Ab initio SCF-CI studies of the intermolecular interaction between two hydrogen molecules near the Van der Waals minimum

SCF-CI calculations have been used to study the intermolecular energy between two hydrogen molecules in four different geometrical configurations. The CI matrix was diagonalized using perturbation techniques. The importance of the perturbation expansion order upon the intermolecular energy could therefore be studied. The wave function includes all singly and doubly excited configurations. The natural orbitals have been determined and their relative importance on the intermolecular energy is considered.     

Ab initio calculation of the first order term of the intermolecular energy near the van der Waals minimum

The first order term of the intermolecular energies between two hydrogen molecules and between Li⁺ and H₂ has been computed by three different methods: two of them are based on a perturbative procedure, including or neglecting the overlap between the orbitals of the interacting molecules or atoms in the calculation of the electrostatic and exchange terms. We can then study the effect of the overlap on each of these terms. The third method is the SCF supermolecule treatment which provides results in very good agreement with the perturbative procedure including the overlap. TheT configuration in the case of two hydrogen molecules and theC _2v configuration for Li⁺+H₂ are stable with respect to the first order term.     

Ab initio SCF MO study of hydrogen bonds between H2S and H2O molecules

The hydrogen bonds between H₂S and H₂O molecules are calculated through anab initio, LCAO MO SCF method using a Gaussian type orbital double-zeta basis set. The capacity of the H₂S molecule to act as an electron acceptor is confirmed. Consultant of the Instituto Mexicano del Petróleo.     

Ab initio potential energy surface and intermolecular vibrations of the naphthalene-argon van der Waals complex

The intermolecular potential energy surface (PES) of the naphthalene-argon (NpAr) complex is constructed using an ab initio method. The molecule-argon interaction energy is computed at the level of the second-order M?ller-Plesset (MP2) theory combined with the augmented correlation consistent polarized valence double-ζ basis set. The analytical PES fitted to a large set of single energy values is further improved with the help of correction functions determined by calculations of the interaction energy at the coupled cluster level including single and double excitations supplemented by triple excitations performed for a limited set of intermolecular configurations. The PES determined is very flat near its four equivalent global minima of -493 cm(-1) located from both sides of the Np plane at a distance of 3.435 A? and shifted from the center of Np by ±0.43 A? along its long symmetry axis. The large-amplitude motion of Ar in the complex is investigated, and dynamical consequence of a strong intermode coupling is discovered in the excited vibrational states. The theoretical results obtained allow for the reassignment of the spectral bands observed in the electronic transition S(1) ← S(0) of the NpAr complex.     

Ab initio calculations of H2-H2 potential surfaces near the van der Waals minimum

The interaction energy between two hydrogen molecules near the van der Waals minimum is computed, for four relative orientations, and for intermolecular distances ranging from 5.5 to 16 au. A partially optimized basis set limited to 52 independent gaussian functions was used throughout the energy calculations. A new method based on the polarized atomic orbital technique has been used to reduce subsequently the size of the CI calculations which makes this method tractable for heavier molecular systems than H₂-H₂.     

Ab initio ground- and excited-state intermolecular potential energy surfaces for the NO-Ne and NO-Ar van der Waals complexes

The ground- [NO(X(2)Π)] and excited-state [NO(A(2)Σ(+))] intermolecular potential energy surfaces (IPESs) of the NO-Ne and NO-Ar van der Waals complexes are evaluated using the RCCSD(T) spin-restricted coupled cluster method and d-aug-cc-pVQZ basis set extended with a set of 3s3p2d1f1g midbond functions. These bases are selected from the results of a systematic basis-set convergence study carried out for the NO(A(2)Σ(+))-Ar state. We fit the interaction energies to analytic functions and compare the results to those previously available. The NO-Ar (NO-Ne) IPESs are characterized by absolute minima of -120 and -75 cm(-1) (-58 and -5 cm(-1)) at the ground and first excited state, respectively, located close to the T-shaped geometries for the ground states and at linear dispositions in the case of the excited states. The potentials are further used in the evaluation of the rovibrational spectra of the complexes, and the results are compared to those available in the literature.     

