair Li+(HF)3Cl- by replacing a Cl- anion with an ejected electron {e-}; thus Li(HF)3 can be described as Li+(HF)3{e-}. The entity {e-} is trapped under the electrostatic field created by three HF bond dipoles; and at the same time, the HF bonds are polarized and weakened. A triplet anion {e-}(HF)3Li+(HF)3{e-} is stable and is a possible anion unit of electrides.

The anion-water bonds and hydrogen bonds between water molecules in X(-)(H(2)O)(n) (X = F and Cl, n = 3-7) clusters are analyzed by evaluating the charge-transfer (CT) and dispersion terms for every pair of ions and molecules with the perturbation theory based on the locally projected molecular orbitals. In particular, the relative stabilities and the bond strengths in all 11 distinct cubic X(-)(H(2)O)(7) isomers are analyzed by classifying the ligand water (L) with the numbers of the donating (n) and accepting (m) OHs as LD(n)A(m). The number of LD(0)A(2) waters determines the relative stability. It is demonstrated that the strengths of the anion-ligand bonds are strongly influenced by two other hydrogen bonds of the water molecules adjacent to the ligand. When the model theory of Mulliken's charge-transfer interaction is applied to the anion-ligand and water-water hydrogen bonds, the dependence of the bond strengths on the chains of the hydrogen bonds is explained.

The single excitation second order perturbation expansion (SPT) based on the locally projected self-consistent field molecular orbitals for molecular interaction is numerically tested for isomers of uracil–water and for 24 water clusters. For the extensive basis sets, the calculated binding energy BESPT becomes close to the counterpoise corrected binding energy BE SCF CP . The required cpu time is less than that for a single supermolecule calculation. For smaller basis sets, by restricting the excited orbitals to the strictly monomer basis set (SMBS), BESPT becomes approximate to BE SCF CP . The use of the SMBS for the excited orbitals has a clear advantage over the orthogonal but delocalized excited orbitals.

Photoabsorption cross-sections of water dimer and trimer anions are calculated with an ab initio molecular orbital method. Because the electron detachment energy of these small water cluster anions is less than 0.3 eV, all of the photoabsorption spectra in the near-infrared and visible region are due to the bound-free transitions. The theoretical spectra of two isomers of dimer anion and three isomers of trimer anion are compared, which suggest the possibility of the identification of the isomer structure by the photoabsorption spectra. The convergence on the basis sets is also examined by adding diffuse functions.

The geometric structures and stability of Na+Arn clusters were studied with ab initio molecular orbital methods. The clusters of n = 6, 8, and 10 have a high symmetry of Oh, D4d, and D4d, respectively. On the other hand, the clusters of n = 3 and 4 are deformed from the expected high symmetric configuration. It is the attractive force between rare gas atoms that breaks the symmetry. The many-body terms also play an important role in determining the detailed structures. The size dependence of the calculated thermochemical parameters are consistent with the reported mass spectral pattern which shows the first sequence of the magic number at n = 6 and 8.

Approximate theoretical normal and resonant Auger spectra for a series of methylcyano ketones were calculated. Compared with our previous procedure, a set of initial molecular orbitals (MOs) for Auger decay probability calculations of the normal Auger process was modified by changing from a set of ground state MOs to a set of core-holed MOs. For the resonant Auger process, a set of MOs was also modified in the same manner. Furthermore, the bond dissociation factor, which we introduced in the previous article, was also calculated to estimate the bond strength after Auger decay. The site-selectivity for a series of methylcyano ketones was qualitatively explained, but a significant state-specificity was not observed. Molecular size dependence after Auger decay was also discussed.

An approximation method is proposed to simulate the molecular Auger spectra. Auger transition rates are estimated with atomic populations of valence orbitals on an excited atom by Mulliken and Lowdin population analyses. Normal Auger energies and relative rates for H 2 O and NH 3 molecules are evaluated using a full CI wave functions among the valence orbitals constructed from SCF orbitals for the neutral ground state and for the core-hole excited state. Theoretical spectra show fairly good correspondence with the experimental spectra. It is demonstrated that the present approach is applicable for qualitative assignment and analysis of Auger spectra for larger molecules.

