A three-dimensional ab initio potential energy surface and predicted infrared spectra for the He-N2O complex

We report a three-dimensional ab initio potential energy surface for He-N(2)O using a supermolecular method at the coupled-cluster singles and doubles with noniterative inclusion of connected triple level. Besides the intermolecular stretching and bending modes, we included the Q(3) normal mode for the nu(3) antisymmetric stretching vibration of N(2)O molecule in order to simulate the observed infrared spectra in the nu(3) region of N(2)O, especially to explain the frequency shift of the band origin in the infrared spectra. The harmonic oscillator approximation is used for the potential curve of the Q(3) mode of the isolate N(2)O molecule. The intermolecular potential energy surfaces are calculated for five potential-optimized discrete variable representation grid points of the Q(3) mode. The three-dimensional discrete variable representation method was employed to calculate the rovibrational states without separating the inter- and intramolecular nuclear motions. The calculated transition frequencies and line intensities of the rotational transitions in the nu(3) region of N(2)O for the van der Waals ground vibrational state are in good agreement with the observed infrared spectra. The calculated band shifts are found to be 0.1704 and 0.1551 cm(-1) for (4)He-N(2)O and (3)He-N(2)O, respectively, which agree well with the observed values of 0.2532 and 0.2170 cm(-1).     

Three-dimensional ab initio potential-energy surface and rovibrational spectra of the H2-Kr complex

We report a three-dimensional ab initio potential-energy surface for the H2-Kr complex calculated using a supermolecular method. The electronic calculations were performed at the coupled-cluster singles and doubles level with noniterative inclusion of connected triples levels with a large basis set including midbond functions and the full counterpoise correction for the basis-set superposition error. The intermolecular potential energy between the H2 molecule and the Kr atom were evaluated at five potential-optimized discrete variable representation (DVR) grid points generated from the potential-energy curve of H2. The potential for other bond lengths of H2 could be deduced using polynomial interpolations. The complex is found to have a linear preferred structure with a rather flat energy barrier. The three-dimensional DVR method and the Lanczos propagation algorithm were employed to calculate the rovibrational states without separating the inter- and intramolecular nuclear motions. In addition, the rovibrational spectra from the H2 fundamental vibrational band were calculated. The calculated shift for the band origin is -1.50 cm-1, which is in good agreement with the experimental value of -1.706 cm-1, and the calculated transition frequencies in Q1(0) and S1(0) bands are within 3% of the observed values.     

Near-infrared spectra and rovibrational dynamics on a four-dimensional ab initio potential energy surface of (HBr)2

Supersonic jet investigations of the (HBr)(2) dimer have been carried out using a tunable diode laser spectrometer to provide accurate data for comparison with results from a four-dimensional (4-D) ab initio potential energy surface (PES). The near-infrared nu(1) (+/-), nu(2) (+/-), and (nu(1)+nu(4))(-) bands of (H (79)Br)(2), (H (79)Br-H (81)Br), and (H (81)Br)(2) isotopomers have been recorded in the range 2500-2600 cm(-1) using a CW slit jet expansion with an upgraded near-infrared diode laser spectrometer. The 4-D PES has been calculated for (HBr)(2) using second-order M?ller-Plesset perturbation theory with an augmented and polarized 6-311G basis set. The potential is characterized by a global minimum occurring at the H bond structure with the distance between the center of masses (CM) of the monomer being R(CM)=4.10 A with angles theta(A)=10 degrees, theta(B)=100 degrees and a well depth of 692.2 cm(-1), theta(A) is the angle the HBr bond of monomer A makes with the vector from the CM of A to the CM of B, and theta(B) is the corresponding angle monomer B makes with the same CM-CM vector. The barrier for the H interchange occurs at the closed C(2h) structure for which R(CM)=4.07 A, theta(A)=45 degrees, theta(B)=135 degrees, and the barrier height is 73.9 cm(-1). The PES was fitted using a linear-least squares method and the rovibrational energy levels of the complex were calculated by a split pseudospectral method. The spectroscopic data provide accurate molecular parameters for the dimer that are then compared with the results predicted on the basis of the 4-D ab initio PES.     