Ab initio characterization of the Mg-HF van der Waals complex

The equilibrium structure and the three-dimensional potential energy surface of the Mg-HF van der Waals complex in its ground electronic state have been determined from accurate ab initio calculations using the coupled-cluster method, CCSD(T), in conjunction with the basis sets of triple- through quintuple-zeta quality. The core-electron correlation, high-order valence-electron correlation, and scalar relativistic effects were investigated. The Mg-HF complex was confirmed to be linear at equilibrium, with a vibrationless dissociation energy (into Mg and HF) D(e) of 280 cm(-1). The vibration-rotation energy levels of two isotopologues, (24)Mg-HF and (24)Mg-DF, were predicted using the variational method. The predicted spectroscopic constants can be useful in a further analysis of high-resolution vibration-rotation spectra of the Mg-HF complex.     

Ab initio intermolecular potential energy surface, bound states, and microwave spectra for the van der Waals complex Ne-HCCCN

We report two ab initio intermolecular potential energy surfaces for Ne-HCCCN using a supermolecular method. The calculations were performed at the fourth-order M?ller-Plesset (MP4) and the coupled cluster singles-and-doubles with noniterative inclusion of connected triples [CCSD(T)] levels with the full counterpoise correction for the basis set superposition error and a large basis set including bond functions. The complex was found to have a planar T-shaped structure minimum and a linear minimum with the Ne atom facing the H atom. The two-dimensional discrete variable representation method was employed to calculate the rovibrational bound states. In addition, the microwave spectra including intensities for the ground vibrational state were predicted. The results show that the spectrum is dominated by b-type (DeltaK(a) = +/-1) transitions with very weak a-type (DeltaK(a) = 0) transitions. The calculated results at the CCSD(T) potential are in good agreement with those at MP4 potential.     

Ab initio characterization of the Ne-I2 van der Waals complex: intermolecular potentials and vibrational bound states

A theoretical study of the potential energy surface and bound states is performed for the ground state of the NeI(2) van der Waals (vdW) complex. The three-dimensional interaction energies are obtained from ab initio coupled-cluster, coupled-cluster single double (triple)/complete basis set, calculations using large basis sets, of quadruple- through quintuple-zeta quality, in conjunction with relativistic effective core potentials for the heavy iodine atoms. For the analytical representation of the surface two different schemes, based on fitting and interpolation surface generation techniques, are employed. The surface shows a double-minimum topology for linear and T-shaped configurations. Full variational quantum mechanical calculations are carried out using the model surfaces, and the vibrationally averaged structures and energetics for the NeI(2) isomers are determined. The accuracy of the potential energy surfaces is validated by a comparison between the present results and the corresponding experimental data available. In lieu of more experimental measurements, we also report our results/predictions on higher bound vibrational vdW levels, and the influence of the employed surface on them is discussed.     

Study of the hydrogen bond and of the proton transfer between two H2S molecules

The potential energy surface for the H₂S dimer is calculated as the sum of the SCF-MO-LCGO energy with a new, modified, basis set and the estimated dispersion energy. Proton affinities for SH^– and H₂S, and, as their difference, the energy of the proton transfer between two H₂S molecules, are also calculated. Despite the limited basis set used, the results are consistent with experimental data.This work was partly supported by the Polish Academy of Sciences within the project PAN-3.     

Ab initio Calculations of the potential energy surface of the reaction of singlet methylene with the hydrogen molecule

The potential energy surface for the insertion of singlet methylene into H₂ has been computed on theab initio SCF level as well as with inclusion of electron correlation by means of the CEPA method. The results are compared with those of previous semiempirical,ab initio SCF and CI calculations. The system is a prototype of a reaction where an allowed and a symmetry-forbidden path can compete. The electron correlation energy was found to be very different for different regions of the surface, but did not have much influence on the optimum reaction path. From the computed heat of the reaction, the heat of formation of singlet methylene was estimated to be 101.5 kcal/mol. According to the calculations the reaction does not need any activation energy.     

