MP2 Potential Research Articles

Anharmonicity in the Vibrational Spectra of Naphthalene and Naphthalene-d8: Experiment and Theory.

In this paper, we report the gas phase infrared (IR) spectra of naphthalene and naphthalene-d8 recorded in the mid-infrared region (3200-500 cm-1) using a heated multipass long path gas cell. Several combination bands appear as shoulders and satellite peaks in the 3200-2600 cm-1region along with the C-H stretch fundamental bands. Experimental IR spectra of these molecules were systematically analyzed with vibrational self-consistent field (VSCF) theory, vibrational second order perturbation theory (VPT2) and vibrational couple cluster method (VCCM) with two different potential energy surfaces obtained using B3LYP and MP2 methods. A comparative study between these two PESs was made to match the observed spectra. Final assignment of the IR spectra of naphthalene and naphthalene-d8 was done using the VCCM with MP2 potential, which provided the best match.

Hybrid MP2/MP4 potential surfaces in VSCF calculations of IR spectra: Applications for organic molecules

This study introduces an improved hybrid MP2/MP4 ab initio potential for vibrational spectroscopy calculations which is very accurate, yet without high computational demands. The method uses harmonic vibrational calculations with the MP4(SDQ) potential to construct an improved MP2 potential by coordinate scaling. This improved MP2 potential is used for the anharmonic VSCF calculation. The method was tested spectroscopically for four molecules: butane, acetone, ethylene and glycine. Very good agreement with experiment was found. For most of the systems, the more accurate harmonic treatment considerably improved the MP2 anharmonic results.

Theoretical Unimolecular Kinetics for CH4 + M ⇄ CH3 + H + M in Eight Baths, M = He, Ne, Ar, Kr, H2, N2, CO, and CH4

Ensembles of classical trajectories are used to study collisional energy transfer in highly vibrationally excited CH(4) for eight bath gases. Several simplifying assumptions for the CH(4) + M interaction potential energy surface are tested against full dimensional direct dynamics trajectory calculations for M = He, Ne, and H(2). The calculated energy transfer averages are confirmed to be sensitive to the shape of the repulsive wall of the intermolecular potential, with an exponential repulsive wall required for quantitative predictions. For the diatomic baths, the usual "separable pairwise" approximation for the interaction potential is unable to describe the orientation dependence of the interaction potential accurately, and the ambiguity in the resulting parametrizations contributes an additional uncertainty to the predicted energy transfer averages of 20-40%. On the other hand, the energy transfer averages are shown to be insensitive to the level of theory used to describe the intramolecular CH(4) potential, with a computationally efficient semiempirical tight binding potential for hydrocarbons performing equally well as an MP2 potential. The relative collisional energy transfer efficiencies of the eight bath gases are discussed and shown to be a function of temperature. The ensemble-averaged energy transferred in deactivating collisions <ΔE(d)> for each bath is used to parametrize a single-exponential-down model for collisional energy transfer in master equation calculations. The predicted decomposition rate coefficients for CH(4) agree well with available experimental rate coefficients for M = He, Ar, Kr, and CH(4). The effect of vibrational anharmonicity on the predicted rate coefficients is considered briefly.

Ionization of N2O4 in Contact with Water: Mechanism, Time Scales and Atmospheric Implications

Ionization of N(2)O(4) in and on thin water films on surfaces is believed to be a key step in the hydrolysis of NO(2) which generates HONO, a significant precursor to the OH free radical in the lower atmosphere. Molecular dynamics simulations using "on the fly" high-level MP2 potentials are carried out for ONONO(2) x (H(2)O)(n) clusters, n < or = 8, used to mimic the surface reaction, in order to investigate the ionization process and determine its time-scale and mechanism around room temperature. The results are (i) the isolated molecule does not convert to the NO(+)NO(3)(-) ion pair, even for long times; (ii) for ONONO(2) x (H(2)O)(n) with n = 1 and 2, ionization takes place in several picoseconds; (iii) for n > or = 3, ionization is essentially immediate, implying that the neutral species does not have sufficient lifetime to be considered a significant intermediate in the reaction; and (iv) even at ice temperatures, T < or = 250 K, ionization for n > or = 3 is immediate. The implications for hydrolysis of oxides of nitrogen on surfaces in the atmosphere are discussed.

