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2,4,6-trichloroanisole and 2,4,6-tribromoanisole were investigated by means of electron transmission spectroscopy and two different types of dissociative electron attachment spectrometers. The results obtained were interpreted with the support of density functional theory calculations. The dominant dissociative decay channels of the temporary molecular negative ions lead to the formation of Cl- and Br-in the low electron energy region. Formation of long-lived parent anions is observed at thermal electron energies. Their relative intensity depends on the experimental time window, ∼36 μs in the case of the static magnet mass analyzer and ∼200 μs for the quadrupole mass analyzer employed. The results obtained may be useful for rapid detection of these compounds in wine and pharmaceutical industries, as well as other branches connected to the food industry, e.g., packaging. [ABSTRACT FROM AUTHOR]

Correlated ab initio methods (CASPT2 and CCSD(T)) in conjunction with the ANO-RCC basis sets were used to calculate potential energy curves (PECs) of the ground, valence, and Rydberg electronic states of CsH with the inclusion of the scalar relativistic effects. The spectroscopic constants of bound states were calculated from the PECs and compared with previous theoretical and/or available experimental data. Absorption and emission spectra arising from the transition between X¹Σ+ and A¹Σ+ states were modelled using vibrational and rotational energy levels and corresponding nuclear wave functions obtained via the direct numerical integration of one-dimensional rovibrational Schrödinger equation in the CASPT2/ANO-RCC electronic potentials. The anharmonic shape of the A¹Σ+ potential and the shape of the pertinent vibrational wave functions have an interesting impact on the final shape of the spectrum and result in the complicated fine structure of individual emission bands. [ABSTRACT FROM AUTHOR]

We present results of ab initio molecular dynamics study of the structural transformation occurring in hot liquid sulfur under high pressure, which corresponds to the recently observed chain-breakage phenomenon and to the electronic transition reported earlier. The transformation is temperatureinduced and separates two distinct polymeric forms of liquid sulfur: high-temperature form composed of short chain-like fragments with open endings and low-temperature form with very long chains. We offer a structural description of the two liquid forms in terms of chain lengths, cross-linking, and chain geometry and investigate several physical properties. We conclude that the transformation is accompanied by changes in energy (but not density) as well as in diffusion coefficient and electronic properties--semiconductor-metal transition. We also describe the analogy of the investigated process to similar phenomena that take place in two other chalcogens selenium and tellurium. Finally, we remark that the behavior of heated liquid sulfur at ambient pressure might indicate a possible existence of a critical point in the low-pressure region of sulfur phase diagram. [ABSTRACT FROM AUTHOR]

Systematically resolving different crystalline phases starting from the atomic positions, a mandatory step in algorithms for the prediction of structures or for the simulation of phase transitions, can be a non-trivial task. Extending to amorphous phases and liquids which lack the discrete symmetries, the problem becomes even more difficult, involving subtle topological differences at medium range that, however, are crucial to the physico-chemical and spectroscopic properties of the corresponding materials. Typically, system-tailored order parameters are devised, like global or local symmetry indicators, ring populations, etc. We show that a recently introduced metric provides a simple and general solution to this intricate problem. In particular, we demonstrate that a map can be traced displaying distances among water phases, including crystalline as well as amorphous states and the liquid, consistently with experimental knowledge in terms of phase diagram, structural features, and preparation routes. [ABSTRACT FROM AUTHOR]

The transformation pathway in high-pressure solid nitrogen from N2 molecular state to polymeric cg-N phase was investigated by means of ab initio molecular dynamics and metadynamics simulations. In our study, we observed a transformation mechanism starting from molecular Immm phase that initiated with formation of trans-cis chains. These chains further connected within layers and formed a chain-planar state, which we describe as a mixture of two crystalline structures--trans-cis chain phase and planar phase, both with Pnma symmetry. This mixed state appeared in molecular dynamics performed at 120 GPa and 1500 K and in the metadynamics run at 110 GPa and 1500 K, where the chains continued to reorganize further and eventually formed cg-N. During separate simulations, we also found two new phases--molecular P21/c and two-three-coordinated chain-like Cm. The transformation mechanism heading towards cg-N can be characterized as a progressive polymerization process passing through several intermediate states of variously connected trans-cis chains. In the final stage of the transformation chains in the layered form rearrange collectively and develop new intraplanar as well as interplanar bonds leading to the geometry of cg-N. Chains with alternating trans and cis conformation were found to be the key entity--structural pattern governing the dynamics of the simulated molecular-polymeric transformation in compressed nitrogen. [ABSTRACT FROM AUTHOR]

We explore dipole polarizabilities of the singly and doubly charged anions F-, Cl-, O2-, and S2- in an external, harmonic oscillator (HO) confining potential .... We find that in contrast to F- and Cl- those for O2- and S2- are unrealistically high due to the instability of the corresponding restricted Hartree-Fock (RHF) solutions. Yet, already a relatively weak HO confining potential stabilizes their RHF solutions and eliminates any possible broken-symmetry solutions. The coupledcluster theory with single, double and noniterative triple excitations (CCSD(T)) then yields considerably reduced polarizabilities for O2- and S2- relative to their unconfined values. We showed that polarizabilities of O2- and S2- are more sensitive to the strength of a confinement potential than are those for F- and Cl-. This enables us to relate the confining parameter m with the known experimental polarizabilities for selected crystals (our "training set") and to find a specific confining parameter m for which the CCSD(T) polarizability equals the experimental in-crystal polarizability of an anion in the training set. The latter may then be used as an alternative approach for determining the incrystal polarizabilities of anions by exploiting the fact that the characteristic m values depend linearly on the ionic radius of a cation participating in specific crystals containing these anions. Using this method we then calculate the isotropic dipole polarizabilities for F-, Cl-, O2-, and S2- embedded in the LiF, LiCl, NaF, NaCl, KF, KCl, ZnO, ZnS, MgO, MgS, CaO, CaS, SrO, SrS, BaO, BaS, and other crystals containing halogen, oxygen, or sulphur anions. We compare our results with those obtained via alternative models of the in-crystal anionic polarizabilities. [ABSTRACT FROM AUTHOR]

