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Observing changes in molecular structure requires atomic-scale Ångstrom and femtosecond spatio-temporal resolution. We use the Fourier transform (FT) variant of laser-induced electron diffraction (LIED), FT-LIED, to directly retrieve the molecular structure of H2O+ with picometer and femtosecond resolution without a priori knowledge of the molecular structure nor the use of retrieval algorithms or ab initio calculations. We identify a symmetrically stretched H2O+ field-dressed structure that is most likely in the ground electronic state. We subsequently study the nuclear response of an isolated water molecule to an external laser field at four different field strengths. We show that upon increasing the laser field strength from 2.5 to 3.8 V/Å, the O–H bond is further stretched and the molecule slightly bends. The observed ultrafast structural changes lead to an increase in the dipole moment of water and, in turn, a stronger dipole interaction between the nuclear framework of the molecule and the intense laser field. Our results provide important insights into the coupling of the nuclear framework to a laser field as the molecular geometry of H2O+ is altered in the presence of an external field. [ABSTRACT FROM AUTHOR]

The solvent effects on various isomers of the palladium complexes in dichloromethane was investigated by continuum self-consistent reaction field (SCRF) calculations and compared with the discrete second-order Moller-Plesset (MP2) calculations. It was shown that the free energy of solvation in the continuum model correctly accounts for the electrostatic energy.

The solvent effects on various isomers of the palladium PdH(sub 3-)Cl((NH3)(sub 2))/[PdH2Cl(NH3)](super -)(NH4)(super +) complexes in dichloromethane were investigated by discrete second-order Moller-Plesset (MP2) calculations. The importance of various contributions to the solute-solvent interaction energy was correlated with the chemical properties of the ligand to which the solvent molecule is attached.

We introduce a new method for the computation of the transition moments between the excited electronic states based on the expectation value formalism of the coupled cluster theory [B. Jeziorski and R. Moszynski, Int. J. Quantum Chem. 48, 161 (1993)]. The working expressions of the new method solely employ the coupled cluster operator T and an auxiliary operator S that is expressed as a finite commutator expansion in terms of T and T†. In the approximation adopted in the present paper, the cluster expansion is limited to single, double, and linear triple excitations. The computed dipole transition probabilities for the singlet-singlet and triplet-triplet transitions in alkali earth atoms agree well with the available theoretical and experimental data. In contrast to the existing coupled cluster response theory, the matrix elements obtained by using our approach satisfy the Hermitian symmetry even if the excitations in the cluster operator are truncated, but the operator S is exact. The Hermitian symmetry is slightly broken if the commutator series for the operator S are truncated. As a part of the numerical evidence for the new method, we report calculations of the transition moments between the excited triplet states which have not yet been reported in the literature within the coupled cluster theory. Slater-type basis sets constructed according to the correlation-consistency principle are used in our calculations. [ABSTRACT FROM AUTHOR]

Explicitly correlated quantum chemical calculations require calculations of five types of two-electron integrals beyond the standard electron repulsion integrals. We present a novel scheme, which utilises general ideas of the McMurchie-Davidson technique, to compute these integrals when the so-called "range-separated" correlation factor is used. This correlation factor combines the well-known short range behaviour resulting from the electronic cusp condition, with the exact long-range asymptotics derived for the helium atom [Lesiuk, Jeziorski, and Moszynski, J. Chem. Phys. 139, 134102 (2013)]. Almost all steps of the presented procedure are formulated recursively, so that an efficient implementation and control of the precision are possible. Additionally, the present formulation is very flexible and general, and it allows for use of an arbitrary correlation factor in the electronic structure calculations with minor or no changes. [ABSTRACT FROM AUTHOR]

Theory of one-electron transition density matrices has been formulated within the time-independent coupled cluster method for the polarization propagator [R. Moszynski, P. S. Zuchowski, and B. Jeziorski, Coll. Czech. Chem. Commun. 70, 1109 (2005)]. Working expressions have been obtained and implemented with the coupled cluster method limited to single, double, and linear triple excitations (CC3). Selected dipole and quadrupole transition probabilities of the alkali earth atoms, computed with the new transition density matrices are compared to the experimental data. Good agreement between theory and experiment is found. The results obtained with the new approach are of the same quality as the results obtained with the linear response coupled cluster theory. The one-electron density matrices for the ground state in the CC3 approximation have also been implemented. The dipole moments for a few representative diatomic molecules have been computed with several variants of the new approach, and the results are discussed to choose the approximation with the best balance between the accuracy and computational efficiency. [ABSTRACT FROM AUTHOR]

