Three-Dimensional ab Initio Potential and Ground State Dynamics of the HeI2 Complex

Computational Modeling: Up-to-Date Approaches and Cutting-Edge Applications from Clusters, Nanostructures to Bulk Systems

The contributions in this special theme collection, in honor to Prof. P. Villarreal, cover a broad variety of computational methodologies and experimental techniques, containing studies on gas phase, clusters and condensed phase systems.

Potential Energy Surface and Bound States of Ne-Li2+(X2Σg+) van der Waals Complex Based on Ab Initio Calculations

Theoretical studies of the potential energy surface and vibrational bound states calculations were performed for the ground state of the Ne-Li2+(X2Σg+) van der Waals (vdW) complex. The intermolecular interactions were investigated by using an accurate monoconfigurational RCCSD(T) method and large basis sets (aug-cc-pVnZ, n = T, Q, 5), extrapolated to the complete basis set (CBS) limit. In turn, the obtained raw data from RCCSD(T)/CBS(Q5) calculations were numerically interpolated using the Morse + vdW model and the Reproducing Kernel Hilbert Space (RKHS) polynomial method to generate analytic expressions for the 2D-PES. The RKHS interpolated PES was then used to assess the bound states of the Ne-Li2+(X2Σg+) system through nuclear quantum calculations. By studying the aspect of the potential energy surface, the analysis sheds light on the behavior of the Ne-Li2+(X2Σg+) complex and its interactions between repulsive and attractive forces with other particles. By examining the vibrational states and wave functions of the system, the researchers were able to gain a better understanding of the behavior of the Ne-Li2+(X2Σg+) complex. The calculated radial and angular distributions for all even and odd symmetries are discussed in detail. We observe that the radial distributions exhibit a more complicated nodal structure, representing stretching vibrational behavior in the neon atom along its radial coordinate. For the highest bound states, the situation is very different, and the energies surpass the angular barrier.

Insight into the halogen-bond nature of noble gas-chlorine systems by molecular beam scattering experiments, ab initio calculations and charge displacement analysis

We have carried out molecular-beam scattering experiments and high-level ab initio investigations on the potential energy surfaces of a series of noble-gas-Cl2 adducts. This effort has permitted the construction of a simple, reliable and easily generalizable analytical model potential formulation, which is based on a few physically meaningful parameters of the interacting partners and transparently shows the origin, strength, and stereospecificity of the various interaction components. The results demonstrate quantitatively beyond doubt that the interaction between a noble-gas (Ng) atom - even He - and Cl2 in a collinear configuration is characterized by weak halogen bond (XB) formation, accompanied by charge transfer (CT) from the Ng to chlorine. This characteristic, which stabilizes the adduct, rapidly disappears on going towards the T-shaped configuration, dominated by pure van der Waals (vdW) forces. Similarly, a pure vdW interaction takes place - with no CT component in any configuration - if Cl2 is present in the lowest πg* → σu* excited state, because the change in electron density that accompanies the excitation eliminates the Cl2 polar flattening and σ hole, making the XB interaction inaccessible.

Investigations of the Rg-BrCl (Rg=He, Ne, Ar, Kr, Xe) binary van der Waals complexes: ab initio intermolecular potential energy surfaces, vibrational states and predicted pure rotational transition frequencies

The intermolecular potential energy surfaces (PESs) of the ground electronic state for the Rg-BrCl (Rg=He, Ne, Ar, Kr, Xe) van der Waals complexes have been constructed by using the coupled-cluster method in combination with the augmented quadruple-zeta correlation-consistent basis sets supplemented with an additional set of bond functions. The features of the anisotropic PESs for these complexes are remarkably similar, which are characterized by three minima and two saddle points between them. The global minimum corresponds to a collinear Rg-Br-Cl configuration. Two local minima, correlate with an anti-linear Rg-Cl-Br geometry and a nearly T-shaped structure, can also be located on each PES. The quantum bound state calculations enable us to investigate intermolecular vibrational states and rotational energy levels of the complexes. The transition frequencies are predicted and are fitted to obtain their corresponding spectroscopic constants. In general, the periodic trends are observed for this complex family. Comparisons with available experimental data for the collinear isomer of Ar-BrCl demonstrate reliability of our theoretical predictions, and our results for the other two isomers of Ar-BrCl as well as for other members of the complex family are also anticipated to be trustable. Except for the collinear isomer of Ar-BrCl, the data presented in this paper would be beneficial to improve our knowledge for these experimentally unknown species.

Preferential stabilization of HeI2 van der Waals isomers: the effect of energetics and temperature

Energetics and temperature dependence on the preferential stabilization of the linear/T-shaped HeI₂ isomers.