Recently we have demonstrated that the applicability of the Tkatchenko-Scheffler (TS) method for calculating dispersion corrections to density-functional theory can be extended to ionic systems if the Hirshfeld method for estimating effective volumes and charges of atoms in molecules or solids (AIM's) is replaced by its iterative variant [T. Bucko, S. Lebègue, J. Hafner, and J. Ángyán, J. Chem. Theory Comput. 9, 4293 (2013)]. The standard Hirshfeld method uses neutral atoms as a reference, whereas in the iterative Hirshfeld (HI) scheme the fractionally charged atomic reference states are determined self-consistently. We show that the HI method predicts more realistic AIM charges and that the TS/HI approach leads to polarizabilities and C6 dispersion coefficients in ionic or partially ionic systems which are, as expected, larger for anions than for cations (in contrast to the conventional TS method). For crystalline materials, the new algorithm predicts polarizabilities per unit cell in better agreement with the values derived from the Clausius-Mosotti equation. The applicability of the TS/HI method has been tested for a wide variety of molecular and solid-state systems. It is demonstrated that for systems dominated by covalent interactions and/or dispersion forces the TS/HI method leads to the same results as the conventional TS approach. The difference between the TS/HI and TS approaches increases with increasing ionicity. A detailed comparison is presented for isoelectronic series of octet compounds, layered crystals, complex intermetallic compounds, and hydrides, and for crystals built of molecules or containing molecular anions. It is demonstrated that only the TS/HI method leads to accurate results for systems where both electrostatic and dispersion interactions are important, as illustrated for Li-intercalated graphite and for molecular adsorption on the surfaces in ionic solids and in the cavities of zeolites. [ABSTRACT FROM AUTHOR]

The structures and chemical bonding of TaBn- (n = 3-8) clusters are investigated systematically to elucidate the formation of the planar metal-centred aromatic borometallic cluster, Ta©B10- (the © sign is used to designate the central position of the doped atom in monocyclic structures in M©Bn-type planar clusters), which was found previously to have the highest coordination number for a metal atom in a planar geometry. Photoelectron spectroscopy is combined with ab initio calculations to determine the global minima of the TaBn- clusters. We find that from TaB3- to TaB5- the boron atoms nucleate around the central Ta atom to form fan-like structures. A structural transition occurs at TaB6-, which is found to have a hexagonal structure, but with a boron atom in the centre and the Ta atom on the periphery. TaB7- is shown to have a three-dimensional boat-like structure, which can be viewed as a Ta atom coordinated to an elongated B7 cluster from above. The global minimum of the TaB8- cluster is found to be pyramidal with the Ta atom interacting with a B8 monocyclic ring. Starting from this structure, additional boron atoms simply enlarge the boron ring to form the slightly pyramidal TaB9- cluster and eventually the perfectly planar Ta-centred B10-ring aromatic cluster, Ta©B10-. It is shown that boron atoms do not nucleate smoothly around a Ta atom on the way to the decacoordinated Ta©B10- molecular wheel, but rather the competition between B-B interactions and Ta-B interactions determines the most stable structures of the smaller TaBn- (n = 3-8) clusters. [ABSTRACT FROM AUTHOR]

Beta-diketones are a versatile class of compounds that can complex almost any metal in the periodic table of elements. Their metal complexes are found to be fairly stable and generally have sufficient vapor pressure for deposition techniques requiring volatile metal sources. Motivated by the potential role of low energy electrons in focused electron beam induced deposition, we have carried out a crossed electron/molecular beam study on the dissociative electron attachment and non-dissociative electron attachment (NDEA) to hexafluoroacetylacetone (HFAc) and its bidentate metal complexes: bis-hexafluoroacetylacetonate copper(II), Cu(hfac)2 and bis-hexafluoroacetylacetonate palladium(II), Pd(hfac)2. The relative ion yield curves for the native precursor to the ligand as well as its stable, 16 valence electron Pd(II) complex and open shell, 17 valence electron Cu(II) complex, are presented and compared. For HFAc, the loss of HF leads to the dominant anion observed, and while NDEA is only weakly pronounced for Pd(hfac)2 and loss of hfac- is the main dissociation channel, [Cu(hfac)2]- formation from Cu(hfac)2 dominates. A comparison of the ion yield curves and the associated resonances gives insight into the role of the ligand in the attachment process and highlights the influence of the central metal atom. [ABSTRACT FROM AUTHOR]

We performed a joint photoelectron spectroscopy and ab initio study of two carbon-doped boron clusters, CB9- and C2B8-. Unbiased computational searches revealed similar global minimum structures for both clusters. The comparison of the experimentally observed and theoretically calculated vertical detachment energies revealed that only the global minimum structure is responsible for the experimental spectra of CB9-, whereas the two lowest-lying isomers of C2B8- contribute to the experimental spectra. The planar 'distorted wheel' type structures with a single inner boron atom found for CB9- and C2B8- are different from the quasi-planar structure of B10-, which consists of two inner atoms and eight peripheral boron atoms. The adaptive natural density partitioning chemical bonding analysis revealed that CB9- and C2B8 clusters exhibit π aromaticity and σ antiaromaticity, which is consistent with their planar distorted structures. [ABSTRACT FROM AUTHOR]

We present experimental and theoretical study of electron ionization and dissociative ionization to the gas phase amino acids valine, leucine, and isoleucine. A crossed electron/molecular beams technique equipped with quadrupole mass analyzer has been applied to measure mass spectra and ion efficiency curves for formation of particular ions. From experimental data the ionization energies of the molecules and the appearance energies of the fragment ions were determined. Ab initio calculations (Density Functional Theory and G3MP2 methods) were performed in order to calculate the fragmentation paths and interpret the experimental data. The experimental ionization energies of parent molecules [P]+ 8.91 ± 0.05, 8.85 ± 0.05, and 8.79 ± 0.05 eV and G3MP2 ionization energies (adiabatic) of 8.89, 8.88, and 8.81 eV were determined for valine, leucine, and isoleucine, respectively, as well as the experimental and theoretical threshold energies for dissociative ionization channels. The comparison of experimental data with calculations resulted in identification of the ions as well as the neutral fragments formed in the dissociative reactions. Around 15 mass/charge ratio fragments were identified from the mass spectra by comparison of experimental appearance energies with calculated reaction enthalpies for particular dissociative reactions. [ABSTRACT FROM AUTHOR]

It is shown that the convergence of anharmonic infrared spectral intensities with respect to the basis set size is much enhanced in explicitly correlated calculations as compared to traditional configuration interaction type wave function expansion. Explicitly correlated coupled cluster (CC) calculations using Slater-type geminal correlation factor (CC-F12) yield well-converged dipole derivatives and vibrational intensities for hydrogen fluoride with basis set involving f functions on the heavy atom. Combination of CC-F12 with singles, doubles, and non-iterative triples (CCSD(T)-F12) with small corrections due to quadruple excitations, core-electron correlation, and relativistic effects yields vibrational line positions, dipole moments, and transition dipole matrix elements in good agreement with the best experimental values. [ABSTRACT FROM AUTHOR]