State-of-the-art ab initio techniques have been applied to compute the potential energy curves for the electronic states in the A1Σu+, c3Πu, and a3Σu+ manifold of the strontium dimer, the spin-orbit and nonadiabatic coupling matrix elements between the states in the manifold, and the electric transition dipole moment from the ground X1Σg+ to the nonrelativistic and relativistic states in the A+c+a manifold. The potential energy curves and transition moments were obtained with the linear response (equation of motion) coupled cluster method limited to single, double, and linear triple excitations for the potentials and limited to single and double excitations for the transition moments. The spin-orbit and nonadiabatic coupling matrix elements were computed with the multireference configuration interaction method limited to single and double excitations. Our results for the nonrelativistic and relativistic (spin-orbit coupled) potentials deviate substantially from recent ab initio calculations. The potential energy curve for the spectroscopically active (1)0u+ state is in quantitative agreement with the empirical potential fitted to high-resolution Fourier transform spectra [A. Stein, H. Knöckel, and E. Tiemann, Eur. Phys. J. D 64, 227 (2011)]. The computed ab initio points were fitted to physically sound analytical expressions, and used in converged coupled channel calculations of the rovibrational energy levels in the A+c+a manifold and line strengths for the A1Σu+←X1Σg+ transitions. Positions and lifetimes of quasi-bound Feshbach resonances lying above the 1S0 + 3P1 dissociation limit were also obtained. Our results reproduce (semi)quantitatively the experimental data observed thus far. Predictions for on-going and future experiments are also reported. [ABSTRACT FROM AUTHOR]

Gas-phase nuclear magnetic resonance (NMR) spectra demonstrating the effect of weak intermolecular forces on the NMR shielding constants of the interacting species are reported. We analyse the interaction of the molecular hydrogen isotopomers with He, Ne, and Ar, and the interaction in the He-CO2 dimer. The same effects are studied for all these systems in the ab initio calculations. The comparison of the experimental and computed shielding constants is shown to depend strongly on the treatment of the bulk susceptibility effects, which determine in practice the pressure dependence of the experimental values. Best agreement of the results is obtained when the bulk susceptibility correction in rare gas solvents is evaluated from the analysis of the He-rare gas interactions, and when the shielding of deuterium in D2-rare gas systems is considered. [ABSTRACT FROM AUTHOR]

Theory of long-range interactions between an atom in its ground S state and a linear molecule in a degenerate state with a nonzero projection of the electronic orbital angular momentum is presented. It is shown how the long-range coefficients can be related to the first and second-order molecular properties. The expressions for the long-range coefficients are written in terms of all components of the static and dynamic multipole polarizability tensor, including the nondiagonal terms connecting states with the opposite projection of the electronic orbital angular momentum. It is also shown that for the interactions of molecules in excited states that are connected to the ground state by multipolar transition moments additional terms in the long-range induction energy appear. All these theoretical developments are illustrated with the numerical results for systems of interest for the sympathetic cooling experiments: interactions of the ground state Rb(2S) atom with CO(3Π), OH(2Π), NH(1Δ), and CH(2Π) and of the ground state Li(2S) atom with CH(2Π). [ABSTRACT FROM AUTHOR]