Electronic spectra of ammoniated ammonium radicals, NH4(NH3)n, produced through the photolysis of ammonia clusters, are investigated with a laser photodepletion spectroscopy. Vibrationaly resolved bands beginning at energy of 9305 ± 15 cm-1 are successfully assigned to NH4NH3. These bands are ascribed to the 2 2A −1 2A transition derived from the 3p 2F2−3s 2A1 excitation of NH4. A second group of bands beginning at energy 10073 ± 15 cm-1 is assigned to the 12E−12A transition. Electronic spectra of a series of NH4(NH3)n (n = 1−8) are also recorded with low resolution in the energy region of 4500−16000 cm-1. A drastic decrease of the excitation energy from 15062 cm-1 for NH4 to 5800 cm-1 for NH4(NH3)4 is observed, while no appreciable spectral change is found for n ≥ 5. The large spectral change is ascribed to the spontaneous ionization of NH4 in ammonia clusters. Successive binding energies of NH4(NH3)n in the excited state are determined from the spectral band positions.

Accurate potential energy and dipole moment curves of the ground state carbon monoxide were calculated with the multi-reference configuration interaction method. Vibrational and spectroscopic constants were determined and compared with the values derived from experimental data. Einstein's A and B coefficients of the rovibronic transitions of Δv= 1, 2 and 3 were evaluated. The near-infrared absorption spectra in the solar atmosphere observed by a satellite were simulated with the calculated rovibronic transitions, and the temperature of the gas was estimated around 4500 to 5500 K .

Weakly bound reactant and product complexes in the hydrogen abstraction reactions of an OH radical with CH 4 , C 2 H 6 , and C 3 H 8 are investigated with ab initio molecular orbital methods. The calculated binding energy of the CH 4 and OH reactant complex at CCSD(T)/aug-cc-pVTZ (aug-cc-pVDZ) levels is 0.54 (0.74) kcal/ mol. The zero-point vibrational energy correction at MP2 level with the corresponding basis sets reduces the binding energy to 0.16 (0.08) kcal/mol, which is substantially smaller than the recent experimental estimation (about 0.60 kcal/mol). A product complex for the CH 4 + OH system, which is more stable than the reactant complex, has the binding energy of 0.77 (0.79) kcal/mol at CCSD(T)/aug-cc-pVTZ (aug-cc-pVDZ) levels with MP2 zero-point energy corrections. The reactant and product complexes are also found for the C 2 H 6 + OH and C 3 H 8 + OH reactions at the CCSD(T)/aug-cc-pVDZ level of theory. The binding energies tend to increase with the number of carbon atoms of the hydrocarbon. The reaction rates and their temperature dependence are estimated, and they are more than 1 order of magnitude larger than the experimentally reported values.

The hydrogen bonds and their networks in the water clusters (H2O)20 and (H2O)25 are characterized using the charge-transfer (E(W(a),W(d))(CT)) and dispersion (E(W(a),W(d))(Disp)) terms for every pair of water molecules (Wa, Wd) in the clusters. The terms are evaluated by the perturbation theory based on the ab initio locally projected molecular orbitals (LPMO PT) developed by the present author. The relative binding energies among the isomers evaluated by the LPMO PT agree with those of the high level ab initio wave function based theories. A strong correlation between E(W(a),W(d))(CT) and E(W(a),W(d))(Disp) for the hydrogen bonded pairs is found. The pair-wise interaction energies are characterized by the types of hydrogen-donor (Wd) and hydrogen-acceptor (Wa) water molecules. The strongest pair is that of the D2A1 water molecule as a hydrogen-acceptor and the D1A2 water molecule as a hydrogen-donor, where the DnAm water molecule implies that the water molecule has n hydrogen bonding O-H and m accepting HO. The intra-molecular deformation as well as the O···O distance is also dependent on the types of hydrogen bonded pairs. The ring structures in the cluster are classified by the pattern of alignment of the hydrogen bonds. The lengthening of the hydrogen-bonding OH of Wd is strongly correlated with the charge-transfer (E(W(a),W(d))(CT)) energy.