The effect of a finite size model for both the nuclear charge and magnetic moment distributions on calculated EPR hyperfine structure have been studied using a relativistic four-component method based on density functional theory. This approach employs a restricted kinetically balanced basis (mDKS-RKB) and includes spin-polarization using noncollinear spin-density exchange-correlation functionals in the unrestricted fashion. Benchmark calculations have been carried out for a number of small molecules containing Zn, Cd, Ag, and Hg. The present results are compared with those obtained at the Douglas-Kroll-Hess second order (DKH-2) method. The dependence of the results on the quality of the orbital and auxiliary basis sets has been studied. It was found that some basis sets contain irregularities that deteriorate the results. Especial care has to be taken also on the construction of the auxiliary basis for fitting the total electron and spin-densities. [ABSTRACT FROM AUTHOR]

Electron impact ionization of the gas phase 3-furanol, tetrahydro (3-hydroxytetrahydrofuran, 3HTHF) and 2-furanmethanol, tetrahydro (α-tetrahydrofurfuryl alcohol, THFA) molecules has been studied both experimentally and theoretically. The electron induced positive ion formation has been investigated experimentally using a crossed electron/neutral beams technique in combination with a quadrupole mass spectrometry. The mass spectra of both molecules have been determined at the incident electron energy of 70 eV. The ionization efficiency curves for each parent cation and a number of fragment cations have been measured near the threshold, and the corresponding appearance energies have been derived using an iterative fitting procedure based on the Wannier threshold law, taking into account the incident electron energy resolution. The appearance energies of the parent cations were experimentally determined to be (9.620±0.058) eV for (C4H8O2+/3HTHF) and (9.43±0.12) eV for (C5H10O2+/THFA), which are in a good agreement with G3MP2 calculated results: 9.480 and 9.419 eV, respectively. The most abundant cations in the mass spectra were determined to be 57 amu for 3HTHF and 71 amu for THFA, with the corresponding experimentally determined appearance energies of (10.22±0.10) eV and (9.574±0.062) eV, respectively. With the help of the energies calculated at B3LYP and G3MP2 levels of theory, the possible fragmentation patterns were discussed. [ABSTRACT FROM AUTHOR]

Some linear and nonlinear optical (NLO) properties of Ni(SCH)4 and several of its derivatives have been computed by employing a series of basis sets and a hierarchy of methods (e.g., HF, DFT, coupled cluster, and multiconfigurational techniques). The electronic structure of Ni(SCH)4 has been also analyzed by using CASSCF/CASPT2, ab initio valence bond, and DFT methods. In particular we discuss how the diradicaloid character (DC) of Ni(SCH)4 significantly affects its NLO properties. The quasidegeneracy of the two lowest-energy singlet states 1 1Ag and 1 1B1u, the clear DC nature of the former, and the very large number of low-lying states enhance the NLO properties values. These particular features are used to interpret the NLO properties of Ni(SCH)4. The DC of the considered derivatives has been estimated and correlated with the NLO properties. CASVB computations have shown that the structures with Ni(II) are the dominant ones, while those with Ni(0) and Ni(IV) have negligible weight. The weights of the four diradical structures were discussed in connection with the weight of the structures, where all the electrons are paired. Comparative discussion of the properties of Ni(SCH)4 with those of tetrathia fulvalene demonstrates the very large effect of Ni on the properties of the Ni-dithiolene derivatives. A similar remarkable effect on the NLO properties is produced by one or two methyl or C3S groups. The considered Ni-dithiolene derivatives have exceptionally large NLO properties. This feature in connection with their other physical properties makes them ideal candidates for photonic applications. [ABSTRACT FROM AUTHOR]

In a recent publication [J. Chem. Phys. 127, 084102 (2007)], the nearly variational DEWE approach (DEWE denotes Discrete variable representation of the Watson Hamiltonian using the Eckart frame and an Exact inclusion of a potential energy surface expressed in arbitrarily chosen coordinates) was developed to compute a large number of (ro)vibrational eigenpairs for medium-sized semirigid molecules having a single well-defined minimum. In this publication, memory, CPU, and hard disk usage requirements of DEWE, and thus of any DEWE-type approach, are carefully considered, analyzed, and optimized. Particular attention is paid to the sparse matrix-vector multiplication, the most expensive part of the computation, and to rate-determining steps in the iterative Lanczos eigensolver, including spectral transformation, reorthogonalization, and restart of the iteration. Algorithmic improvements are discussed in considerable detail. Numerical results are presented for the vibrational band origins of the 12CH4 and 12CH2D2 isotopologues of the methane molecule. The largest matrix handled on a personal computer during these computations is of the size of (4·108)×(4·108). The best strategy for determining vibrational eigenpairs depends largely on the actual details of the required computation. Nevertheless, for a usual scenario requiring a large number of the lowest eigenpairs of the Hamiltonian matrix the combination of the thick-restart Lanczos method, shift-fold filtering, and periodic reorthogonalization appears to result in the computationally most feasible approach. [ABSTRACT FROM AUTHOR]

High-pressure structural transformations of carbon at terapascal pressures are studied using metadynamics and ab initio methods. Diamond transforms to a mechanically stable cubic structure (P4132) at 2.5 TPa and 300 K. At 4000 K and 2 TPa, simple cubic carbon SC1 (Pm-3m) is obtained from cubic diamond. The high-pressure tetrahedrally coordinated BC8 (Ia-3) structure of carbon is obtained by decompression of the SC1 structure at 1 TPa and 5000 K. At 3000 K, with decompression of SC1 carbon to 1 TPa, two new metastable tetrahedrally coordinated structures form, MP8 (P2/c) and OP8 (Pccn) with higher density than that of cubic diamond. The results show the presence of strong kinetic effects and suggest that phase transformations and structures of carbon at extreme pressures are more complex than previously thought. [ABSTRACT FROM AUTHOR]

We present a computational study of pressure-induced structural phase transitions in bulk CdSe. Thanks to the use of the metadynamics technique we were able to observe the phase transitions at room temperature close to the experimental transition pressure. We discuss the transition mechanisms from four-coordinated wurtzite and zinc blende to six-coordinated rock salt, as well as the reverse transitions, where we found a mixed wurtzite/zinc blende stacking. [ABSTRACT FROM AUTHOR]