State-of-the-art ab initio techniques have been applied to compute the potential energy surface for the lithium atom interacting with the lithium hydride molecule in the Born-Oppenheimer approximation. The interaction potential was obtained using a combination of the explicitly correlated unrestricted coupled-cluster method with single, double, and noniterative triple excitations [UCCSD(T)-F12] for the core-core and core-valence correlation and full configuration interaction for the valence-valence correlation. The potential energy surface has a global minimum 8743 cm-1 deep if the Li-H bond length is held fixed at the monomer equilibrium distance or 8825 cm-1 deep if it is allowed to vary. In order to evaluate the performance of the conventional CCSD(T) approach, calculations were carried out using correlation-consistent polarized valence X-tuple-zeta basis sets, with X ranging from 2 to 5, and a very large set of bond functions. Using simple two-point extrapolations based on the single-power laws X-2 and X-3 for the orbital basis sets, we were able to reproduce the CCSD(T)-F12 results for the characteristic points of the potential with an error of 0.49% at worst. The contribution beyond the CCSD(T)-F12 model, obtained from full configuration interaction calculations for the valence-valence correlation, was shown to be very small, and the error bars on the potential were estimated. At linear LiH-Li geometries, the ground-state potential shows an avoided crossing with an ion-pair potential. The energy difference between the ground-state and excited-state potentials at the avoided crossing is only 94 cm-1. Using both adiabatic and diabatic pictures, we analyze the interaction between the two potential energy surfaces and its possible impact on the collisional dynamics. When the Li-H bond is allowed to vary, a seam of conical intersections appears at C2v geometries. At the linear LiH-Li geometry, the conical intersection is at a Li-H distance which is only slightly larger than the monomer equilibrium distance, but for nonlinear geometries it quickly shifts to Li-H distances that are well outside the classical turning points of the ground-state potential of LiH. This suggests that the conical intersection will have little impact on the dynamics of Li-LiH collisions at ultralow temperatures. Finally, the reaction channels for the exchange and insertion reactions are also analyzed and found to be unimportant for the dynamics. [ABSTRACT FROM AUTHOR]

Nonadiabatic theory of molecular spectra of diatomic molecules is presented. It is shown that in the fully nonadiabatic framework, the rovibrational wave functions describing the nuclear motions in diatomic molecules can be obtained from a system of coupled differential equations. The rovibrational wave functions corresponding to various electronic states are coupled through the relativistic spin-orbit coupling interaction and through different radial and angular coupling terms, while the transition intensities can be written in terms of the ground state rovibrational wave function and bound rovibrational wave functions of all excited electronic states that are electric dipole connected with the ground state. This theory was applied in the nearly exact nonadiabatic calculations of energy levels, line positions, and intensities of the calcium dimer in the A 1Σu+(1 1S+1 1D), c 3Πu(1 3P+1 1S), and a 3Σu+(1 3P+1 1S) manifolds of states. The excited state potentials were computed using a combination of the linear response theory within the coupled-cluster singles and doubles framework for the core-core and core-valence electronic correlations and of the full configuration interaction for the valence-valence correlation, and corrected for the one-electron relativistic terms resulting from the first-order many-electron Breit theory. The electric transition dipole moment governing the A 1Σu+←X 1Σg+ transitions was obtained as the first residue of the frequency-dependent polarization propagator computed with the coupled-cluster method restricted to single and double excitations, while the spin-orbit and nonadiabatic coupling matrix elements were computed with the multireference configuration interaction wave functions restricted to single and double excitations. Our theoretical results explain semiquantitatively all the features of the observed Ca2 spectrum in the A 1Σu+(1 1S+1 1D), c 3Πu(1 3P+1 1S), and a 3Σu+(1 3P+1 1S) manifolds of states. [ABSTRACT FROM AUTHOR]

Equations of the random-phase approximation (RPA) for high-spin open-shell molecules are derived using the time-dependent variational principle and a unitary exponential representation of the high-spin determinantal wave function parametrized with a nonredundant set of variational parameters. The restricted open-shell RPA theory is applied to derive expressions for the polarization propagator, and for the dispersion energy of high-spin open-shell complexes. It is also used to define the intramonomer correlation expansions of the RPA dispersion energy using various Møller–Plesset-type partitionings of the Hamiltonian. A close relation between the present treatment and the multipole expansion approach of Hettema and Wormer [H. Hettema and P. E. S. Wormer, J. Chem. Phys. 93, 3389 (1990)] is established and discussed. Numerical results for various high-spin open-shell–closed-shell and open-shell–open-shell complexes are presented. Comparison of the dispersion energy computed in the random phase approximation with highly correlated results from the full configuration interaction or coupled-cluster singles, doubles, and approximate triples calculations shows that the random phase approximation accounts for the major part of the intramonomer correlation effects in the dispersion energy. For open-shell–closed-shell complexes the convergence of the intramonomer correlation expansion through the second order is very good, while for the more difficult case of open-shell–open-shell complexes is much less satisfactory, and full RPA calculations are necessary to get reliable results. © 2003 American Institute of Physics. [ABSTRACT FROM AUTHOR]