Full-dimensional vibrational calculations for H5O2+ using an ab initio potential energy surface

We report quantum diffusion Monte Carlo (DMC) and variational calculations in full dimensionality for selected vibrational states of H(5)O(2) (+) using a new ab initio potential energy surface [X. Huang, B. Braams, and J. M. Bowman, J. Chem. Phys. 122, 044308 (2005)]. The energy and properties of the zero-point state are focused on in the rigorous DMC calculations. OH-stretch fundamentals are also calculated using "fixed-node" DMC calculations and variationally using two versions of the code MULTIMODE. These results are compared with infrared multiphoton dissociation measurements of Yeh et al. [L. I. Yeh, M. Okumura, J. D. Myers, J. M. Price, and Y. T. Lee, J. Chem. Phys. 91, 7319 (1989)]. Some preliminary results for the energies of several modes of the shared hydrogen are also reported.     

A new ab initio intermolecular potential energy surface and predicted rotational spectra of the Ne-H2S complex

We report a new three-dimensional ab initio intermolecular potential energy surface for the Ne-H(2)S complex with H(2)S monomer fixed at its experimental average structure. Using the supermolecular approach, the intermolecular potential energies were evaluated at CCSD(T) (coupled cluster with single and double and perturbative triple excitations) level with large basis sets including bond functions. The full counterpoise procedure was employed to correct the basis set superposition error. The planar T-shaped global minimum is located at the intermolecular distance of 3.51 ? with a well depth of 71.57 cm(-1). An additional planar local minimum was found to be separated from the global minimum with an energy barrier of 23.11 cm(-1). In addition, two first-order and one second-order saddle points were also located. The combined radial discrete variable representation/angular finite basis representation method and the Lanczos algorithm were employed to evaluate the rovibrational energy levels for eight isotopic species of the Ne-H(2)S complexes. The rotational transition frequencies for the eight isotopomers were also determined for the ground and first vibrational excited states, which are all in very good agreement with the available experimental values.     

A new ab initio intermolecular potential energy surface and predicted rotational spectra of the Ar-H2S complex

We report a reliable three-dimensional ab initio intermolecular potential energy surface for the Ar-H(2)S complex with H(2)S monomer fixed at its experimental average structure. The potential energies were evaluated using the supermolecular approach at the coupled-cluster level with a large basis set including bond functions. The full counterpoise procedure was used to correct the basis set superposition error. The potential has a planar T-shaped global minimum with a well depth of 177.48 cm(-1) at the intermolecular distance of 3.72 ?. An additional planar local minimum is also found and is separated from the global minimum with an energy barrier with a height of 47.46 cm(-1). The combined radial discrete variable representation/angular finite basis representation method and the Lanczos algorithm were employed to calculate the rovibrational energy levels for three isotopic species of Ar-H(2)S complexes (Ar-H(2)(32)S, Ar-H(2)(33)S, and Ar-H(2)(34)S). The rotational transition frequencies and structural parameters for the three isotopomers were also determined for the ground and the first excited states, which are all in good agreement with the available experimental values.     

F2 dimer: Improved intermolecular potential energy surface using ab initio calculations

A computational study on the intermolecular potential energy of 44 different orientations of F₂ dimers is presented. Basis set superposition error (BSSE) corrected potential energy surface is calculated using the supermolecular approach at CCSD(T) and QCISD(T) levels of theory. The interaction energies obtained using the aug‐cc‐pVDZ and aug‐cc‐pVTZ basis sets are extrapolated to the complete basis set limit using the latest extrapolation scheme. The basis set effect is checked and it is found that the extrapolated intermolecular energies provide the best compromise between the accuracy and computational cost. Among 1320 energy points of F₂–F₂ system covering more relative orientations, the most stable structure of the dimers was obtained with a well depth of ?146.62 cm^?1 that related to cross configuration, and the most unstable structure is related to linear orientation with a well depth of ?52.63 cm^?1. The calculated second virial coefficients are in good agreement with experimental data. The latest extrapolation scheme of the complete basis set limit at the CCSD(T) level of theory is used to determine the intermolecular potential energy surface of the F₂ dimer. Comparing the results obtained by the latest scheme with those by older schemes show that the new approach provides the best compromise between accuracy and computational cost.     