The potential energy surface of H2O–H2 is of great importance for quantum chemistry as a test case for H2O-molecule interactions. It is also required for a detailed understanding of important astrophysical processes, namely, the collisional excitation of water, including the pumping of water masers and the formation of molecular hydrogen on icy interstellar dust grains. We have calculated the interaction for H2O–H2 by performing both rigid-rotor (five-dimensional) and non-rigid-rotor (nine-dimensional) calculations using the coupled-cluster theory at the level of singles and doubles with perturbative corrections for triple excitations [CCSD(T)] with moderately large but thoroughly selected basis set. The resulting surface was further calibrated using high precision explicitly correlated CCSD(T)-R12 calculations on a subset of the rigid-rotor intermolecular geometries. The vibrationally averaged potential is presented in some details and is compared with the most recent rigid-rotor calculations. We explain, in particular, as to why vibrationally averaged rigid-rotor geometries are a better choice than equilibrium geometries. Our fit of the vibrationally averaged surface provides for the first time an accuracy of ∼3 cm-1 in the van der Waals minimum region of the interaction. The overall accuracy of the nine-dimensional surface and fit is lower but remains of the order of 3%–4% of the anisotropy in the domain spanned by the vibrational functions. [ABSTRACT FROM AUTHOR]

Full explicitly correlated F12 coupled cluster theory with single and double excitations and with Slater-type geminal as a correlation factor is introduced and implemented within the standard approximation. The variant “C” that does not require integrals over the commutator between the kinetic operator and the correlation factor has been used. All the necessary integrals are analytically calculated. With variant C also, first results are reported for the correlation factor being the interelectronic distance coordinate, i.e., for original R12 method. Calculations have been performed for a set of eight molecules including CH2(1A1), CH4, NH3, H2O, HF, CO, N2, and F2, as well as for the constituting atoms. Atomization energies are reported too. [ABSTRACT FROM AUTHOR]

Second order R12 theory is presented and derived alternatively using the second quantized hole-particle formalism. We have shown that in order to ensure the strong orthogonality between the R12 and the conventional part of the wave function, the explicit use of projection operators can be easily avoided by an appropriate partitioning of the involved operators to parts which are fully describable within the computational orbital basis and complementary parts that involve imaginary orbitals from the complete orbital basis. Various Hamiltonian splittings are discussed and computationally investigated for a set of nine molecules and their atomization energies. If no generalized Brillouin condition is assumed, with all relevant partitionings the one-particle contribution arising in the explicitly correlated part of the first order wave function has to be considered and has a significant role when smaller atomic orbital basis sets are used. The most appropriate Hamiltonian splitting results if one follows the conventional perturbation theory for a general non-Hartree-Fock reference. Then, no couplings between the R12 part and the conventional part arise within the first order wave function. The computationally most favorable splitting when the whole complementary part of the Hamiltonian is treated as a perturbation fails badly. These conclusions also apply to MP2-F12 approaches with different correlation factors. [ABSTRACT FROM AUTHOR]

Using a crossed electron/molecule beam technique the dissociative electron attachment (DEA) to gas phase L-valine, (CH3)2CHCH(NH2)COOH, is studied by means of mass spectrometric detection of the product anions. Additionally, ab initio calculations of the structures and energies of the anions and neutral fragments have been carried out at G2MP2 and B3LYP levels. Valine and the previously studied aliphatic amino acids glycine and alanine exhibit several common features due to the fact that at low electron energies the formation of the precursor ion can be characterized by occupation of the π* orbital of the carboxyl group. The dominant negative ion (M–H)- (m/Z=116) is observed at electron energies of 1.12 eV. This ion is the dominant reaction product at electron energies below 5 eV. Additional fragment ions with m/Z=100, 72, 56, 45, 26, and 17 are observed both through the low lying π* and through higher lying resonances at about 5.5 and 8.0–9.0 eV, which are characterized as core excited resonances. According to the threshold energies calculated here, rearrangements play a significant role in the formation of DEA fragments observed from valine at subexcitation energies. [ABSTRACT FROM AUTHOR]

Density fitting approach to Coulomb integrals for infinite systems with translational periodicity is reformulated in direct space. Despite of the Coulomb infinite decay of some integrals, direct-space calculation is shown to be feasible. Moreover, we show that the direct-space ansatz is completely equivalent to our previous formulation in reciprocal space. Computational demands scale linearly with the number of unit cells. In addition, direct-space treatment has some practical advantages over the reciprocal-space formulation. The efficiency of our scheme is demonstrated on systems with translational periodicity in one dimension. Computation time takes only a small fraction of the conventional calculation with exact integrals. We show that for infinite systems auxiliary basis sets of equally good quality as for molecules can be constructed in a systematic way. [ABSTRACT FROM AUTHOR]

We have measured appearance energies of Arn+, n≤30, by electron impact of gas phase clusters. Quantum-chemical calculations have been performed to determine the adiabatic and vertical ionization energies of argon clusters up to n=4 and 6, respectively. The experimental appearance energy of the dimer ion approaches, under suitable cluster source conditions, the adiabatic ionization energy. The agreement with values obtained by photoionization and threshold photoelectron-photoion coincidence (TPEPICO) spectra demonstrates that autoionizing Rydberg states are accessible by electron impact. Appearance energies of larger clusters, though, exceed the TPEPICO values by about 0.5 eV. [ABSTRACT FROM AUTHOR]

The hydrogen and water molecules are ubiquitous in the Universe. Their mutual collisions drive water masers and other line emission in various astronomical environments, notably molecular clouds and star-forming regions. We report here a full nine-dimensional interaction potential for H2O-H2 calibrated using high-accuracy, explicitly correlated wave functions. All degrees of freedom are included using a systematic procedure transferable to other small molecules of astrophysical or atmospherical relevance. As a first application, we present rate constants for the vibrational relaxation of the υ2 bending mode of H2O obtained from quasiclassical trajectory calculations in the temperature range of 500–4000 K. Our high-temperature (T>=1500 K) results are found compatible with the single experimental value at 295 K. Our rates are also significantly larger than those currently used in the astrophysical literature and will lead to a thorough reinterpretation of vibrationally excited water emission spectra from space. [ABSTRACT FROM AUTHOR]

