The calculated infrared spectrum of Cl- H2O using a full dimensional ab initio potential surface and dipole moment surface

We report a full dimensional ab initio-based global potential energy surface (PES) and dipole moment surface (DMS) for Cl(-)H(2)O. Both surfaces are symmetric with respect to interchange of the H atoms. The PES is a fit to thousands of electronic energies calculated using the coupled-cluster method (CCSD(T)) with a moderately large basis (aug-cc-pVTZ). The infrared spectrum and vibrational dynamics are reported and compared to experiment. These results are analyzed by examination of wave function and the dipole surface.     

The theoretical prediction of infrared spectra of trans- and cis-hydroxycarbene calculated using full dimensional ab initio potential energy and dipole moment surfaces

Accurate infrared spectra of the two hydroxycarbene isomers are computed by diagonalizing the Watson Hamiltonian including up to four mode couplings using full dimensional potential energy and dipole moment surfaces calculated at the CCSD(T)/cc-pVTZ (frozen core) and CCSD6-311G(**) (all electrons correlated) levels, respectively. Anharmonic corrections are found to be very important for these elusive higher-energy isomers of formaldehyde. Both the energy levels and intensities of stretching fundamentals and all overtone transitions are strongly affected by anharmonic couplings between the modes. The results for trans-HCOHHCOD are in excellent agreement with the recently reported IR spectra, which validates our predictions for the cis-isomers.     

A new ab initio ground-state dipole moment surface for the water molecule

A valence-only (V) dipole moment surface (DMS) has been computed for water at the internally contracted multireference configuration interaction level using the extended atom-centered correlation-consistent Gaussian basis set aug-cc-pV6Z. Small corrections to these dipole values, resulting from core correlation (C) and relativistic (R) effects, have also been computed and added to the V surface. The resulting DMS surface is hence called CVR. Interestingly, the C and R corrections cancel out each other almost completely over the whole grid of points investigated. The ground-state CVR dipole of H(2) (16)O is 1.8676 D. This value compares well with the best ab initio one determined in this study, 1.8539+/-0.0013 D, which in turn agrees well with the measured ground-state dipole moment of water, 1.8546(6) D. Line intensities computed with the help of the CVR DMS shows that the present DMS is highly similar to though slightly more accurate than the best previous DMS of water determined by Schwenke and Partridge [J. Chem. Phys. 113, 16 (2000)]. The influence of the precision of the rovibrational wave functions computed using different potential energy surfaces (PESs) has been investigated and proved to be small, due mostly to the small discrepancies between the best ab initio and empirical PESs of water. Several different measures to test the DMS of water are advanced. The seemingly most sensitive measure is the comparison between the ab initio line intensities and those measured by ultralong pathlength methods which are sensitive to very weak transitions.     

Spectroscopic properties of MgH2, MgD2, and MgHD calculated from a new ab initio potential energy surface

A three-dimensional potential energy surface for the ground electronic state of MgH2 has been constructed from 9030 symmetry-unique ab initio points calculated using the icMRCI+Q method with aug-cc-pVnZ basis sets for n=3, 4, and 5, with core-electron correlation calculated at the MR-ACPF level of theory using cc-pCVnZ basis sets, with both calculations being extrapolated to the complete basis set limit. Calculated spectroscopic constants of MgH2 and MgD2 are in excellent agreement with recent experimental results: for four bands of MgH2 and one band of MgD2 the root-mean-square (rms) band origin discrepancies were only 0.44 and 0.06 cm(-1), respectively, and the rms relative discrepancies in the inertial rotational constants (B[v]) were only 0.0196% and 0.0058%, respectively. Spectroscopic constants for MgHD were predicted using the same potential surface.     