Temperature Dependence of HeBr2 Isomers' Stability through Rovibrational Multiconfiguration Time-Dependent Hartree Calculations

The multiconfiguration time-dependent Hartree (MCTDH) method using a six-dimensional Hamiltonian that includes all rotational and vibrational degrees of freedom and an ab initio potential energy surface was employed to calculate the rovibronic states of the HeBr₂ van der Waals complex. All rotational states of energies within 7 cm ^-1 with respect to the energy of the linear ground state were calculated without restriction of the total angular momentum. In total, we obtained 500 and 320 rotationally excited states of the ground vibrational T-shaped and linear isomers of the HeBr₂, respectively, and compared them with those predicted by the rigid rotor model. A thermodynamic model was then introduced to determine the relative stability of the two conformers as a function of the temperature. On the basis of the present results, the linear conformers were found to be energetically more stable than the T-shaped ones by 1.14 cm^-1 at T = 0 K, whereas conversion from linear to T-shaped complexes was observed at temperatures above 2.87 K.

Theoretical investigation of the He-I2(E3Πg) ion-pair state: ab initio intermolecular potential and vibrational levels

We present a theoretical study on the potential energy surface and vibrational bound states of the E electronic excited state of the HeI(2) van der Waals system. The interaction energies are computed using accurate ab initio methods and large basis sets. Relativistic small-core effective core potentials in conjunction with a quintuple-zeta quality basis set are employed for the heavy iodine atoms in multireference configuration interaction calculations for the (3)A' and (3)A" states. For the representation of the potential energy surface we used a general interpolation technique for constructing potential surfaces from ab initio data based on the reproducing kernel Hilbert space method. The surface presents global and local minima for T-shaped configurations with well-depths of 33.2 and 4.6 cm(-1), respectively. Vibrational energies and states are computed through variational quantum mechanical calculations. We found that the binding energy of the HeI(2)(E) T-shaped isomer is 16.85 cm(-1), in excellent agreement with recent experimental measurements. In lieu of more experimental data we also report our predictions on higher vibrational levels and we analyze the influence of the underlying surface on them. This is the first attempt to represent the potential surface of such a highly excited electronic state of a van der Waals complex, and it demonstrates the capability of the ab initio technology to provide accurate results for carrying out reliable studies to model experimental data.

Full-dimensional multi configuration time dependent Hartree calculations of the ground and vibrationally excited states of He2,3Br2 clusters

Quantum dynamics calculations are reported for the tetra-, and penta-atomic van der Waals He(N)Br(2) complexes using the multiconfiguration time-dependent Hartree (MCTDH) method. The computations are carried out in satellite coordinates, and the kinetic energy operator in this set of coordinates is given. A scheme for the representation of the potential energy surface based on the sum of the three-body HeBr(2) interactions at CSSD(T) level plus the He-He interaction is employed. The potential surfaces show multiple close lying minima, and a quantum description of such highly floppy multiminima systems is presented. Benchmark, full-dimensional converged results on ground vibrational/zero-point energies are reported and compared with recent experimental data available for all these complexes, as well as with previous variational quantum calculations for the smaller HeBr(2) and He(2)Br(2) complexes on the same surface. Some low-lying vibrationally excited eigenstates are also computed by block improved relaxation calculations. The binding energies and the corresponding vibrationally averaged structures are determined for different conformers of these complexes. Their relative stability is discussed, and contributes to evaluate the importance of the multiple-minima topology of the underlying potential surface.

Probing the dependence of long-range, four-atom interactions on intermolecular orientation: 3. Hydrogen and iodine

Two-laser, action spectroscopy experiments have been performed in the I(2)B-X, υ'-0 spectral region on H(2)···I(2) and D(2)···I(2) complexes to investigate the dependence of the H(2)/D(2) + I(2) intermolecular interactions on orientation. The spectra contain features associated with at least two different conformers of the ground-state H(2)/D(2)···I(2)(X,υ'' = 0) complexes; one conformer has a preferred T-shaped geometry with the H(2)/D(2) moiety localized in a potential minimum that is orthogonal to the I-I bond axis, and the second conformer has a linear geometry with the H(2)/D(2) moiety positioned in minima at either end of the I(2) molecule, along the bond axis. Those features associated with complexes containing para-H(2)(j = 0), ortho-H(2)(j = 1), ortho-D(2)(j = 0), and para-D(2)(j = 1) are also assigned. The linear conformers are found to be more strongly bound than the T-shaped conformers with binding energies of 118.9(1.9) cm(-1) versus 91.3-93.3 cm(-1) for the ortho-H(2)···I(2) complexes and 144.2(2.1) cm(-1) versus 107.9 cm(-1) for the para-D(2)···I(2) complexes, respectively. Electronic structure calculations of the complexes containing ICl and I(2) with H(2), He, Ne, and Ar were performed to reveal the nature of the interactions and to shed insight into the origins of the different binding energies. The most stable minima in the H(2)/D(2) + I(2)(B,υ') excited-state potentials have T-shaped geometries. Calculated energies and probability amplitudes of the excited-state levels provide insight into the different excited-state intermolecular vibrational levels accessed by transitions of the two ground-state conformers.