New double many-body expansion potential energy surface for ground-state HCN from a multiproperty fit to accurate ab initio energies and rovibrational calculations

An accurate single-sheeted double many-body expansion potential energy surface has been obtained for the ground electronic state of the hydrogen cyanide molecule via a multiproperty fit to ab initio energies and rovibrational data. This includes 106 rovibrational levels and 2313 discrete points, which are fit with a rmsd of 4 cm(-1) and 2.42 kcal mol(-1), respectively, and seven zero first-derivatives that are reproduced at three stationary points. Since the potential also describes accurately the appropriate asymptotic limits at the various dissociation channels, it is commended both for the simulation of rovibrational spectra and reaction dynamics.     

Intermolecular potential energy surface of the N2-CO dimer: ab initio investigation and analytical representation

In this work, for the first time, an analytical four-dimensional representation for the intermolecular potential of the N(2)-CO dimer is constructed from ab initio calculations. The most stable structure of dimer is found to be a distorted T-shape conformation with CO forming the top and N(2) the leg of T. Important structures of the dimer are characterized, and surprisingly, it is found that in contrast with general assumptions, the potential energy surface of the N(2)-CO dimer has a single symmetry unique minimum. The energy profile of a minimum energy path that connects two T-shaped saddle points to the minimum structure is derived. Important structures are characterized along this path to represent the concerted internal rotation of monomers within the complex. The second virial coefficient is calculated from the fitted PES, and reasonable agreement is found with recent experimental results.     

An ab initio correlated study of the potential energy surface for the HOBr.H2O complex

The potential energy surface (PES) for the HOBr.H(2)O complex has been investigated using second- and fourth-order M?ller-Plesset perturbation theory (MP2, MP4) and coupled cluster theory with single and doubles excitations (CCSD), and a perturbative approximation of triple excitations (CCSD-T), correlated ab initio levels of theory employing basis sets of triple zeta quality with polarization and diffuse functions up to the 6-311++G(3dp,3df ) standard Pople's basis set. Six stationary points being three minima, two first-order transition state (TS) structures and one second-order TS were located on the PES. The global minimum syn and the anti equilibrium structure are virtually degenerated [DeltaE(ele-nuc) approximately 0.3 kcal mol(-1), CCSD-T/6-311++G(3df,3pd) value], with the third minima being approximately 4 kcal mol(-1) away. IRC analysis was performed to confirm the correct connectivity of the two first-order TS structures. The CCSD-T/6-311++G(3df,3pd)//MP2/6-311G(d,p) barrier for the syn<-->anti interconversion is 0.3 kcal mol(-1), indicating that a mixture of the syn and anti forms of the HOBr.H(2)O complex is likely to exist.     

Vibrational spectra, ab initio calculations, and ring-puckering potential energy function for gamma-crotonolactone

The infrared and Raman spectra of liquid and vapor gamma-crotonolactone have been collected. Both the experimental data and ab initio calculations show that the molecule is rigidly planar in its electronic ground state. This conclusion agrees with the previously reported microwave studies and is attributed to the conjugation between the C=C and C=O double bonds of the ring. The ring-puckering potential energy function was generated from ab initio calculations and was confirmed by the vapor-phase Raman spectra to be nearly harmonic. Density functional theory (DFT) calculations predict a harmonic ring-puckering frequency of 203 cm(-1) as compared to the observed vapor-phase Raman value of 208 cm(-1). The DFT calculations were also used to compute the infrared and Raman spectra of gamma-crotonolactone, and these agree very well with the experimental spectra.     