Molecular dynamics simulations of fluid methane properties using ab initio intermolecular interaction potentials

Intermolecular interaction energy data for the methane dimer have been calculated at a spectroscopic accuracy and employed to construct an ab initio potential energy surface (PES) for molecular dynamics (MD) simulations of fluid methane properties. The full potential curves of the methane dimer at 12 symmetric conformations were calculated by the supermolecule counterpoise-corrected second-order Møller-Plesset (MP2) perturbation theory. Single-point coupled cluster with single and double and perturbative triple excitations [CCSD(T)] calculations were also carried out to calibrate the MP2 potentials. We employed Pople's medium size basis sets [up to 6-311++G(3df, 3pd)] and Dunning's correlation consistent basis sets (cc-pVXZ and aug-cc-pVXZ, X = D, T, Q). For each conformer, the intermolecular carbon-carbon separation was sampled in a step 0.1 A for a range of 3-9 A, resulting in a total of 732 configuration points calculated. The MP2 binding curves display significant anisotropy with respect to the relative orientations of the dimer. The potential curves at the complete basis set (CBS) limit were estimated using well-established analytical extrapolation schemes. A 4-site potential model with sites located at the hydrogen atoms was used to fit the ab initio potential data. This model stems from a hydrogen-hydrogen repulsion mechanism to explain the stability of the dimer structure. MD simulations using the ab initio PES show quantitative agreements on both the atom-wise radial distribution functions and the self-diffusion coefficients over a wide range of experimental conditions.

Dynamics of proton recombination with NO3− anion in water clusters

Recombination events of a proton with NO3- at (H2O)8 clusters are studied by molecular dynamics, using "on-the-fly" reliable ab initio MP2 potentials. The main findings are: (1) the lifetime of the ions is less than 1.2 picoseconds; (2) the recombination step invariably involves H3O+, not H5O2+; and (3) an essentially unique transition-state structure of H3O+/NO3- for recombination is found in all cases. Proton migration involves both H3O+ and H5O2+ species: Grotthuss and other mechanisms contribute.

Anharmonic vibrational spectroscopy calculations with electronic structure potentials: comparison of MP2 and DFT for organic molecules

Density functional theory (DFT) technique is the most commonly used approach when it comes to computation of vibrational spectra of molecular species. In this study, we compare anharmonic spectra of several organic molecules such as allene, propyne, glycine, and imidazole, computed from ab initio MP2 potentials and DFT potentials based on commonly used BLYP and B3LYP functionals. Anharmonic spectra are obtained using the direct vibrational self-consistent field (VSCF) method and its correlation-corrected extension (CC-VSCF). The results of computations are compared with available experimental data. It is shown that the most accurate vibrational frequencies are obtained with the MP2 method, followed by the DFT/B3LYP method, while DFT/BLYP results are often unsatisfactory.

An intrinsic reaction coordinate calculation of the torsional potential in ethane: Comparison of the computationally and experimentally derived torsional transitions and the rotational barrier

Intrinsic reaction coordinate (IRC) calculations of the torsional potentials of C2H6, CH3CD3, and C2D6 have been carried out at the MP2/6-31++G** level. The C2H6 potential was corrected at the coupled-cluster single double (triple) [CCSD(T)] level with extrapolation to the complete basis set limit (CBS). For CH3CD3 and C2D6, the MP2 potentials were scaled by 0.862 to approximate CCSD(T)/CBS results. The IRC potential for the D3h→D3d relaxation in C2H6 was fit to a two-term Fourier series containing V3 and V6 coefficients for which the barrier height, V3, was set to the CCSD(T)/CBS value (941 cm−1), and V6 was optimized at 6.7 cm−1. Sixfold torsional potentials were constructed from the CCSD(T)/CBS profiles and the resulting eigenvalues were used to calculate the Δn(ν4)=2 transitions, which are compared with experimental assignments. Comparisons are also made with observed IR transitions. Our best estimate of the rotational barrier is 941 cm−1. This value, as well as other high-level ab initio results, is about 50 cm−1 smaller than the V3 parameter obtained from the analysis of microwave and Raman data. The one-dimensional IRC potential neglects subtle coupling of ethane’s torsional motion to other modes and is likely responsible for the small, but systematic, differences between the calculated and experimental results.

