High-accuracy Potential Energy Surface Research Articles

Scattering resonances in the rotational excitation of HDO by Ne and normal-H2: theory and experiment.

The rotational excitation of a singly deuterated water molecule (HDO) by a heavy atom (Ne) and a light diatomic molecule (H2) is investigated theoretically and experimentally in the near-threshold regime. Crossed-molecular-beam measurements with a variable crossing angle are compared to close-coupling calculations based on high-accuracy potential energy surfaces. The two lowest rotational transitions, 000 → 101 and 000 → 111, are probed in detail and a good agreement between theory and experiment is observed for both transitions in the case of HDO + Ne, where scattering resonances are however blurred out experimentally. In the case of HDO + H2, the predicted theoretical overlapping resonances are faithfully reproduced by experiment for the 000 → 111 transition, while the calculated strong signal for the 000 → 101 transition is not detected. Future work is needed to reconcile this discrepancy.

Collisional Excitation and Non-LTE Modeling of Interstellar Chiral Propylene Oxide

Abstract The first set of theoretical rotational cross sections for propylene oxide (CH3CHCH2O) colliding with cold He atoms has been obtained at the full quantum level using a high-accuracy potential energy surface. By scaling the collision reduced mass, rotational rate coefficients for collisions with para-H2 are deduced in the temperature range 5–30 K. These collisional coefficients are combined with radiative data in a non-LTE radiative transfer model in order to reproduce observations of propylene oxide made toward the Sagittarius B2(N) molecular cloud with the Green Bank and Parkes radio telescopes. The three detected absorption lines are found to probe the cold (∼10 K) and translucent (n H ∼ 2000 cm−3) gas in the outer edges of the extended Sgr B2(N) envelope. The derived column density for propylene oxide is N tot ∼ 3 × 1012 cm−2, corresponding to a fractional abundance relative to total hydrogen of ∼2.5 × 10−11. The present results are expected to help our understanding of the chemistry of propylene oxide, including a potential enantiomeric excess, in the cold interstellar medium.

An experimentally-accurate and complete room-temperature infrared HCN line-list for the HITRAN database

A hydrogen cyanide line list (MOMeNT-90) developed for the HITRAN spectroscopic database covering 0–7500 cm−1 range (λ>1330 nm) is presented. The line list is a combination of the variationally calculated line intensities with line centers obtained from experimentally derived energy levels. There are four features of this line list which distinguishes it from the previously calculated ones. First, the intensities are variationally calculated using a new, high-accuracy potential energy surface (PES) obtained via fitting the PES using experimental energy levels. Second, a new ab initio dipole moment surface was calculated at a high level of quantum chemical theory. Based on the wave functions calculated with the new PES and use of the new dipole moment surface, line intensities are reported which of similar accuracy to those obtained experimentally. Third, the calculated states are mapped to the existing complete set of experimental eigenenergies, resulting in an assigned and complete HCN line list down to the HITRAN intensity threshold of 10−34 cm/molecule. Fourth, extensive validation of the line list is provided through line-by-line comparisons of the results with measured HCN spectra which confirms the accuracy of the intensities used to construct the line list. The line list is augmented with parameters needed to calculate line widths for pressure-dependent simulations.

Interaction of H2O with CO: potential energy surface, bound states and scattering calculations

Collisions between H2O and CO play a crucial role in the gaseous component of comets and protoplanetary disks. We present here a five-dimensional potential energy surface (PES) for the H2O-CO collisional complex. Ab initio calculations were carried out using the explicitly-correlated closed-shell single- and double-excitation coupled cluster approach with the non-iterative perturbative treatment of triple excitations [CCSD(T)-F12a] method with the augmented correlation-consistent aug-cc-pVTZ basis sets. The most stable configuration of the complex, where the carbon atom of CO is pointing towards the OH bond of water, has a binding energy De = 646.1 cm-1. The end-over-end rotational constant of the H2O-CO complex was extracted from bound state calculations and it was found to be B0 = 0.0916 cm-1, in excellent agreement with experimental measurements. Finally, cross sections for the rotational excitation of CO by H2O are computed for s-wave (J = 0) scattering at the full close-coupling level of theory. These results will serve as a benchmark for future studies.

ExoMol line lists – XV. A new hot line list for hydrogen peroxide

A computed line list for hydrogen peroxide, H216O2, applicable to temperatures up to T = 1250 K is presented. A semi-empirical high-accuracy potential energy surface is constructed and used with an ab initio dipole moment surface as input trove to compute 7.5 million rotational-vibrational states and around 20 billion transitions with associated Einstein-A coefficients for rotational excitations up to J = 85. The resulting APTY line list is complete for wavenumbers below 6000 cm−1 (λ < 1.67 μm) and temperatures up to 1250 K. Room-temperature spectra are compared with laboratory measurements and data currently available in the HITRAN data base and literature. Our rms with line positions from the literature is 0.152 cm−1 and our absolute intensities agree better than 10 per cent. The full line list is available from the CDS data base as well as at www.exomol.com.