Ab initio potential energy and dipole moment surfaces of (H2O)2

A full-dimensional ab initio potential energy surface (PES) and dipole moment surface (DMS) are reported for the water dimer, (H2O)2. The CCSD(T)-PES is a very precise fit to 19,805 ab initio energies obtained with the coupled-cluster (CCSD(T)) method, using an aug-cc-pVTZ basis. The standard counterpoise correction was applied to approximately eliminate basis set superposition errors. The fit is based on an approach that incorporates the permutational symmetry of identical atoms [Huang, X.; Braams, B.; Bowman, J. M. J. Chem.Phys. 2005, 122, 044308]. The DMS is a fit to the dipole moment obtained with M?ller-Plesset (MP2) theory, using an aug-cc-pVTZ basis. The PES has an RMS fitting error of 31 cm(-1) for energies below 20,000 cm(-1), relative to the global minimum. This surface can describe various internal floppy motions, including various monomer inversions, and isomerization pathways. Ten characteristic stationary points have been located on the surface, four of which are transition structures and the rest are higher order saddle points. Their geometrical and vibrational properties are presented and compared with available previous theoretical work. The CCSD(T)-PES and MP2-DMS dissociate correctly (and symmetrically) to two H2O monomers, with D(e) = 1665.7 cm(-1) (19.93 kJ/mol). Accurate quantum calculations of the zero-point energy of the dimer (using diffusion Monte Carlo) and the monomers (using a vibrational configuration interaction approach) are reported, and these together with D(e) give a value of D0 of 1042 cm(-1) (12.44 kJ/mol). A best estimated value is 1130 cm(-1) (13.5 kJ/mol).     

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).     

Ab initio potential energy and dipole moment surfaces for H5O2 +

Full-dimensional ab initio potential energy surface (PES) and dipole moment surface (DMS) are reported for H(5)O(2) (+). Tens of thousands of coupled-cluster [CCSD(T)] and second-order Moller-Plesset (MP2) calculations of electronic energies, using aug-cc-pVTZ basis, were done. The energies were fit very precisely in terms of all the internuclear distances, using standard least-square procedures, however, with a fitting basis that satisfies permutational symmetry with respect to like atoms. The H(5)O(2) (+) PES is a fit to 48 189 CCSD(T) energies, containing 7962 polynomial coefficients. The PES has a rms fitting error of 34.9 cm(-1) for the entire data set up to 110 000 cm(-1). This surface can describe various internal floppy motions, including the H atom exchanges, monomer inversions, and monomer torsions. First- and higher-order saddle points have been located on the surface and compared with available previous theoretical work. In addition, the PES dissociates correctly (and symmetrically) to H(2)O+H(3)O(+), with D(e)=11 923.8 cm(-1). Geometrical and vibrational properties of the monomer fragments are presented. The corresponding global DMS fit (MP2 based) involves 3844 polynomial coefficients and also dissociates correctly.     

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.     

Analysis of the HO2 vibrational spectrum on an accurate ab initio potential energy surface

The complete vibrational spectrum of the HO2(X(2)A' ') radical, up to the H + O2 dissociation limit, has been determined quantum mechanically on an accurate potential energy surface (PES), based on approximately 15000 ab initio points at the icMRCI+Q/aug-cc-pVQZ level of theory. The vibrational states are found to be assignable at low energies but become more irregular as the energy approaches the dissociation limit. However, even at very high energies, regularity still exists, in sharp contrast to earlier results based on the double many-body expansion (DMBE) IV potential. Several Fermi resonances have been identified, and the spectrum is fit with a spectroscopic Hamiltonian. In addition, the vibrational dynamics is analyzed using a periodic orbit approach.     

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.     

An accurate ab initio potential energy surface and calculated spectroscopic constants for BeH2, BeD2, and BeHD

A three-dimensional potential energy surface for the ground electronic state of BeH(2) has been determined by three-dimensional spline interpolation over 6864 symmetry-unique ab initio points calculated at the icMRCI/aug-cc-pV5Z level and corrected for core-electron correlation computed at the MR-ACPF/cc-pCV5Z level. Calculated spectroscopic constants of BeH(2) and BeD(2) are in excellent agreement with recent experimental results: for 11 bands of BeH(2) and 5 bands of BeD(2) the root mean square (rms) band origin discrepancies were only 0.15(+/-0.09) and 0.46(+/-0.19) cm(-1), respectively, and the rms relative discrepancies in the inertial rotational constants (B([v])) were only 0.028% and 0.023%, respectively. Spectral constants for BeHD were predicted using the same potential surface. The effect of different interpolation methods on predicted potential function values and on the calculated level energies and spectroscopic constants has been examined.     