A gradient-corrected density functional and MP2 study of phenol–ammonia and phenol–ammonia(+) hydrogen-bonded complexes

Abstract A gradient-corrected DFT and MP2 study of phenol–ammonia neutral and cationic dimers were carried out. Various combinations of exchange and correlation functionals were employed within the DFT approach: B3LYP, MPW1PW91, PBE1PBE, and the modified B3LYP(M) [J. Mol. Struct. 416 (1997) 1]. The BSSE-corrected PES of the neutral dimer was also explored at B3LYP, B3LYP(M) and MP2/6-31++G(2d,p) levels. A single minimum was located on the neutral complex PES corresponding to the hydrogen-bonded non-proton-transferred form, while the only minimum on cationic complex PES corresponds to a phenoxyl-ammonium proton-transferred form, regardless on the level of theory. Dissociation energies corrected for BSSE and including the relaxation energy terms are reported for both complexes. The calculated vertical IE values at all DFT levels of theory are in excellent agreement with the experimental data. While the anharmonic vibrational analysis based on all 1D DFT vibrational potentials overestimate the ν (OH) mode frequency shift upon complexation, the value obtained from the 1D MP2 potential for the BSSE-corrected MP2/6-31++G(2d,p) PES is in excellent agreement with the experimental data (457 vs. 469 cm −1 ). Various contributions to the overall anharmonic ν (OH) frequency shifts are considered, as well as the BSSE influence which was found to be very small. As revealed by the CFP 1D ν (OH) DFT and MP2 vibrational potentials, the electrostatic interaction alone can not account neither for the substantial red shift of this mode upon hydrogen bonding, and especially for the substantial intensity enhancement.

An ab initio potential energy surface of Ne–LiH

The intermolecular potential energy surface for Ne–LiH is calculated using the fourth-order Møller–Plesset perturbation approach with a large basis set consisting of the midpoint bond function set 3s3p2d. Two local potential minima are found to be located at the linear Ne–LiH ( θ=180°) and Ne–HLi ( θ=0°) geometry. The MP2 potential is also calculated to check the convergence of the potential. In addition, the vibrational energy levels are calculated using the vibrational SCF-CI method. Both MP2 and MP4 potentials support three bound vibrational states for the Ne–LiH system.

Spectro-temporal analysis of complex tones: two cortical processes dependent on retention of sounds in the long auditory store

Objectives: To examine whether two cortical processes concerned with spectro-temporal analysis of complex tones, a ‘C-process’ generating CN1 and CP2 potentials at cf. 100 and 180 ms after sudden change of pitch or timbre, and an ‘M-process’ generating MN1 and MP2 potentials of similar latency at the sudden cessation of repeated changes, are dependent on accumulation of a sound image in the long auditory store. Methods: The durations of steady (440 Hz) and rapidly oscillating (440–494 Hz, 16 changes/s) pitch of a synthesized ‘clarinet’ tone were reciprocally varied between 0.5 and 4.5 s within a duty cycle of 5 s. Potentials were recorded at the beginning and end of the period of oscillation in 10 non-attending normal subjects. Results: The CN1 at the beginning of pitch oscillation and the MN1 at the end were both strongly influenced by the duration of the immediately preceding stimulus pattern, mean amplitudes being 3–4 times larger after 4.5 s as compared with 0.5 s. Conclusions: The processes responsible for both CN1 and MN1 are influenced by the duration of the preceding sound pattern over a period comparable to that of the ‘echoic memory’ or long auditory store. The store therefore appears to occupy a key position in spectro-temporal sound analysis. The C-process is concerned with the spectral structure of complex sounds, and may therefore reflect the ‘grouping’ of frequency components underlying auditory stream segregation. The M-process (mismatch negativity) is concerned with the temporal sound structure, and may play an important role in the extraction of information from sequential sounds.

Morphing ab initio potentials: A systematic study of Ne–HF

A procedure for “morphing” an ab initio potential energy surface to obtain agreement with experimental data is presented. The method involves scaling functions for both the energy and the intermolecular distance. In the present work, the scaling functions are parametrized and determined by least-squares fitting to the experimental data. The method is tested on the system Ne–HF, for which high-resolution infrared spectra are available. It is shown to work well even with relatively low-level ab initio calculations. Several basis sets are investigated at the CCSD(T) correlation level, including various aug-cc-pVnZ basis sets and the specially-tailored Ne–HF basis set of ONeil et al. All give good results after morphing, but the changes needed to match experiment are much smaller for the ONeil basis set. The use of MP2 calculations is also investigated: again, the MP2 potential is quite satisfactory after morphing, but requires much more modification than the CCSD(T) potential.

Interatomic potentials for ArLi + obtained by SCF, MP2 and density functional theory

The interatomic potential for the ground state (X 1ϵ) of ArLi + was determined by SCF, MP2 and density functional theory methods, using large Gaussian-type basis functions. It was found that the MP2 potential was in excellent agreement with an accurate extended group function model; the density functional theory results were less satisfactory but better than the corresponding SCF results.