Elastic and rotationally inelastic cross sections have been measured for He+H[sub 2]O scattering at two collision energies, 66.3 and 99.0 meV, using the crossed molecular beam technique. The inelastic events are detected by time-of-flight analysis of the scattered He atoms. The data are converted to elastic differential cross sections and inelastic angular-dependent energy loss spectra in the center-of-mass system. They are compared with averaged, full close-coupling calculations of state-to-state cross sections for rotational excitation based on a newly calculated ab initio potential using symmetry-adapted perturbation theory. The agreement with the elastic differential cross sections is excellent. The energy loss spectra are reproduced satisfactorily and among the largest differential cross sections that contributed to the measurements are excitations around all three possible axes for δJ = 1 but a preference of the excitation around the in-plane C axis for δJ = 2 transitions. [ABSTRACT FROM AUTHOR]

Symmetry-adapted perturbation theory has been applied to compute the intermolecular potential energy surface of the He–CO[sub 2] complex. The ab initio potential has a global minimum of ε[sub m]=-50.38 cm[sup -1] at R[sub m]=5.81 bohr for the “T”-shaped geometry of the complex, and a local one of ε[sub m]=-28.94 cm[sup -1] at R[sub m]=8.03 bohr for the linear He···O==C==O structure. The computed potential energy surface has been analytically fitted and used in converged variational calculations to generate bound rovibrational states of the He–CO[sub 2] complex and the infrared spectrum corresponding to the simultaneous excitation of the ν[sub 3] vibration and internal rotation in the CO[sub 2] subunit within the complex. The complex was shown to be a semirigid asymmetric top and the rovibrational energy levels could be classified with the asymmetric top quantum numbers. The computed frequencies of the infrared transitions in the ν[sub 4] band of the spectrum are in very good agreement with the high resolution experimental data of Weida et al. [J. Chem. Phys. 101, 8351 (1994)]. The energy levels corresponding to the ν[sub 5] bending mode of the complex have been used to compute the transition frequencies in the ν[sub 5] hot band of He–CO[sub 2]. A tentative assignment of the transitions observed in the ν[sub 5] band with the quantum numbers of the asymmetric rotor is presented. As a further test of the ab initio potential we also report the pressure broadening coefficients of the R branch rotational lines of the ν[sub 3] spectrum of CO[sub 2] in a helium bath at various temperatures. Very good agreement is found with the wealth of experimental results for various rotational states of CO[sub 2] at different temperatures. Finally, we also tested the potential by computing the second virial coefficients at various temperatures. Again, the agreement between theory an... [ABSTRACT FROM AUTHOR]

Reports on the use of converged close-coupling and coupled-states calculations to obtain state-to-state rate constants and pressure broadening coefficients for the collisional rotational (de)-excitation of acetylene by helium. Monitoring of the population decay in the prepared state and the transfer to other rotational states by laser induced fluorescence.

Examines the applicability of various density functional theory (DFT) methods to describe the anisotropy of the intermolecular potential energy surfaces of hydrogen-bonded and van der Waals complexes. Comparison between supermolecule coupled-cluster method and perturbational symmetry-adapted perturbation theory.

Convergence properties of symmetry-adapted perturbation expansions for nonadditive interactions are tested by performing high-order calculations for three spin-aligned hydrogen atoms. It is shown that the strong symmetry forcing characteristic of the Hirschfelder–Silbey theory leads to a rapidly convergent perturbation series. The symmetrized Rayleigh–Schrödinger perturbation theory employing a weak symmetry forcing is shown to provide in low orders accurate approximations to the nonadditive part of the interaction energy. In very high orders the convergence of this perturbation expansion becomes very slow, and the series converges to an unphysical limit, very close to the exact interaction energy. The nonadditive part of the interaction energy for the lowest quartet state of H3 is interpreted in terms of the first-order exchange, induction, exchange-induction, exchange-dispersion, induction-dispersion, and dispersion contributions. It is shown that even for such a simple trimer the correct description of these components is necessary to obtain quantitative agreement with variational full configuration interaction results. © 1996 American Institute of Physics. [ABSTRACT FROM AUTHOR]