An attempt is made to improve the currently accepted muonic value for the 197Au nuclear quadrupole moment [+0.547(16)×10-28 m2] for the 3/2+ nuclear ground state obtained by Powers et al. [Nucl. Phys. A230, 413 (1974)]. From both measured Mössbauer electric quadrupole splittings and solid-state density-functional calculations for a large number of gold compounds a nuclear quadrupole moment of +0.60×10-28 m2 is obtained. Recent Fourier transform microwave measurements for gas-phase AuF, AuCl, AuBr, and AuI give accurate bond distances and nuclear quadrupole coupling constants for the 197Au isotope. However, four-component relativistic density-functional calculations for these molecules yield unreliable results for the 197Au nuclear quadrupole moment. Relativistic singles-doubles coupled cluster calculations including perturbative triples [CCSD(T) level of theory] for these diatomic systems are also inaccurate because of large cancellation effects between different field gradient contributions subsequently leading to very small field gradients. Here one needs very large basis sets and has to go beyond the standard CCSD(T) procedure to obtain any reliable field gradients for gold. From recent microwave experiments by Gerry and co-workers [Inorg. Chem. 40, 6123 (2001)] a significantly enhanced 197Au nuclear quadrupole coupling constant in (CO)AuF compared to free AuF is observed. Here, these cancellation effects are less important, and relativistic CCSD(T) calculations finally give a nuclear quadrupole moment of +0.64×10-28 m2 for 197Au. It is argued that it is currently very difficult to improve on the already published muonic value for the 197Au nuclear quadrupole moment. [ABSTRACT FROM AUTHOR]

The partial ionization cross section for the formation of SF3+ fragment ions following electron impact on SF6 is known to have a pronounced structure in the cross section curve slightly above 40 eV. We used the mass-analyzed ion kinetic energy (MIKE) scan technique to demonstrate the presence of a channel contributing to the SF3+ partial ionization cross section that we attribute to the Coulomb explosion of doubly charged metastable SF42+ ions into two singly charged ions SF3+ and F+, with a threshold energy of about 45.5 eV. Thus the observed unusual shape of the SF3+ partial ionization cross section is the result of two contributions, (i) the direct formation of SF3+ fragment ions via dissociative ionization of SF6 with a threshold energy of 22 eV and (ii) the Coulomb explosion of metastable SF42+ ions with a threshold energy of about 45.5 eV. A detailed analysis of the MIKE spectrum reveals an average kinetic energy release of about 5 eV in the Coulomb explosion of the SF42+ ions with evidence of a second channel corresponding to an average kinetic energy release of about 1.1 eV. © 2004 American Institute of Physics. [ABSTRACT FROM AUTHOR]

Probing electronic femtosecond (fs) coherence among segmental sites that are congested by static and dynamic site disorder and subject to structural relaxation is a big, experimental challenge in the study of photophysics of poly(p-phenylenevinylene). In this work, fs-wave-packet fluorescence interferometry experiments are presented that measure macroscopic coherent kernels and their phase-relaxation in the low-temperature, bottom-state regime of the density-of-states below the migrational threshold energy where downhill site-to-site transfer is marginal. By using freely propagating and tunable 70 is excitation/probing pulses and employing narrow-hand spectral filtering of wave packets, fluorescence interferograms with strongly damped beatings can be observed. The coherences formally follow the in-phase superpositions of two site-optical free-induction-decays and originate from distinct pairs of coherent doorway-states, different in energy and space, each of them being targeted, by two discrete quantum-arrival-states 1α and 1β, via independent, isoenergetic 0→1 fluorescence transitions. The coherent transients are explained as site-to-site polarization beatings, caused by the interference of two fluorescence correlation signals. The numerical analysis of the damping regime, based upon second-order perturbational solutions, reveals the lower limit value of homogeneous dephasing in the range from T2 ≃ 100 fs to T2 ≃ 200 fs depending on the site-excitation energy of the bottom-states. The experiments enable to look into the formation of the relaxed state as a special molecular process of electron-phonon coupling and hence open-up a quite new perspective in the puzzle of multichromophore optical dynamics and structural relaxation in conjugated polymers. [ABSTRACT FROM AUTHOR]

Electron impact ionization of the chlorodifluoromethane molecule is studied using crossed beams of high-resolution electrons and an effusive molecular beam of CHF[sub 2]Cl. Ionization energies (IEs) for many positive ions from CHF[sub 2]Cl (CHF[sub 2]Cl[sup +],CF[sub 2]Cl[sup +],CHFCl[sup +],CFCl[sup +],CHF[sub 2][sup +],CF[sub 2][sup +],HCl[sup +],Cl[sup +],CF[sup +],CH[sup +],F[sup +] C[sup +]) are determined from a careful examination of the threshold behavior of the ionization cross sections. Reaction pathways for the dissociative ionization products are suggested using known thermodynamic quantities. Surprisingly, it is observed that the ionization threshold for the parent positive ion IE(CHF[sub 2]Cl[sup +]/CHF[sub 2]Cl)=12.50(±0.05) eV lies above that for the fragment ions CHF[sub 2][sup +], CHFCl[sup +], and CF[sup +] [IE(CHF[sub 2][sup +]/CHF[sub 2]Cl)=12.24(±0.03) eV, IE(CHFCl[sup +]/CHF[sub 2]Cl)=12.3(±0.05) eV, and IE(CF[sup +]/CHF[sub 2]Cl)=11.5(±0.1) eV]. Experiments using a three sector field BEE mass spectrometer provide evidence for the existence of two states of the parent CHF[sub 2]Cl[sup +] ion, one exhibiting a short lifetime of about 2 μs and another parent ion state which appears to be stable on the time scale of the experiment (>20 μs) with an IE of 12.50 eV. © 2003 American Institute of Physics. [ABSTRACT FROM AUTHOR]

Dissociative electron attachment (DEA) to nitroethane (C[sub 2]H[sub 5]NO[sub 2]) in the gas phase is studied using a crossed electron–molecule beams technique. The ion yields for particular fragment ions were measured in the electron energy range from 0 to 9.5 eV with an electron energy resolution of about 120 meV. In contrast to previous studies additional negative ions, i.e., HCN[sup -] and NO[sup -], were observed. Measures for the absolute partial cross sections were obtained for the ions using a calibration technique involving a comparison with the known cross section of the 0.8 eV peak in DEA to CCl[sub 4]. The dominant negative ion product is NO[sub 2][sup -] with a cross section of approximately 11×10[sup -22] m[sup 2] at 0.75 eV. The values of the cross section estimates for the other ions are about σ(C[sub 2]H[sub 3]NO[sub 2][sup -])=4×10[sup -24] m[sup 2] at ∼0 eV, σ(O[sup -])=5×10[sup -24] m[sup 2] at 5.7 eV, σ(OH[sup -])=2×10[sup -24] m[sup 2] at 5.7 eV, σ(CN[sup -])=5×10[sup -25] m[sup 2] at 1.7 eV, σ(CNO[sup -])=3×10[sup -25] m[sup 2] at ∼4.5 eV, σ(HCN[sup -])=2×10[sup -25] m[sup 2] at ∼4.5 eV and σ(NO[sup -])=3×10[sup -25] m[sup 2] at ∼8 eV. © 2003 American Institute of Physics. [ABSTRACT FROM AUTHOR]