Revelation of non-statistical behavior in HO2 vibration by a new ab initio potential energy surface

The hydroperoxyl radical (HO2) has long been considered as a prototype for statistical vibrational dynamics. In this work, however, it is shown that the bound state energy levels (up to the dissociation threshold) and low-lying resonances of the HO2 system (J=0) obtained on a new ab initio potential energy surface exhibit surprisingly large regularity. The implications of the non-statistical behavior of the HO2 system in unimolecular and bimolecular reactions are discussed.     

Five-dimensional ab initio potential energy surface and predicted infrared spectra of H2-CO2 van der Waals complexes

The authors present a new five-dimensional potential energy surface for H2-CO2 including the Q3 normal mode for the nu3 antisymmetric stretching vibration of the CO2 molecule. The potential energies were calculated using the supermolecular approach with the full counterpoise correction at the CCSD(T) level with an aug-cc-pVTZ basis set supplemented with bond functions. The global minimum is at two equivalent T-shaped coplanar configurations with a well depth of 219.68 cm-1. The rovibrational energy levels for four species of H2-CO2 (paraH2-, orthoH2-, paraD2-, and orthoD2-CO2) were calculated employing the discrete variable representation (DVR) for radial variables and finite basis representation (FBR) for angular variables and the Lanczos algorithm. Our calculations showed that the off-diagonal intra- and intermolecular vibrational coupling could be neglected, and separation of the intramolecular vibration by averaging the total Hamiltonian with the wave function of a specific vibrational state of CO2 should be a good approximation with high accuracy. The calculated band origin shift in the infrared spectra in the nu3 region of CO2 is -0.113 cm-1 for paraH2-CO2 and -0.099 cm-1 for orthoH2-CO2, which agrees well with the observed values of -0.198 and -0.096 cm-1. The calculated rovibrational spectra for H2-CO2 are consistent with the available experimental spectra. For D2-CO2, it is predicted that only a-type transitions occur for paraD2-CO2, while both a-type and b-type transitions are significant for orthoD2-CO2.     

Intermolecular potential energy surface and rovibrational spectra of the He-N2O complex from ab initio calculations

We report an ab initio intermolecular potential energy surface calculation on the He-N(2)O complex with N(2)O at its ground state using a supermolecular approach. The calculation was performed at the coupled-cluster [CCSD(T)] level, with the full counterpoise correction for the basis set superposition error and a large basis set including midpoint bond functions. The CCSD(T) potential is found to have two minima corresponding to the T-shaped and linear He-ONN structures. The T-shaped minimum is the global minimum. The two-dimensional discrete variable representation method was employed to calculate the rovibrational energy levels for (4)He-N(2)O and (3)He-N(2)O with N(2)O at its ground and nu(3) excited states. The results indicate that the CCSD(T) potential supports five and four vibrational bound states for the (4)He-N(2)O and (3)He-N(2)O, respectively. Moreover, the calculations on the line intensities of the rotational transitions in the nu(3) region of N(2)O for the ground vibrational state shows that the (3)He-N(2)O spectrum is dominated by a-type transitions (DeltaK(a)=0), while the (4)He-N(2)O spectrum is contributed by both the a-type and b-type (DeltaK(a)=+/-1) transitions. The calculated transition frequencies and the intensities are in good agreement with the observed results.     