Spectroscopic identification of higher-order rare gas-dihalogen complexes with different geometries: He(2,3)...Br(2) and He(2,3)...ICl

Rovibronic transitions of multiple conformers of the He(2)...(79)Br(2)(X, v'' = 0), He(3)...(79)Br(2)(X, v'' = 0), He(2)...I(35)Cl(X, v'' = 0), and He(3)...I(35)Cl(X, v'' = 0) complexes stabilized in a pulsed, supersonic expansion are observed in action spectra recorded in the B-X region of the dihalogens. In addition to features associated with He(2)...(79)Br(2) and He(2)...I(35)Cl complexes with the rare gas atoms localized in the toroidal potential well lying in a plane perpendicular to the dihalogen bond, those associated with a ground-state conformer that has one He atom localized in the toroidal potential and the other He atom localized in the linear well at the end of the dihalogen moiety are also identified. Transitions of at least three conformers of the He(3)...Br(2) complex and two conformers of the He(3)...ICl complex are also observed. The relative populations of the different conformers are found to depend on where along the supersonic expansion the spectra are recorded, and thus on the local temperature regime sampled. The He(2)...(79)Br(2) and He(2)...I(35)Cl conformers with one He atom in each well are found to be the more stable conformers.

Ab initiovibrational predissociation dynamics of He–I2(B) complex

Three-dimensional quantum mechanical calculations on the vibrational predissociation dynamics of HeI2 B state complex are performed using a potential energy surface accurately fitted to unrestricted open-shell coupled cluster ab initio data, further enabling extrapolation for large I2 bond lengths. A Lanczos iterative method with an optimized complex absorbing potential is used to determine energies and lifetimes of the vibrationally predissociating He,I2(B,v') complex for v'<or=26 of I2 vibrational excitations. The calculated predissociating state energies agree with recent experimental results within 0.5 cm(-1). This excellent agreement is remarkable since no adjustment was made with respect to the experiments. The present ab initio approach, however, shows its limitations in the fact that the computed lifetimes that are highly sensitive to subtle details of the potential energy surfaces such as anisotropy, are a factor of 1.5 larger than the available experimental data.

Ab initio potential energy surface and spectrum of the B(Π3) state of the HeI2 complex

The three-dimensional interaction potential for I2(B 3Pi0u+)+He is computed using accurate ab initio methods and a large basis set. Scalar relativistic effects are accounted for by large-core relativistic pseudopotentials for the iodine atoms. Using multireference configuration interaction calculations with subsequent treatment of spin-orbit coupling, it is shown for linear and perpendicular structures of the complex that the interaction potential for I2(B 3Pi0u+)+He is very well approximated by the average of the 3A' and 3A" interaction potentials obtained without spin-orbit coupling. The three-dimensional 3A' and 3A" interaction potentials are computed at the unrestricted open-shell coupled-cluster level of theory using large basis sets. Bound state calculations based on the averaged surface are carried out and binding energies, vibrationally averaged structures, and frequencies are determined. These results are found to be in excellent accord with recent experimental measurements from laser-induced fluorescence and action spectra of HeI2. Furthermore, in combination with a recent X-state potential, the spectral blueshift is obtained and compared with available experimental values.

Experimental and theoretical investigations of the He⋯I2 rovibronic spectra in the I2 B–X, 20–0 region

The laser-induced fluorescence and action spectra of I2 in a helium supersonic expansion have been recorded in the I2 B-X, 20-0 region. Two features are identified within the spectra. The lower-energy feature arises from transitions between states that are localized in a T-shaped conformation on both the X- and B-state potentials. The higher-energy feature reflects transitions from states that are localized in a linear conformation on the X state to states that have energies that are larger than the barrier for free rotation of the rare gas atom about the I2 molecule on the B-state potential. Ground-state binding energies of 16.6(6) and 16.3(6) cm-1 were determined for the T-shaped and linear conformers, respectively. These spectra are compared to those calculated using the experimentally determined rotational temperatures. Based on the agreement between the experimental and calculated spectra, the binding energies of the J'=0 states with 0 and 2-6 quanta of excitation in the He...I2 bending mode on the B state were determined. Several models for the B-state potential were used to investigate the origins of the shape of the contour of the higher-energy feature in the spectra for He...I2 and He...Br2. The shape of the contours was found to be relatively insensitive to the choice of potential. This leads us to believe that the spectra of these systems are relatively insensitive to the parameters of the B-state potential energy surface and are more sensitive to properties of the halogen molecule.