Interaction energies between the coinage metal atoms (Cu, Ag, and Au) and lone-pair donating molecules (H[sub 2]O and NH[sub 3]) are calculated by the spin adapted restricted open-shell Hartree–Fock coupled cluster method with the scalar relativistic effects accounted for by the Douglas–Kroll approximation. All ammonia complexes CuNH[sub 3], AgNH[sub 3], and AuNH[sub 3] are found to be of C[sub 3v] symmetry with the counterpoise corrected interaction energies equal to -16.68, -6.87, and -14.64 mH for Cu, Ag, and Au, respectively. In the case of the water molecule the complexes are much weaker with the counterpoise corrected interaction energies equal to -3.78, -1.81, and -1.77 mH, for the three metal atoms, respectively. Moreover, all complexes with the water molecule are nonplanar. For both lone-pair donating molecules the structure and energetics of their complexes with the coinage metal atoms is mostly due to electron correlation effects. The relativistic effects are found to increase the bonding energies in the series of the ammonia complexes, whereas they reduce the bonding energy in the AgOH[sub 2] complex and are essentially negligible for CuOH[sub 2] and AuOH[sub 2]. The calculated complex geometries and interaction energies are discussed in terms of different models. The pattern of interaction energies is discussed in terms of the balance between long-range induction and dispersion contributions and short-range forces. Also the possibility of some charge transfer from the lone-pair donor to the metal atom is considered and supported by analysis of the ionization potential and electron affinity data. The relativistic reduction of the size of the coinage metal atoms is found to be of importance as well. The calculated structural data are used to interpret the experimental observation concerning the existence of well resolved resonantly enhanced multiphoton ionization (REMPI) spectra of the ammonia–silver complexes and the absence of the corresponding spectra of the water–silver complex. This experimental difference between the ammonia and water complexes is explained in terms of the very flat interaction energy function for the wagging motion of the water molecule in AgOH[sub 2]. This large amplitude vibration makes the structure of the complex undefined and is responsible for the nonexistence of the vibrationally resolved REMPI spectrum of the complex.© 2003 American Institute of Physics. [ABSTRACT FROM AUTHOR]

Surface induced dissociation of the protonated ethanol molecular ion, protonated ethanol dimer, and protonated ethanol trimer on a stainless steel surface covered by hydrocarbons was investigated over the incident ion energy range 10-80 eV. Mass spectra of the product ions were recorded and from them the dependencies of the relative abundance of the product ions on the collision energy (CERMS, collision energy resolved mass spectra) were obtained. The dissociation kinetics follows the unimolecular decomposition model of a surface-excited polyatomic projectile behaving as a statistical ensemble. The CERMS curves of the three projectiles, normalized to the same number of internal degrees of freedom, fit each other over the respective energy regions, and matched together provide information on the break-down pattern of the surface-excited protonated ethanol trimer over a wide energy range. This clearly shows that surface collisions can be used to impart in a defined manner a certain excitation energy to the projectile ion. The binding energy of an ethanol monomer in the protonated trimer and in the protonated dimer was estimated from the data as 0.95 and 1.6 eV, respectively, in good agreement with earlier data. [ABSTRACT FROM AUTHOR]

C[sub 3]H[sup 2+, sub 5], formed by electron impact ionization of propane, undergoes metastable decay into C[sub 2]H[sub 2, sup +] + CH[sub 3, sup +]. We have monitored this reaction in a magnetic mass spectrometer of reversed geometry that is equipped with two electric sectors (BEE geometry). Three different techniques were applied to identify the fragment ions and determine the kinetic-energy release (KER) of spontaneous Coulomb explosion of C[sub 3]H[sup 2+, sub 5] in the second and third field free regions of the mass spectrometer. The KER distribution is very narrow, with a width of about 3% [root-mean square standard deviation]. An average KER of 4.58 ± 0.15 eV is derived from the distribution. High level ab initio quantum-chemical calculations of the structure and energetics of C[sub 3]H[sup 2+, sub 5] are reported. The activation barrier of the reverse reaction, CH[sub 3, sup +] + C[sub 2]H[sub 2, sup +] (vinylidene), is computed. The value closely agrees with the experimental average KER, thus indicating that essentially all energy available in the reaction is partitioned into kinetic energy. [ABSTRACT FROM AUTHOR]

Dissociations and reactions induced by impact of acetonitrile monomer ions (CH[sub 3]CN[sup +], CD[sub 3]CN[sup +]), dimer ions [(CH[sub 3]CN)[sub 2, sup +], (CD[sub 3]CN)[sub 2, sup +]] and trimer ions [(CD[sub 3]CN)[sub 3, sup +]] on a hydrocarbon-covered stainless-steel surface were investigated over the projectile energy range of 3-70 eV. Both simple dissociations of the projectile ion and chemical reactions of H-atom transfer from the surface material (followed by dissociations of the protonated projectile ion formed) were observed for the monomer ions. Results obtained for the dimer ions (CD[sub 3]CN)[sub 2, sup +] indicate the formation of the protonated acetonitrile ions via surface-induced reactions in two ways: (i) an intracluster ionmolecule reaction followed by dissociation to form CD[sub 3]CND[sup +], and (b) a hydrogen pick-up reaction from the surface material during the interaction of the dimer ion with the surface leading to CD[sub 3]CNH[sup +]. A simple model based on the Brauman double-well potential—suggested earlier to explain the occurrence of analogous reactions in acetone cluster ion/surface interactions—accounts well for the formation of both product ions. Moreover, in adition to these protonated species, considerable amounts of nondissociated dimer ions were observed after acetonitrile dimer cation/ surface collisions with energies up to 25 eV. Similarly, both trimer ions (up to 20 eV) and dimer ions (up to 30 eV) were observed in acetonitrile trimer cation/surface interactions. This indicates that unimolecular dissociation kinetics governs the product formation for these cluster ion/surface interactions. [ABSTRACT FROM AUTHOR]