SiH2Cl2: ab initio anharmonic force field, dipole moments, and infrared vibrational transitions

The vibrational spectra of SiH2Cl2 have been recorded in the 1000-13,000 cm(-1) region, utilizing the Fourier-transform spectroscopy and Fourier-transform intracavity laser absorption spectroscopy. Totally 61 band centers and intensities are derived from the infrared spectra. An ab initio quartic force field is obtained by applying the second-order Moller-Plesset perturbation theory and correlation-consistent polarized valence triplet-zeta basis sets [J. Chem. Phys. 90, 1007 (1989); 98, 1358 (1993)]. Most observed bands are assigned by the vibration analysis based on the second-order perturbation theory. Reduced-dimensional ab initio dipole moment functions (two dimensional and three dimensional) have also been calculated to investigate the absolute band intensities of the SiH2 chromophore. The calculated values agree reasonably with the observed ones.     

A global, high accuracy ab initio dipole moment surface for the electronic ground state of the water molecule

A highly accurate, global dipole moment surface (DMS) is calculated for the water molecule using ab initio quantum chemistry methods. The new surface is named LTP2011 and is based on all-electron, internally contracted multireference configuration interaction, including size-extensivity corrections in the aug-cc-pCV6Z basis set. Dipoles are computed as energy derivatives and small corrections due to relativistic effects included. The LTP2011 DMS uses an appropriate functional form that guarantees qualitatively correct behaviour even for most high energies configuration (up to about 60,000 cm(-1)), including, in particular, along the water dissociation channel. Comparisons with high precision experimental data show agreement within 1% for medium-strength lines. The new DMS and all the ab initio data are made available in the supplementary material.     

Collective solvent coordinates for the infrared spectrum of HOD in D2O based on an ab initio electrostatic map

An ab initio MP2 vibrational Hamiltonian of HOD in an external electrostatic potential parametrized by the electric field and its gradient-tensor is constructed. By combining it with the fluctuating electric field induced by the D(2)O solvent obtained from molecular dynamics simulations, we calculate the infrared absorption of the O-H stretch. The resulting solvent shift and infrared line shape for three force fields (TIP4P, SPC/E, and SW) are in good agreement with the experiment. A collective coordinate response for the solvent effect is constructed by identifying the main electrostatic field and gradient components contributing to the line shape. This allows a realistic stochastic Liouville equation simulation of the line shapes which is not restricted to Gaussian frequency fluctuations.     

New ab initio potential energy surface and the vibration-rotation-tunneling levels of (H2O)2 and (D2O)2

We report a new full-dimensional potential energy surface (PES) for the water dimer, based on fitting energies at roughly 30,000 configurations obtained with the coupled-cluster single and double, and perturbative treatment of triple excitations method using an augmented, correlation consistent, polarized triple zeta basis set. A global dipole moment surface based on Moller-Plesset perturbation theory results at these configurations is also reported. The PES is used in rigorous quantum calculations of intermolecular vibrational frequencies, tunneling splittings, and rotational constants for (H2O)2 and (D2O)2, using the rigid monomer approximation. Agreement with experiment is excellent and is at the highest level reported to date. The validity of this approximation is examined by comparing tunneling barriers within that model with those from fully relaxed calculations.     

Quantum vibrational analysis and infrared spectra of microhydrated sodium ions using an ab initio potential

We present a full-dimensional potential energy surface and a dipole moment surface (DMS) for hydrated sodium ion. These surfaces are based on an n-body expansion for both the potential energy and the dipole moment, truncated at the two-body level for the H(2)O-Na(+) interaction and also for the DMS. The water-water interaction is truncated at the three-body level. The new full-dimensional two-body H(2)O-Na(+) potential is a fit to roughly 20,000 coupled-cluster single double (triple)/aug-cc-pVTZ energies. Properties of this two-body potential and the potential describing (H(2)O)(n)Na(+) clusters, with n up to 4 are given. We then report anharmonic, coupled vibrational calculations with the "local-monomer model" to obtain infrared spectra and also 0 K radial distribution functions for these clusters. Some comparisons are made with the recent infrared predissociation spectroscopy experiments of Miller and Lisy [J. Am. Chem. Soc. 130, 15381 (2008).].     

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