Symmetry-adapted perturbation theory has been applied to compute the interaction-induced polarizability for the helium diatom. The computed polarizability invariants have been analytically fitted, and used in quantum-dynamical calculations of the binary collision-induced Raman spectra. The predicted intensities of the depolarized spectrum are in good agreement with the experimental data [M.H. Proffitt et al., Can. J. Phys. 59, 1459 (1981)]. The computed polarized spectrum shows agreement with the experiment within the large experimental uncertainties. The calculated trace polarizability was also checked by comparison of computed second dielectric virial coefficients with the experimental data. The ab initio dielectric virial coefficients, including first and second quantum corrections, agree well with the experimental data from indirect measurements. © 1996 American Institute of Physics. [ABSTRACT FROM AUTHOR]

Symmetry-adapted perturbation theory of pairwise nonadditive interactions in trimers is formulated, and pure three-body polarization and exchange components are explicitly separated out. It is shown that the three-body polarization contributions through the third order of perturbation theory naturally separate into terms describing the pure induction, mixed induction–dispersion, and pure dispersion interactions. Working equations for these components in terms of molecular integrals and linear and quadratic response functions are derived. These formulas have a clear, partly classical, partly quantum mechanical, physical interpretation. The asymptotic expressions for the second- and third-order three-body polarization contributions through the multipole moments and (hyper)polarizabilities of the isolated monomers are reported. Finally, assuming the random phase approximation for the response functions, explicit orbital formulas for the three-body polarization terms are derived. The exchange terms are also classified, and the simplest approximations (neglecting intramonomer correlation effects) are written as explicitly connected commutator expressions involving second-quantized operators. The corresponding orbital formulas are also reported. © 1995 American Institute of Physics. [ABSTRACT FROM AUTHOR]

Symmetry-adapted perturbation theory has been applied to compute the intermolecular potential energy surface of the He–CO complex. The interaction energy is found to be dominated by the first-order exchange contribution and the dispersion energy. The ab initio potential has a single minimum of εm=-24.895 cm-1 for the linear CO–He geometry at Rm=6.85 bohr. The computed potential energy surface has been analytically fitted and used in converged variational calculations to generate bound rovibrational states of the He–CO molecule and the infrared spectrum, which corresponds to the simultaneous excitation of vibration and internal rotation in the CO subunit within the complex. The predicted positions and intensities of lines in the infrared spectrum are in good agreement with the experimental spectrum [C.E. Chuaqui et al., J. Chem. Phys. 101, 39 (1994)]. The theoretical potential was also checked by comparison of computed excess second virial coefficients with the experimental data. The ab initio interaction virial coefficients, including quantum corrections, lie within the experimental error bars over a wide range of temperatures. © 1995 American Institute of Physics. [ABSTRACT FROM AUTHOR]

Symmetry-adapted perturbation theory has been applied to compute the intermolecular potential energy surface of the He–C2H2 complex. The interaction energy is found to be dominated by the first-order exchange contribution and the dispersion energy. In both contributions it was necessary to include high-level intramolecular correlation effects. Our potential has a global minimum of εm=-22.292 cm-1 near the linear He–HCCH geometry at Rm=8.20 bohr and [variant_greek_theta]m=14.16°, and a local minimum at a skew geometry (Rm=7.39 bohr, [variant_greek_theta]m=48.82°, and εm=-21.983 cm-1). The computed potential energy surface has been analytically fitted and used in converged variational calculations to generate bound rovibrational states of the He–C2H2 molecule and the near-infrared spectrum, which corresponds to the simultaneous excitation of the vibration and hindered rotation of the C2H2 monomer within the complex. The nature of the bound states and of the spectrum predicted from the ab initio potential are discussed. © 1995 American Institute of Physics. [ABSTRACT FROM AUTHOR]