Exploring the new three-dimensional ab initio interaction energy surface of the Ar-HF complex: rovibrational calculations for Ar-HF and Ar-DF with vibrationally excited diatoms

Several features and the performance of the recently published [P. Jankowski and M. Ziolkowski, Mol. Phys. 104, 2293 (2006)] three-dimensional intermolecular potential energy surface for the Ar-HF complex have been investigated. This full-dimensional surface has been obtained using the method of the local expansion of the exact interaction energy surface [P. Jankowski, J. Chem. Phys. 121, 1655 (2004)] in the Taylor series with respect to intramolecular coordinates. The interaction energies have been calculated with the coupled-cluster supermolecular method with single, double, and noniterative triple excitations. The convergence of the interaction energy with respect to the size of the basis set is discussed. The two-dimensional surfaces resulting from averaging of the full-dimensional surface over the intramolecular vibration of HF have been obtained and directly compared to the empirical H6(4,3,2) set of surfaces proposed by Hutson [J. Chem. Phys. 96, 6752 (1992)]. A very good agreement has been observed. The averaged potentials have been used to calculate the rovibrational energy levels of the Ar-HF and Ar-DF complexes and compared to the experimental data. The accuracy of rovibrational calculations achieved with the new surface is much better than with any of the ab initio surfaces available so far. Predictions of the rovibrational energy levels and spectroscopic constants have also been done for Ar-HF with HF in the v=4,5 vibrational states, and for Ar-DF and DF in the v=3,4 states. The full-dimensional surface studied in this paper is the first ab initio surface which is fully compatible with the empirical H6(4,3,2) surface proposed by Hutson.     

Ab initio potential energy surface and rovibrational spectrum of Ar-HCCCN

We report an ab initio intermolecular potential energy surface of the Ar-HCCCN complex using a supermolecular method. The calculations were performed using the fourth-order M?ller-Plesset theory 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 Ar atom facing the H atom. The T-shaped minimum is the global minimum with the well depth of 236.81 cm(-1). A potential barrier separating the two minima is located at R=5.57 A and theta=20.39 degrees with the height of 151.59 cm(-1). The two-dimensional discrete variable representation was employed to calculate the rovibrational energy levels for Ar-HCCCN. The rovibrational spectra including intensities for the ground state and the first excited intermolecular vibrational state are also presented. The results show that the spectra are mostly b-type (Delta K(a)=+/-1) transitions with weak a-type (Delta K(a)=0) transitions in structure, which are in good agreement with the recent experimental results [A. Huckauf, W. Jager, P. Botschwina, and R. Oswald, J. Chem. Phys. 119, 7749 (2003)].     

Analytical potential from ab initio calculations for the Fe+-H2O and Feo-H2O systems

Analytical intermolecular potentials for the Fe⁺?H₂O and Fe^o?H₂O systems have been determined from ab initio calculations. Interaction energies for a lot of points along the two potential energy surfaces were calculated using Huzinga's MINI ?2 basis set. The results obtained were fitted to an analytical function containing 11 adjustable parameters that we have already used with success for the Fe²⁺?H₂O system. The goodness of the generated intermolecular potentials is discussed.     

Rotational spectrum, potential energy surface, and bound states of the weakly bound complex He-N2O

Pure rotational transitions of the weakly bound complex He-N(2)O and three minor isotopomers (He-(14)N(15)NO, He-(15)N(14)NO, and He-(15)N(15)NO) were measured in the frequency region from 6 to 20 GHz. Predictions for the microwave transition frequencies were based on the infrared work by Tang and McKellar [J. Chem. Phys. 117, 2586 (2002)]. In the case of (14)N containing isotopomers, nuclear quadrupole hyperfine structure of the rotational transitions was observed and analyzed. The resulting spectroscopic parameters were used to determine geometrical and dynamical information about the complex. An ab initio potential energy surface was calculated at the coupled cluster level of theory with single and double excitations and perturbative inclusion of triple excitations. This surface was constructed using the augmented correlation consistent polarized valence triple zeta basis set for all atoms with the inclusion of bond functions for the van der Waals bond. Bound state calculations were done to determine the energies of low-lying rovibrational levels that are supported by the potential energy surface. The resulting transition energies agree with the experimental values to 1% or better.     