A theoretical study of He2ICl van der Waals cluster

The structure, energetics, and dynamics of He2ICl complex in its ground state are studied by means of ab initio electronic structure and quantum-mechanical calculations. Interaction energies for selected He2ICl configurations are calculated at the coupled-cluster [CCSD(T)] level of theory using a large-core pseudopotential for the I atom and the aug-cc-pVTZ and aug-cc-pV5Z basis sets for the Cl and He atoms, respectively. The surface is characterized around its lower five minima and the minimum energy pathways through them. The global minimum of the potential corresponds to a "police-nightstick (1)" configuration, the second one to a linear, the next one to tetrahedral configuration, and the following two to "bifork" and "police-nightstick (2)" structures, with well depths of -99.12, -97.42, -88.32, -85.84, and -78.54 cm(-1), respectively. An analytical form based on the sum of the three-body parametrized HeICl interactions plus the He-He interaction is found to represent very well the tetra-atomic CSSD(T) results. The present potential expression is employed to perform variational five-dimensional quantum-mechanical calculations to study the vibrational bound states of the van der Waals He2ICl complex. Results for total angular momentum J = 0 provide the binding energy D0 and the corresponding vibrationally averaged structure for different isomers of the cluster. Comparison of these results with recent experimental observations further justifies the potential used in this work.

An empirical potential-energy surface for the He–I2(BΠu3) van der Waals complex including three-body effects

An empirical intermolecular potential surface is proposed for the He-I2(B 3IIu) complex, modeled as a sum of pairwise He-I Morse interactions plus a three-body interaction term. The potential reproduces with very good agreement the spectral blueshifts and vibrational predissociation lifetimes measured for He-I2(B,v') in the range v' = 10-67 of I2 vibrational excitations. In particular, the accuracy achieved in the description of the experimental data for high v' levels is attributed to the three-body interaction term included in the potential. The behavior of the potential surface with the I-I separation is analyzed and correlated with the experimental findings.

Structure and energy difference of two isomers of He-CH3F

The intermolecular potential surface of He-CH(3)F is investigated through ab initio calculations and microwave and millimeter-wave spectroscopies. The intermolecular potential is calculated at the fourth-order Møller-Plesset level with a large basis set including bond functions. Three minimums exist, the deepest of which is at the carbon end of the C-F axis and has a depth of 46.903 cm(-1), the second deepest is in a T-shaped position relative to the C-F axis with a depth of 44.790 cm(-1), and the shallowest is at the fluorine end of the C-F axis with a depth of 30.929 cm(-1). The barrier to internal rotation of the CH(3)F subunit about its C-F axis is very low, thus leading to essentially free internal rotation and two separate sets of bound states correlating to ortho-CH(3)F (|K| = 3n) for the ground, or A, internal rotor state upon which this study focuses, and to para-CH(3)F (|K| = 3n +/- 1) for the excited, or E, internal rotor state. Bound-state calculations of the A state performed using two different techniques show the lowest-energy state to have the helium localized in the T-shaped well with an energy of -11.460 cm(-1), while two excited configurations of the A state have the helium localized either in the well at the carbon end ("linear") with an energy of -7.468 cm(-1) or in the well at the fluorine end ("antilinear") with an energy of -4.805 cm(-1). Spectroscopic observations confirm the predicted energy-level structure of the ground and first excited states. Sixteen transitions between 12 distinct energy levels have been observed, including pure rotational transitions of both the T-shaped ground state and the linear excited state, as well as rovibrational transitions between the ground state and the linear excited state. The energy difference between the T-shaped state and the linear state is measured to be 132 374.081(16) MHz. There is significant Coriolis mixing of the ground state J(K(a)K(c)) = 2(20) and the linear J(K) = 2(0) levels which aided in the observation of the T to linear transitions. This mixing and the T to linear energy difference are sensitive probes of the relative well depths of the two lowest minimums and are well predicted by the ab initio potential. Improved agreement between experiment and theory is obtained by morphing the correlation energy of the potential. He-CH(3)F is one of just a few atom-molecule complexes for which the ground-state geometry does not coincide with the global potential minimum.