The interaction potential for the HeK+ system has been computed as the sum of attractive and repulsive contributions due to the electrostatic, exchange, induction, and dispersion interactions using the symmetry-adapted perturbation theory and a high-level treatment of electron correlation. The zero of the theoretical potential occurs at 4.704 bohr and the minimum occurs at 5.418 bohr where the potential value is -0.779 mhartree. The potential supports 36 bound rovibrational levels, and the ground state of the HeK+ molecular ion is bound by 125.1 cm-1. For all interatomic distances the ab initio potential agrees very well with the empirical potential obtained by direct inversion of the K+ mobilities in gaseous helium and disagrees with the potential obtained from the ion–beam scattering cross section data. The ab initio potential has been used to compute the transport coefficients of potassium ions in helium gas over a wide range of temperature and reduced field strength. A very good agreement of the calculated transport coefficients with the preponderance of the existing experimental data is observed. This agreement leads us to believe that the present interaction potential is accurate to within a few percent at all separations between 4.0 and 10.0 bohr and, consequently, represents the most accurate potential available for the HeK+ system. [ABSTRACT FROM AUTHOR]

Starting from an ab initio symmetry-adapted perturbation theory potential energy surface we have performed converged variational and close-coupling calculations of the bound rovibrational states and of the positions and widths of rotationally predissociating resonances of HeHF and HeDF van der Waals complexes. The energy levels were used to compute transition frequencies in the near-infrared spectra of these complexes corresponding to the simultaneous excitation of vibration and internal rotation in the HF(DF) subunit in the complex. The computed transition energies and other model independent characteristics of the near-infrared spectra are in excellent agreement with the results of high-resolution measurements of Lovejoy and Nesbitt [C. M. Lovejoy and D. J. Nesbitt, J. Chem. Phys. 93, 5387 (1990)]. In particular, the ab initio potential predicts dissociation energies of 7.38 and 7.50 cm-1 for HeHF and HeDF, respectively, in very good agreement with the Lovejoy and Nesbitt results of 7.35 and 7.52 cm-1. The agreement of the observed and calculated linewidths is less satisfactory. We have found, however, that the linewidths are very sensitive to the accuracy of the short-range contribution to the V1(r,R) term in the anisotropic expansion of the potential. By simple scaling of the latter component we have obtained linewidths in very good agreement with the experimental results. We have also found that this scaling introduces a very small (2%) change in the total potential around the van der Waals minimum. [ABSTRACT FROM AUTHOR]

Symmetry-adapted perturbation theory has been applied to compute the HeHF intermolecular potential energy surface for three internuclear distances in the HF subunit. The interaction energy is found to be dominated by the first-order exchange contribution and by the dispersion energy (including the intramonomer correlation effects). However, smaller corrections as the electrostatics, induction, and second-order exchange are found to be nonnegligible, and the final shape of the potential results from a delicate balance of attractive and repulsive contributions due to the four fundamental intermolecular interactions: electrostatics, exchange, induction, and dispersion. For a broad range of He–HF configurations the theoretical potential agrees very well with the empirical potential of Lovejoy and Nesbitt [C. M. Lovejoy and D. J. Nesbitt, J. Chem. Phys. 93, 5387 (1990)], which was adjusted to reproduce the near-infrared spectrum of the complex. Our potential has a global minimum of εm=-39.68 cm-1 for the linear He–HF geometry at Rm=6.16 bohr, and a secondary minimum of εm=-36.13 cm-1 for the linear He–FH geometry at Rm=5.59 bohr. These values are in very good agreement with the corresponding empirical results: εm=-39.20 cm-1 and Rm=6.17 bohr for the global minimum, and εm=-35.12 cm-1 and Rm=5.67 bohr for the secondary minimum. [ABSTRACT FROM AUTHOR]

The first-order exchange energy for the interactions of closed-shell many-electron systems is expanded as a perturbation series with respect to the Mo\ller–Plesset correlation potentials of the monomers. Explicit orbital formulas for the leading perturbation corrections are derived applying a suitable density matrix formalism. The numerical results obtained using the Mo\ller–Plesset perturbation expansion, as well as nonperturbative, coupled-cluster type procedure, are presented for the interactions of He–F-, He–HF, H2–HF, and Ar–H2. It is shown that the correlation part of the first-order exchange energy increases the uncorrelated results by 10% to 30% for the investigated range of configurations. The analysis of the total interaction energies for selected geometries of these systems shows that at the present level of theory the symmetry-adapted perturbation approach correctly accounts for major intramonomer correlation effects and is capable to accurately reproduce the empirical potential energy surfaces. [ABSTRACT FROM AUTHOR]