Dissociative electron attachment (DEA) to CH[sub 3]NO[sub 2] in the gas phase was studied in the electron energy range from zero up to 10 eV with an energy resolution of 140 meV. For the most intense negative fragments NO[sup -, sub 2], O[sup -], OH[sup -], CN[sup -], CNO[sup -] estimates for the absolute partial cross sections were obtained for the first time [σ(NO[sub 2])≈10[sup -21]m² at 0.62 eV, σ(O[sup -])≈ 10[sup -23]m² at 5.4 eV, σ(OH[sup -])≈10[sup -24]m² at 4 eV, σ(CN[sup -])≈10[sup -24]m² at 1.7 eV, and σ(CNO[sup -]) ≈ 10[sup -25]m² at 4 eV]. In the case of OH[sup -], CN[sup -], and CNO[sup -], ion formation at very low electron energies (≈0 eV) has been observed in contrast to previous studies. The formation of OH[sup -] and CNO[sup -] at these low electron energies is explained in terms of DEA to vibrationally excited molecules. Analyzing measured partial cross sections, the standard enthalpy of formation of the CH[sub 3]NO (nitroso-methane) and the CNO radical has been estimated, as Δ[sub f]H[sup º, sub g],(CH[sub 3]NO)=129±30 kJ/mol and Δ[sub f]/H[sup º,, sub g](CNO)=323 ± 30 kJ/mol, respectively. [ABSTRACT FROM AUTHOR]

The two-dimensional (2D) temporal shape of the (degenerate) three-pulse photon-echo (3-PPE) polarization P[sup (3)](t', τ[sub 12], τ[sub 23] = 0) has been theoretically analyzed for a weakly interacting two-site system (TSS) in the presence of site inhomogeneous broadening. The TSS has been modeled in terms of two identical, energy-degenerate, excitonically coupled pairs of electronic states in the site-representation (i.e., two S[sub 0] and S[sub 1] states forming a 4-level system). The 2D time-domain signal S(t',τ[sub 12],τ[sub 23]=O) has been derived in a closed-form solution which allowed to formulate the analytical Fourier transform analog under sequential δ-pulse excitation. The resulting 2D PE signal in frequency-frequency space S(ω[sub t'], ω[sub τ[sub 12]) is formed in the region of the resonant, optical carrier frequency by, generally, four peaks, arranged at the edges of a square with distances determined by the excitonic coupling. The two diagonal peaks are of one-exciton origin, while the two off-diagonal (cross) peaks involve two-exciton effects, too. The relative heights of the four peaks are in distinct relation with the mutual orientation of the site-localized transition dipoles, thus these 2D line shapes serving as a structural probe for resonantly interacting molecules. The shapes of the peaks reflect the ratio between homogeneous and inhomogeneous dephasings and can be used for structural studies of statically disordered systems of TSS. The semiclassical two-site model is intended to provide a preparatory platform for mimicking electronically coupled segmental pairs dispersed in energy and space along the strongly disordered main chain contour. The results of the model calculation are taken to discuss the potential and the limitations of 2D line shape echo-spectroscopy in the study of short-range coherences and electronic coupling in φ-conjugated polymers. [ABSTRACT FROM AUTHOR]

Using a high resolution two sector field mass spectrometer of reversed geometry we have measured metastable fractions and mass-analyzed ion kinetic energy peaks for metastable decay reactions involving monomer evaporation via (O[sub 2])[sub n][sup +[sup *]]→(O[sub 2])[sub n-1][sup +]+O[sub 2]. Both, average kinetic energy release data derived from the peak shapes and the time dependence of the metastable fractions show a characteristic dependence on cluster size indicating a change in the metastable fragmentation mechanism when going from the dimer to the dekamer ion. Moreover, the data contain information about the transition state temperature and thus one can use finite heat bath theory to calculate the binding energies of the decaying cluster ions. The results obtained are in fair agreement with (i) previous results based on gas phase ion equilibria measurements and with (ii) the corresponding bulk value. © 2002 American Institute of Physics. [ABSTRACT FROM AUTHOR]

Electron impact ionization cross sections measured close to threshold are reported for both the monomers and dimers of ozone and OC1O using a new high resolution electron impact apparatus. The present appearance energies AE(O[sup +, sub 3]-/O[sub 3])=12.70±0.02 eV, AE (OC1O[sup +]/OC1O)= 10.55±0.02 and AE(C1O[sup +]/OC10 = 13.37±0.03 eV derived from the measured ionization cross sections are in excellent agreement with the vertical threshold values determined for these ions by high resolution PES and PIMS photoionization studies. The corresponding appearance energies determined for the dimer ions, 10.10±0.3 eV for (O[sub 3])[sup +, sub 2] and 9.87±0.2 eV for (OC1O)[sup +, sub 2], are both red shifted with respect to the monomer case. The bond energy (0.70-0.3+0.5) eV of (OC1O)[sup +, sub 2] estimated from these data is similar to that of other dimer ions, whereas the bond energy of (O[sub 3]-O[sup +, sub 3]) with (2.55-0.4+0.6) eV is rather large suggesting an unusual structure for the cationic ozone dimer ion. Based on quantum chemical calculations on various levels we are led to the conclusion that the ion produced by ionization of the ozone dimer is no longer a conventional dimer ion where the two monomer units are still present (as is the case for the OC1O system), but rather an ion of form O[sub 2]...O[sup +, sub 4] or a twisted ring structure of (O[sub 6])[sup +]. [ABSTRACT FROM AUTHOR]

An implementation of a two-component all-electron (jj/ωω) treatment of both scalar and spin–orbit relativistic effects in the MOLFDIR program suite is presented. Relativity is accounted for by Douglas–Kroll transformed one-electron operators: scalar (spin-free) and so called mean-field spin–orbit terms. The interelectronic interaction is represented by the nonrelativistic Coulomb operator. High-level correlated calculations of properties of several systems (FO, ClO, Cl, O[sub 2][sup +], O[sub 2][sup -], Tl, and TlH) where spin–orbit effects play a dominant role are presented and compared with other data. Agreement with Dirac–Coulomb(–Gaunt) reference values is in general very good. © 2001 American Institute of Physics. [ABSTRACT FROM AUTHOR]

We have measured the kinetic energy release (KER) distributions for electron-induced dissociation of mass-selected molecular parent and fragment ions of propane. They are compared with distributions determined for spontaneous (metastable) dissociation. The average KER for induced dissociation of C[sub 3]H[sub 8][sup +] into C[sub 3]H[sub 7][sup +] is 13.2±1.2 meV, about 42% larger than for the spontaneous reaction. This large difference is attributed to the dramatically reduced time at which the induced reaction can be sampled. In contrast, the KER for dissociation of C[sub 3]H[sub 7][sup +] into C[sub 3]H[sub 5][sup +], which is characterized by a large reverse activation energy, is hardly affected by the experimental time scale. © 2000 American Institute of Physics. [ABSTRACT FROM AUTHOR]

Reports on the use of the highly monochromatized electrons in a crossed beam experiments to investigate electron attachment to oxygen clusters. Increase in the attachment cross section for the reaction with decreasing electron energy compatible with s-wave electron capture; Vibrational levels of the anion populated by attachment of an electron to a single oxygen.