Ab initio investigation of the NH(X)-N2 van der Waals complex

The NH-N(2) van der Waals complex has been examined at the CCSD(T) level of theory using aug-cc-pVDZ and aug-cc-pVTZ basis sets. The full basis set superposition error correction was applied. Two minimum energy structures were located for the electronic ground state. The global minimum corresponds to a linear geometry of the complex (NH-N-N), with D(e)=236 cm(-1) and R(c.m.)=4.22 A. The secondary minimum corresponds to a T-shaped geometry of C(2v) symmetry, where the nitrogen atom of the H-N moiety points toward the center of mass of the N(2) unit, aligned with the a-inertial axis of the complex. The binding energy and R(c.m.) value for the secondary minimum were 144 cm(-1) and 3.63 A, respectively. This potential energy surface is consistent with the properties of matrix-isolated NH-N(2), and it is predicted that linear NH-N(2) will be a stable complex in the gas phase at low temperatures.     

Lithium ions in the van der Waals gap of Bi2Se3 single crystals

Insertion/extraction of lithium ions into/from Bi₂Se₃ crystals was investigated by means of cyclic voltammetry. The process of insertion is reflected in the appearance of two bands on voltammograms at ∼1.7 and ∼1.5 V, corresponding to the insertion of Li⁺ ions into octahedral and tetrahedral sites of the van der Waals gap of these layered crystals. The process of extraction of Li⁺ ions from the gap results in the appearance of four bands on the voltammograms. The bands 1 and 2 at ∼2.1 and ∼2.3 V correspond to the extraction of a part of Li⁺ guest ions from the octahedral and tetrahedrals sites and this extraction has a character of a reversible intercalation/deintercalation process. A part of Li⁺ ions is bound firmly in the crystal due to the formation of negatively charged clusters of the (LiBiSe₂.Bi₃Se₄⁻) type. A further extraction of Li⁺ ions from the van der Waals gap is associated with the presence of bands 3 and 4 placed at ∼2.5 and ∼2.7 V on the voltammograms as their extraction needs higher voltage due to the influence of negative charges localized on these clusters.     

Ab initio calculations on octahedral (CH) x (NH)6−x

Octahedral 38-, 44- and 48-electron systems are closed-shells and could be stable. The latter two systems have a high energy and dissociate via a non-symmetric path. (NH)₆ in a chair conformation should be stable.     

Ab initio calculations of intermolecular potentials. The ground state of the Ar?H2 van der Waals molecule

Hartree–Fock computations of the potential surface of Ar? H₂ have been carried out and supplemented with calculations of the dispersion energy, with use of the counterpoise method to remove the basis set superposition error. The collinear and perpendicular bisector geometries are considered. The resulting potentials agree quite well with the actual experimental data.     

Vibrational frequencies and force constants of N2O4 in complexes with molecules of different solvents determined by Raman spectra andAb initio calculation

The Raman spectra of N₂O₄ solutions in organic solvents have been recorded. The frequencies ofv ₁,v ₂, andv ₃ bands of N₂O₄ increase with increasing solvent electron-donor properties. Especially large changes ofv ₃ N-N stretching band have been observed (254.5 cm^–1 in n-hexane, 276.5 cm^–1 in 1,4-dioxane). The ab initio calculations have shown that the interaction between N₂O₄ and electron-donor molecules causes an increase of N-N and N-O stretching and O-N-O bending force constants of N₂O₄ in agreement with the results of Raman study.     

Ab initio intermolecular energies between Li+ and H2 near the van der waals minimum: comparison between a perturbative procedure and an SCF supermolecule treatment

The electrostatic, exchange, polarization, and dispersion energies are computed without using the multipole expansion. It is seen that the overlap between the orbitals of the interacting molecules must be taken into account to describe qualitatively the first order term. The neglect of overlap in the polarization term overestimates the attractive energy around the van der Waals minimum. An interpretation of the polarization phenomena in terms of molecular orbitals is proposed. The results are compared with SCF calculations and the use of small basis sets is considered.