The second-order induction energy in the symmetry-adapted perturbation theory is expressed in terms of electron densities and polarization propagators at zero frequency of the isolated monomers. This expression is used to derive many-body perturbation series with respect to the Mo\ller–Plesset type correlation potentials of the monomers. Two expansions are introduced—one based on the standard Mo\ller–Plesset expansion of electron densities and polarization propagators, and the second accounting for the so-called response or orbital relaxation effects, i.e., for the perturbation induced modification of the monomer’s Fock operators. Explicit orbital formulas for the leading perturbation corrections that correctly account for the response effects are derived through the second order in the correlation potential. Numerical results are presented for several representative van der Waals complexes—a rare gas atom and an ion Ar–Na+, Ar–Cl-, and He–F-; a polar molecule and an ion H2O–Na+ and H2O–Cl-; two polar molecules (H2O)2; and a rare gas atom and a polar molecule Ar–HCl and He–HCl. It is shown that in the above systems, the significance of the correlation part of the induction energy varies from a very important one in the complexes of rare gas atoms and ions to a practically negligible one in the complexes of rare gases with polar molecules. [ABSTRACT FROM AUTHOR]

Explicitly connected many-body perturbation expansion for the energy of the first-order exchange interaction between closed-shell atoms or molecules is derived. The influence of the intramonomer electron correlation is accounted for by a perturbation expansion in terms of the Mo\ller–Plesset fluctuation potentials WA and WB of the monomers or by a nonperturbative coupled-cluster type procedure. Detailed orbital expressions for the intramonomer correlation corrections of the first and second order in WA+WB are given. Our method leads to novel expressions for the exchange energies in which the exchange and hybrid integrals do not appear. These expressions, involving only the Coulomb and overlap integrals, are structurally similar to the standard many-body perturbation theory expressions for the polarization energies. Thus, the exchange corrections can be easily coded by suitably modifying the existing induction and dispersion energy codes. As a test of our method we have performed calculations of the first-order exchange energy for the He2, (H2)2, and He–H2 complexes. The results of the perturbative calculations are compared with the full configuration interaction data computed using the same basis sets. It is shown that the Mo\ller–Plesset expansion of the first-order exchange energy converges moderately fast, whereas the nonperturbative coupled-cluster type approximations reproduce the full configuration interaction results very accurately. [ABSTRACT FROM AUTHOR]

Many-body perturbation theory for a direct calculation of the electrostatic interaction energy is developed. Since no multipole expansion is used, the obtained electrostatic energy includes the short-range contributions resulting from the overlap (penetration) of monomers’ charge distributions. The influence of intramonomer electronic correlation is accounted for by the perturbation expansion in terms of the Mo\ller–Plesset type fluctuation potentials for the interacting molecules. Two types of expansions are introduced: one based on the standard Mo\ller–Plesset expansion of the electron density, and the second accounting for the perturbation induced modifications of the monomer’s Fock operators, i.e., for the so-called response or orbital relaxation effects. Explicit orbital expressions for the terms through the fourth order in the intramonomer fluctuation potentials are derived. In this way the leading three-particle correlation contribution to the electrostatic energy is taken into account. Numerical results of perturbative calculations through the fourth order for the He2, (H2)2, and He–H2 systems are presented and compared with the complete configuration interaction results obtained using the same basis sets. It is found that the convergence of the many-body perturbation expansion of the electrostatic energy is fast. The sum of corrections through the fourth order reproduces to within few percent the correlation part of the electrostatic energy computed with the full configuration interaction wave function. [ABSTRACT FROM AUTHOR]

Investigates near-infrared absorption spectrum of the argon-hydrogen deuterium (Ar-HD) gas complex. Details on the potential energy surfaces of the complex; Aspects of the AR-HD dynamics; Projection of transition energies in the Q1(1).

Examines the total differential cross sections for the argon-methane scattering complex. Computation of the collision energy; Basis for the close-coupling calculations; Accuracy of the symmetry-adapted perturbation theory.

Examines the characterization of nonadditive interactions in van der Waals molecules using symmetry-adapted perturbation theory of three-body surfaces. Multipole-expanded expressions for the induction, induction-dispersion and dispersion nonadditivities; Significance of exchange quadrupole electrostatic interaction.