A method to calculate the spectral density of any state vectors with respect to a set of eigenstates of a Hamiltonian is presented. A spectral density operator, whose expectation value on the state vector gives the spectral density, is evaluated indirectly by using the Chebyshev expansion method. A spectral transformation function is introduced to improve resolution at the low energy region, at the expense of the one at the higher region. The predissociation spectrum of CO+ is calculated to demonstrate the method. © 2002 Wiley Periodicals, Inc. J Comput Chem 23: 310–318, 2002

A new time-independent perturbation theory is developed for the multireference problem. In the derivation, neither perturbed wave function nor intermediate normalization condition is required. In the single-reference case, the present approach gives the same perturbation expressions as Rayleigh-Schrodinger perturbation theory. In the multireference case, the perturbation expressions are derived with two kinds of partitions and can be applied to those cases with or without the definition of the zeroth Hamiltonian H 0 . As the size of the reference space is decreased to 1, one multireference expansion returns to the single-reference one with Epstein-Nesbet partition, while another is reduced to a new expansion. These two multireference perturbation expansions are further expressed with the eigenvectors of the Hamiltonian within the reference space for an efficient implementation. The correspondences between the single-reference and multireference perturbation expansion are discussed.

Approximate theoretical normal and resonant Auger spectra for a series of methyl cyanoesters were calculated. To study the reported molecular dependence of the fragmentation patterns after the core excitations, a new measure, bond dissociation factor, was introduced. The site-selectivity and the state-specificity for a series of methyl cyanoesters are qualitatively explained.

The self-consistent field method for molecular interaction (SCF MI) by Gianinetti, Raimondi, and Tornaghi is extended to multi-component systems. A set of equations are written with projection operators, and the accurate approximate equations are derived. The method is applied to water clusters and to a fluoride anion complex with a water dimer. The calculated interaction energies are compared with those estimated with the counterpoise method, and they converge to smaller values for extensive basis sets. The underestimation of the binding energy results from the omission of the most part of charge transfer contribution in the wave function.

Auger electron spectra of PMMA (poly-methylmethacrylate) which show remarkable site-specific ion desorption are calculated using ab initio molecular orbital (MO) calculation and compared with the results of Auger electron-photoion coincidence experiments. Methyl isobutyrate and its trimer are used as model molecules of PMMA. The calculation is based on the single configuration state function and limited configuration interaction methods, and Auger transition probabilities are estimated by overlap between core and valence MOs. The calculated normal Auger spectra obtained on specific atoms reproduce well the experimental resonant Auger spectra for both the carbon and oxygen sites. The careful analysis of the calculated Auger spectra indicates that the desorption of CHn+ ions is strongly concerned with the Auger final states which have holes in the O–CH3 bonding orbitals.

Various isomers of Si4H2 cluster have been investigated with ab initio molecular orbital and density functional theory (DFT) calculations. Nine local minimum isomers on the potential energy surface have been obtained with both Mo/ller–Plesset perturbation theory (MP2) and DFT methods. The Si4 frame is slightly distorted by the dissociative adsorption of H2 on it. The most stable isomer of Si4H2 is a classical structure with both hydrogen atoms bonded to a single silicon atom. The nonclassical H-bridged structures are also found in the calculations, but predicted to be less stable than the nonbridged structures energetically. The formation of the most stable isomer of Si4H2 from Si4 and H2 is proven to be a two-step process and exothermic. The first step is the dissociative adsorption of H2 on Si4 cluster by overcoming an energy barrier of 19.27 kcal/mol, and the second step of conversion from the intermediate to the product will readily proceed with a barrier of only 0.53 kcal/mol.

Ab initio MO calculations were performed for three lowest doublet Π states of CO + using the multireference configuration interaction method. Some spectroscopic constants were evaluated also for other low-lying states. For the X 2 Σ + , A 2 Π, and B 2 Σ + states, the calculated spectroscopic constants are in very good agreement with experimental data within the limit of experimental accuracy. In the photoelectron spectra, a series of the vibrational progression assigned to the D 2 2 Π state persists up to v =9, whereas the calculated adiabatic potential energy holds only three vibrational levels below a barrier at R =1.44 A, which results from an avoided crossing with the upper state, third 2 Π. The observed progression was analyzed in terms of the resonant states obtained from the coupling of two strongly interacting diabatic states, second and third 2 Π.