The prediction of spectroscopic properties from the quartic force field of NOBr

Ab initio molecular electronic structure methods are used to determine the constants describing the infrared spectroscopy of NOBr. Second-order Møller-Plesset perturbation theory (MP2) with a 6311G extended basis set on bromine is used to obtain an equilibrium geometry, harmonic vibrational frequencies and a quartic expansion of the potential energy surface. The cubic and quartic force constants were evaluated in mass-weighted normal coordinates using a finite-difference approach. Standard perturbation theory is then used to determine effective vibrational and rotational Hamiltonians for the fundamental vibrations and selected overtone and combination bands. The quadratic and most of the cubic potential constants are also determined experimentally. The calculated anharmonic constants are found to be in excellent agreement with experiment if a MP2 potential is used.

ChemInform Abstract: Theoretical and Experimental Study of the Non‐s‐cis Form of Unsaturated Ethers: Molecular Structure and Vibrational Assignment of cis‐Methyl‐1‐propenylether

Abstract Ab initio calculations of the internal potential energy function of cis -methyl-1-propenylether, with complete optimization of the structural parameters, have been performed at the SCF level using the 6–31G and 6–31G* bases. By keeping the SCF-optimized geometries, electron correlation was accounted for using the MP2 Moller-Plesset perturbation theory. The most stable isomer was found to occur in a wide double-minimum potential well at a torsional angle o = 152° about the C( sp 2 )O bond; the interconversion barrier between the two skew enantiomers was predicted as low as 53 cm −1 . The s- cis form of the molecule (o=0°) appeared at a relative energy of 1286 cm −1 , in a shallow potential well, only 135 cm −1 deep. The Raman spectra of the solid and liquid phases and the complete IR spectrum of the vapour phase have been recorded. They yield evidence that the molecule occurs in a single, non-planar isomer. An assignment of the vibrational spectrum has been made from the harmonic force constants derived at the SCF 6–31G* level; the calculated frequencies have been corrected by transferring scale factors from propene, methylvinylether and methylisopropenylether, for which parallel calculations of the same accuracy have been made. The average difference between observed and predicted frequencies amounted to 5.7 cm −1 . Structureless gas-phase IR bands observed at 100 and 58 cm −1 are assigned to the skeletal torsion vibration. In a model calculation using the one-vibration approximation and the computed MP2 potential only two closely spaced levels, corresponding to a genuine skew conformation, appear below the interconversion barrier; the 1←0 and 2←0 torsional transitions have frequencies of 40 and 72 cm −1 , while single- and double-jump transitions between higher levels converge to frequencies of about 45 and 90 cm −1 , respectively.

Theoretical and experimental study of the non-S- cis form of unsaturated ethers: Molecular structure and vibrational assignment of cis-methyl-1-propenylether

Ab initio calculations of the internal potential energy function of cis-methyl-1-propenylether, with complete optimization of the structural parameters, have been performed at the SCF level using the 6–31G and 6–31G* bases. By keeping the SCF-optimized geometries, electron correlation was accounted for using the MP2 Møller-Plesset perturbation theory. The most stable isomer was found to occur in a wide double-minimum potential well at a torsional angle ø = 152° about the C( sp 2)O bond; the interconversion barrier between the two skew enantiomers was predicted as low as 53 cm −1. The s- cis form of the molecule (ø=0°) appeared at a relative energy of 1286 cm −1, in a shallow potential well, only 135 cm −1 deep. The Raman spectra of the solid and liquid phases and the complete IR spectrum of the vapour phase have been recorded. They yield evidence that the molecule occurs in a single, non-planar isomer. An assignment of the vibrational spectrum has been made from the harmonic force constants derived at the SCF 6–31G* level; the calculated frequencies have been corrected by transferring scale factors from propene, methylvinylether and methylisopropenylether, for which parallel calculations of the same accuracy have been made. The average difference between observed and predicted frequencies amounted to 5.7 cm −1. Structureless gas-phase IR bands observed at 100 and 58 cm −1 are assigned to the skeletal torsion vibration. In a model calculation using the one-vibration approximation and the computed MP2 potential only two closely spaced levels, corresponding to a genuine skew conformation, appear below the interconversion barrier; the 1←0 and 2←0 torsional transitions have frequencies of 40 and 72 cm −1, while single- and double-jump transitions between higher levels converge to frequencies of about 45 and 90 cm −1, respectively.