An ab initio potential energy surface and vibrational energy levels of ZnH2

A three‐dimensional potential energy surface of the electronic ground state of ZnH₂ (${X}^1\sum _g^ +$ ) molecule is constructed from more than 7500 ab initio points calculated at the internally contracted multireference configuration interaction with the Davidson correction (icMRCI+Q) level employing large basis sets. The calculated relative energies of various dissociation reactions are in good agreement with the previous theoretical/experimental values. Low‐lying vibrational energy levels of ZnH₂, ZnD₂, and HZnD are calculated on the three‐dimensional potential energy surface using the Lanczos algorithm, and found to be in good agreement with the available experimental band origins and the previous theoretical values. © 2009 Wiley Periodicals, Inc. J Comput Chem, 2010     

Potential energy surface and second virial coefficient of methane-water from ab initio calculations

Six-dimensional intermolecular potential energy surfaces (PESs) for the interaction of CH4 with H2O are presented, obtained from ab initio calculations using symmetry-adapted perturbation theory (SAPT) at two different levels of intramonomer correlation and the supermolecular approach at three different levels of electron correlation. Both CH4 and H2O are assumed to be rigid molecules with interatomic distances and angles fixed at the average values in the ground-state vibration. A physically motivated analytical expression for each PES has been developed as a sum of site-site functions. The PES of the CH4-H2O dimer has only two symmetry-distinct minima. From the SAPT calculations, the global minimum has an energy of -1.03 kcal/mol at a geometry where H2O is the proton donor, HO-H...CH4, with the O-H-C angle of 165 degrees, while the secondary minimum, with an energy of -0.72 kcal/mol, has CH4 in the role of the proton donor (H3C-H...OH2). We estimated the complete basis set limit of the SAPT interaction energy at the global minimum to be -1.06 kcal/mol. The classical cross second virial coefficient B12(T) has been calculated for the temperature range 298-653 K. Our best results agree well with some experiments, allowing an evaluation of the quality of experimental results.     

High-dimensional ab initio potential energy surfaces for reaction dynamics calculations

There has been great progress in the development of potential energy surfaces (PESs) for reaction dynamics that are fits to ab initio energies. The fitting techniques described here explicitly represent the invariance of the PES with respect to all permutations of like atoms. A review of a subset of dynamics calculations using such PESs (currently 16 such PESs exist) is then given. Bimolecular reactions of current interest to the community, namely, H + CH(4) and F + CH(4), are focused on. Unimolecular reactions are then reviewed, with a focus on the photodissociation dynamics of H(2)CO and CH(3)CHO, where so-called "roaming" pathways have been discovered. The challenges for electronically non-adiabatic reactions, and associated PESs, are presented with a focus on the OH* + H(2) reaction. Finally, some thoughts on future directions and challenges are given.     

Ab initio ground state phenylacetylene-argon intermolecular potential energy surface and rovibrational spectrum

We evaluate the phenylacetylene-argon intermolecular potential energy surface by fitting a representative number of ab initio interaction energies to an analytic function. These energies are calculated at a grid of intermolecular geometries, using the CCSD(T) method and the aug-cc-pVDZ basis set extended with a series of 3s3p2d1f1g midbond functions. The potential is characterized by two equivalent global minima where the Ar atom is located above and below the phenylacetylene plane at a distance of 3.5781 A? from the molecular center of mass and at an angle of 9.08° with respect to the axis perpendicular to the phenylacetylene plane and containing the center of mass. The calculated interaction energy is -418.9 cm(-1). To check further the potential, we obtain the rovibrational spectrum of the complex and the results are compared to the available experimental data.