Ab initio CIS/6-31G** and CIS/6-3111G** calculations have been carried out in the Astate of tropolone for the stable and saddle point structures. The calculated energies and geometries have been compared with the previous results of Vener et al. The energy barrier for the proton tunneling amounts to 12.0 kcal/mol from the CIS/6-31G** calculations, and 13.0 kcal/mol from the CIS/6-3111G** calculations. The normal mode frequencies have been computed including modes coupled to the proton tunneling mode. Two-dimensional and three-dimensional model potentials, formed from symmetric mode coupling potential and squeezed double well potential, have been fitted to the calculated energy barrier, geometries, and frequencies, and used to analyze proton dynamics. The calculated energy splittings for different vibrationally excited states have been compared with the available experimental data. The CIS/6-3111G** calculation gives good estimation of the tunneling energy splitting in the vibrationally ground state of tropolone. The model PES explain monotonic decrease in tunneling splittings with the excitation of the out-of-plane modes, however underestimate the actual splittings. The increase of the tunneling splitting with the excitation of the v 13 and v 14 modes is qualitatively correct. © 2000 American Institute of Physics. @S0021-9606~00!31113-8#

The harmonic vibrational frequencies for M(H2O)n (M=Li and Na) are calculated with ab initio molecular orbital (MO) methods. Three types of isomers, surface, quasivalence and semi-internal, exhibit the characteristic frequency shifts ΔνOH. The calculated spectral patterns are related to the geometric conformations around the localized electron {e} in the cluster. The downward shifts of OH frequencies are strongly correlated with the lengthening of OH bond distances, as is well known in the hydrogen bonds. The magnitude of the downward shift is as large as that for the hydrogen bond. The correlation of the shifts with the distance between the center of the electron {e} and the hydrogen atom reveals two types of interaction schemes. The characteristics of the structural unit OH{e}HO in M(H2O)n and its OH modes are common with those found in water anion clusters. The interaction between the localized electron and OH bonds can be called an electron-hydrogen bond.

Accurate calculations were performed for several doublet and quartet states of CO+ using the multireference configuration interaction method. With a single set of molecular orbitals the accurate transition dipole moments were evaluated. The calculated lifetimes of the vibronic states of the A 2Π and B 2Σ+ states were in excellent agreement with the available experimental data. The calculated vibrational spacings and rotational constants up to v=40 of the X 2Σ+ state were compared with the latest experimental data, and quantitative agreement was attained.

In addition to isomers having a dipole-bound electron, the internally bound electron isomers of trimer, tetramer and hexamer water anions are found using an ab initio molecular orbital method. The latter isomers have a characteristic OH{e}HO structure. The interaction between the excess electron {e} and the surrounding OH bonds holds the structure stable. The calculated vibrational infrared spectrum for a hexamer anion with two double proton-acceptor water molecules shows a qualitatively similar vibrational spectrum with the one observed. A strong correlation between the vertical detachment energy and the distribution of the excess electron is also found.

Ab initio MO calculations have been performed for the [(CO2)nROH]− (R=H and CH3) anions with n=1 and 2. Three stable structures are found for [(CO2)H2O]−, and two structures for [(CO2)CH3OH]−. All the [(CO2)ROH]− structures are characterized by the charge localization on the CO2 moiety, which interacts with ROH through an O–H⋯O linkage. It is also revealed that the addition of ROH to CO2− leads to the formation of a potential barrier against autodetachment higher than that of a bare CO2−, which results in the increasing stability of [(CO2)ROH]− species. For n=2 the calculations predict the existence of two types of isomers having different degrees of the excess electron localization: CO2−⋅ROH(CO2) and C2O4−⋅ROH isomers. These “electronic isomers” are calculated to be close in energy, while their calculated vertical detachment energies (VDEs) differ by more than 1 eV. The ab initio results are discussed in comparison with recent experimental ones derived from photoelectron spectra of [(CO2)nROH]−.

Energetics of the SiF2++ dication and fragments related to it, SiF++ and SiF+, was calculated using different ab initio approaches, including the semiempirically corrected G2, the complete basis set method, and the coupled cluster method up to the CCSD(T)/aug-cc-pVQZ level. The calculated values of bond energies and ionization potentials were carefully compared with the available experimental data to assess the accuracy of these approaches. In addition, reaction enthalpies for possible fragmentation reactions were calculated. The heat of formation of the SiF2++ dication, resulting from these calculations, is ΔHf(SiF2++) = 546 ± 2 kcal/mol and a theoretical estimate of ΔHf(SiF++) is 649 ± 2 kcal/mol.