Reports on the occurrence of two different chemical reactions initiated by the surface impact of acetone dimer, trimer and tetramer cations on a stainless steel surface. Formation of the reaction product which is the protonated acetone ion; Model used to explain the formation of two product ions.

Reports on the theoretical determination of the absolute electron impact ionization cross-section function for silver clusters Ag[sub n]. Use of a semiclassical approach in the study; Comparison of the results to experimental data; Ionization behavior of clusters as a function of cluster size.

Examines the excitation transfer dynamics from intensity fluctuations in fluorescence interference noise in a two-site system. Dynamics of electronic excitation energy transfer in a two-site molecular system; Calculation of the interferogram of time-integrated total fluorescence; Analysis on fluorescence interference fluctuations.

Dipole moments of SiO, GeO, SnO, PbO, and of the corresponding sulfides are computed by using a variety of high-level correlated methods and polarized basis sets of Gaussian functions. The contribution of relativistic effects has been accounted for by the first-order quasirelativistic approach based on mass-velocity and Darwin terms. The results for sulfides are in good agreement with experiment while for oxides seem to show a progressive deterioration with the increase of the heavy atom nuclear charge. The discrepancies between the calculated and experimental dipole moments for oxides have been carefully analyzed for the PbO molecule in terms of the basis set choice and reliability of the calculated electron correlation and relativistic corrections. The pattern of relativistic corrections computed in the present study is compared with that which follows from recent quasirelativistic and fully relativistic calculations of Dyall. Some complementary calculations of the dipole moment of PbSe and PbTe seem to indicate that both the polarized basis sets and computational methods used in this study should not fail to predict correct values of the dipole moment of PbO and other oxides of the group IVa atoms. The rather unexpected discrepancies have no plausible explanation within the range of methods employed in our study and offer a challenge for more advanced, fully relativistic molecular calculations at the correlated level of approximation. [ABSTRACT FROM AUTHOR]

High-level correlated calculations of the quadrupole moment of the group-Ib hydrides have been carried out with large Gaussian-type-orbital/contracted-Gaussian-type-orbital basis sets. The nonrelativistic results obtained for CuH, AgH, and AuH at the self-consistent-field Hartree–Fock level of approximation and for the origin at the heavy atom are -2.16, -3.17, and -1.94 a.u., respectively. The electron correlation contribution computed by using the coupled-cluster approaches reduces those values to -0.25, -1.14, and -0.18 a.u., respectively. The relativistic corrections have been obtained from the recently developed quasirelativistic method. They have been found to be of increasing importance for the investigated series of hydrides. The final relativistically corrected values of molecular quadrupole moments are found to be small for CuH (+0.05 a.u.) and AgH (-0.36 a.u.) while the corresponding value for AuH is large and positive (+2.00 a.u.). [ABSTRACT FROM AUTHOR]

Examines the electron correlation and relativistic calculations to dipole moments of heavy oxides and sulfides of Group IVa metals. Comparison of dipole moments calculations of oxides and sulfides; Enumeration of oxides and sulfides of Group IVa metals; Use of pseudopotential mechanism for the calculation of the dipole moments of lead oxide.

Studies the electron energy dependence of the dissociative attachment cross section to CHCl[sub 3]. Shape and magnitude of the cross section function; Differences in the absolute cross section; Number of energy peaks; Energy position of the peaks; Activation barrier for the dissociative attachment.

Our recently developed method for the calculation of indirect nuclear spin–spin coupling constants is studied in more detail. For the couplings between nuclei other than N, O, and F (which have lone pairs) the method yields very reliable results. The results for 1J(Si–H) couplings are presented and their dependence on the basis set quality is analyzed. Also, 2J(H–H) and 1J(X–H) couplings (X=C, Si, Ge, Sn) in XH4 molecules are presented and the relativistic effects on 1J(X–H) are discussed. The limitations of the method, which is based on density functional theory, are connected with the inability of the present LDA and GGA exchange-correlation functionals to describe properly the spin-perturbations (through the Fermi-contact mechanism) on atoms to the right of the periodic table (containing lone pairs). However, the deviations from experiment of the calculated couplings for such nuclei are systematic, at least for one-bond couplings, and therefore these calculated couplings should still be useful for NMR structure determinations. © 1996 American Institute of Physics. [ABSTRACT FROM AUTHOR]

Relativistic effects and electron correlation effects on the dipole moments of the coinage metal hydrides are investigated and compared employing one-component (scalar) relativistic approximations based on the mass–velocity and Darwin operator and, alternatively, the Douglas–Kroll-transformed spin-averaged no-pair Hamiltonian. The former of the two operators is found to perform quite accurately for CuH and AgH. For AuH the limits of the Pauli approximation seem to be reached, as can be inferred from a comparison with the values obtained within the spin-averaged Douglas–Kroll no-pair formalism. The coupled cluster calculations in the Douglas–Kroll no-pair approximation for relativistic effects establish the dipole moment values of the coinage metal hydrides as equal to 1.05 a.u. for CuH, 1.14 a.u. for AgH and 0.52 for AuH. The corresponding non-relativistic results are 1.14 a.u., 1.36 a.u., and 1.22 a.u., respectively. Some formal problems arising in applications of the Douglas–Kroll no-pair approximation are discussed. It is shown that the Hellmann–Feynman theorem leads to a rather complicated form of the first-order energy change due to external perturbation. The usual expectation value formula is, however, valid through terms proportional to 1/c4 and can be used in most applications. The invariance property with respect to a shift in the external potential is addressed for the Douglas–Kroll no-pair approximation in a finite basis set. © 1996 American Institute of Physics. [ABSTRACT FROM AUTHOR]