Accurate and predictive values of the bond lengths and angles, dipole moments, and vibrational frequencies of , BCl2(X2A1), and , a3B1) and the corresponding aluminum analogs are calculated by Bec...

Experimental photoelectron spectrum of Si2C2- was assigned by ab initio Monte Carlo simulation with the multicanonical/histogram reweighting technique. At first, two structures, linear and ring, we...

Water cluster complexes with a group 1 metal atom, M(H2O)n (M = Li and Na), for n = 3−6 were studied with ab initio MO methods. The singly occupied molecular orbitals (SOMOs) are classified into th...

The x-ray incoherent scattering factor S(q), which is also called the static structure factor, is very sensitive to electron correlation. In this study a method for calculating S(q) based on coupled cluster singles and doubles (CCSD) approach is developed and the computed S(q) of H2O, CH3OH, CH3CN, C6H6, and C6H12 are compared with experimental results. It is shown that the CCSD method improves theoretical S(q) of large molecules significantly compared with those by configuration interaction singles and doubles (CISD) previously employed.

Employing separate cluster ansatz in time-independent and time-dependent wave-operators, coupled-cluster (CC) response theory is generalized to multireference (MR) expansion spaces. For state energies, this corresponds to the MR secular problem with an arbitrary similarity-transformed effective Hamiltonian, H˜=Ω−1HΩ. The effective Hamiltonian can be generated via size-extensive CC methods. Thus the states in MR linear response theory (MRLRT) maintain the usual CC core-extensive properties. We have used the Gelfand unitary group basis of the spin-adapted configurations to construct the matrix of H˜ in the MR excitation space. As a preliminary application, the CC singles and doubles effective Hamiltonian is applied to excitation and photoionization energies of the CH+ and N2 molecules, and is compared with experimental results and results from other numerical procedures including conventional CC linear response theory (CC-LRT), MR and full configuration interaction (MRCI and FCI) methods. The numerical results indicate that MRLRT reproduces valence and external excited states quantitatively, combining the best features of CC-LRT and MRCI.

The stationary points along the dissociation path are investigated by means of high-level ab initio methods and the reliability of different methods is discussed. The multi-reference AQCC method using the ANO-type basis set is shown to give geometrical parameters and relative energies in very good agreement with experiment. At this level of theory, the C 1 B 2 ( 1 A ′) state has an asymmetrical equilibrium geometry and double-minimum potential with a barrier of 170 cm−1, in good agreement with experimental data.

The potential energy surfaces of internal rotations of two methyl groups were calculated with the ab initio MO method for m-xylene (1,3-dimethylbenzene) in the ground (S0), first excited (S1), and cation ground (D0) states. The internal rotation levels and their wavefunctions on the theoretically calculated potential energy surfaces were obtained by solving Schrodinger's equation for double methyl rotors. The Franck–Condon factors for D0←S1 and S1←S0 transitions were calculated and compared with the experimental spectra. A good agreement was obtained not only in level spacings but also in relative intensities in the spectra. The interaction between two methyl rotors was also found to be small, and the rotational levels can be labeled by the irreducible representation of the direct product G6×G6 group.

A multicanonical Monte Carlo simulation has been performed to investigate average structural changes in the water dimer with temperature. Potential energy was calculated by the ab initio MO method at the RHF/6-31G∗ level at each step of the simulation. By combining the reweighting technique with the multicanonical algorithm various thermodynamic properties have been evaluated over a wide range of temperature from a single production run. The technique is an ideal tool for studying temperature dependence properties, and the temperature dependence of the distribution functions of various geometric parameters has been examined in detail. It was found that the probability of finding structures similar to that of the transition state increases as the temperature is increased.

The equilibrium structure, potential energy surface and van der Waals (vdW) mode vibration of (N2)2 have been studied with high levels of ab initio calculations. The most stable structure is found to be a 45 deg. canted parallel structure of C2h. On the other hand, neither T-shape of C2v nor cross-shape of D2d is a stable structure, but they are transition state structures, which are contrary to the previous calculations. The out-of-plane motion changing from the 45 deg. canted parallel structure of C2h to the cross-shape of D2d has a higher barrier than the in-plane motion. The binding energy of two N2 monomers is about 80 cm−1 and the fundamental frequency of vdW stretching mode is estimated at 22 cm−1. A small energy difference of 5 cm−1 between the C2h and C2v structures implies that two N2 molecules move coherently like a gear rotation in the plane. This internal freedom of motion should make the rotational energy states extremely modulated, and therefore, a very complex spectrum pattern could be exp...

Adsorption of atomic deuterium on ultrathin Ge film deposited over Si(100) has been studied using Fourier-transformed infrared reflection absorption spectroscopy and reflection high-energy electron diffraction (RHEED) measurements. We established that high atomic deuterium exposure of a Ge/Si(100) surface leads to a change in the RHEED pattern from $2\ifmmode\times\else\texttimes\fi{}1$ to $1\ifmmode\times\else\texttimes\fi{}1.$ This change possibly is due to an overwhelming conversion of Ge monodeuterides to dideuterides. Etching of the Ge atoms by deuterium atoms has also clearly been observed.

Structural parameters, binding energies, and frequencies of the infrared- and Raman-active vibrations are calculated for an infinite zigzag chain of hydrogen fluoride molecules by ab initio crystal orbital theory with the analytical energy gradient scheme. The Becke-Lee-Yang-Parr (BLYP), Becke3-Lee-Yang-Parr (B3LYP), and Hartree-Fock (RHF) levels are used in conjunction with the 6-311++G(d,p) basis set. Molecular orbital calculations at the BLYP, B3LYP, RHF, and the second-order Moller-Plesset perturbation (MP2) levels with the same basis set are carried out on linear HF oligomers containing up to six molecules, to examine the chain-length dependence of the energetic and structural properties. The predicted chainlength dependence is found to be significantly smaller in the RHF results than in the BLYP and B3LYP results. The RHF level substantially underestimates the downward frequency shifts in the intramolecular H-F stretching modes on going from the monomer to the polymer, while the shifts calculated at the BLYP and B3LYP levels are much closer to the experimental findings, although they are slightly overestimated. The RHF level strongly underestimates the intramolecular H-F bond length and overestimates the intermolecular F‚‚‚H and F‚‚‚F distances of the HF polymer, while the structural parameters predicted at the BLYP and B3LYP levels are in good agreement with the experimental results. It is concluded that the RHF level seriously underestimates the cooperative binding effects of consecutive hydrogen bonds, whereas the BLYP and B3LYP levels slightly overestimate this behavior; but these latter levels provide much better description than the former. Vibrational assignment of librational modes of HF crystals is reexamined on the basis of the calculated frequencies. The observed frequencies of the librational and pseudo-translational modes fall between the corresponding frequencies calculated at the RHF and density functional levels.

The spin-restricted formulas for the analytical gradients of the second-order Mo/ller–Plesset perturbation (MP2) energy are presented within the framework of ab initio crystal orbital theory of infinite one-dimensional lattices (polymers). The coupled perturbed Hartree–Fock equation for polymers is solved iteratively using the atomic-orbital-based algorithms. The MP2 energy and its gradient contributions are evaluated by the disk-based algorithms with the aid of the two-particle density matrix. The analytical-gradient method at the MP2 level, as well as the analytical first- and second-derivative methods at the Hartree–Fock (HF) level, is applied to calculate the equilibrium structures and harmonic vibrational frequencies of all-trans polyacetylene. The deviations of the calculated frequencies from the observed ones for the in-phase C=C stretching modes are reduced by about 70% on going from HF/6-31G to MP2/6-31G theory.

In the framework of ab initio Hartree–Fock crystal orbital theory of polymers, the formulas for the analytical second derivatives of energy with respect to in-phase ( k =0) nuclear coordinates are derived. The coupled perturbed Hartree–Fock (CPHF) equation is iteratively solved by using the direct (recomputation of two-electron integrals) atomic-orbital-based algorithm. Frequencies of the Brillouin zone center ( k =0) vibrations of all- trans polyethylene are calculated by using the STO-3G, 3-21G and 6-31G* basis sets. The dependence of the frequencies on the number of neighbors included in the lattice summations, on the number of momentum sampling points in the first Brillouin zone, and on the convergence criterion for the CPHF solutions is examined. In our implementation, the use of analytical second derivatives is more efficient than the use of the finite differences of analytical first derivatives.