Effective potential energy surface of HD16O for calculation of highly excited states of nν3 and ν1 + nν3 types

A new spectroscopically determined potential energy surface (PES) for HD16O is presented, and rotational-vibrational transitions are calculated using it for low rotational quantum numbers J ≤ 4. This surface is constructed by adjusting a high-accuracy PES by fitting to experimental energy levels of nν3 and ν1 + nν3 types. Seven hundred and forty rotational levels with energies up to 25600 cm−1 and J ≤ 8 were used for the refinement. To improve the extrapolation properties of the empirical PES, the fitting was applied to experimental and ab initio energy levels.

Collisional excitation of water by hydrogen atoms

We present quantum dynamical calculations that describe the rotational excitation of H2O due to collisions with H atoms. We used a recent, high-accuracy potential energy surface, and solved the collisional dynamics with the close-coupling formalism, for total energies up to 12 000 cm−1. From these calculations, we obtained collisional rate coefficients for the first 45 energy levels of both ortho- and para-H2O and for temperatures in the range T = 5–1500 K. These rate coefficients are subsequently compared to the values previously published for the H2O/He and H2O/H2 collisional systems. It is shown that no simple relation exists between the three systems and that specific calculations are thus mandatory.

WEAK MASER EMISSION OF METHYL FORMATE TOWARD SAGITTARIUS B2(N) IN THE GREEN BANK TELESCOPE PRIMOS SURVEY

A non-LTE radiative transfer treatment of cis-methyl formate (HCOOCH3) rotational lines is presented for the first time using a set of theoretical collisional rate coefficients. These coefficients have been computed in the temperature range 5-30 K by combining coupled-channel scattering calculations with a high accuracy potential energy surface for HCOOCH3-He. The results are compared to observations toward the Sagittarius B2(N) molecular cloud using the publicly available PRIMOS survey from the Green Bank Telescope. A total of 49 low-lying transitions of methyl formate, with upper levels below 25 K, are identified. These lines are found to probe a presumably cold (~30 K), moderately dense (~1e4 cm-3) and extended region surrounding Sgr B2(N). The derived column density of ~4e14 cm-2 is only a factor of ~10 larger than the column density of the trans conformer in the same source. Provided that the two conformers have the same spatial distribution, this result suggests that strongly non-equilibrium processes must be involved in their synthesis. Finally, our calculations show that all detected emission lines with a frequency below 30 GHz are (collisionally pumped) weak masers amplifying the continuum of Sgr B2(N). This result demonstrates the importance and generality of non-LTE effects in the rotational spectra of complex organic molecules at centimetre wavelengths.

Rotational quenching of H2CO by molecular hydrogen: cross-sections, rates and pressure broadening

We compute the rotational quenching rates of the first 81 rotational levels of ortho- and para-H2CO in collision with ortho- and para-H2, for a temperature range of 10-300 K. We make use of the quantum close-coupling and coupled-states scattering methods combined with the high accuracy potential energy surface of Troscompt et al. (2009a). Rates are significantly different from the scaled rates of H2CO in collision with He; consequently, critical densities are noticeably lower. We compare a full close- coupling computation of pressure broadening cross sections with experimental data and show that our results are compatible with the low temperature measurements of Mengel & De Lucia (2000), for a spin temperature of H2 around 50 K.

Reduced dimension discrete variable representation study of cis–trans isomerization in the S1 state of C2H2

Isomerization between the cis and trans conformers of the S(1) state of acetylene is studied using a reduced dimension discrete variable representation (DVR) calculation. Existing DVR techniques are combined with a high accuracy potential energy surface and a kinetic energy operator derived from FG theory to yield an effective but simple Hamiltonian for treating large amplitude motions. The spectroscopic signatures of the S(1) isomerization are discussed, with emphasis on the vibrational aspects. The presence of a low barrier to isomerization causes distortion of the trans vibrational level structure and the appearance of nominally electronically forbidden à (1)A(2)←X̃ (1)Σ(g)(+) transitions to vibrational levels of the cis conformer. Both of these effects are modeled in agreement with experimental results, and the underlying mechanisms of tunneling and state mixing are elucidated by use of the calculated vibrational wavefunctions.

The vibrations and tunnelling of malonaldehyde on a Møller–Plesset surface

The vibrations and tunnelling motion of malonaldehyde have been studied in their full dimensionality using an internal coordinate path Hamiltonian. In this representation there is one large amplitude internal coordinate s and 3N − 7 (=20) normal coordinates Q which are orthogonal to the large amplitude motion at all points. It is crucial that a high accuracy potential energy surface is used in order to obtain a good representation for the tunneling motion; we use a Møller–Plesset (MP2) surface. Our methodology is variational, that is we diagonalize a sufficiently large matrix in order to obtain the required vibrational levels, so an exact representation for the kinetic energy operator is used. In a harmonic valley representation ( s, Q ) complete convergence of the normal coordinate motions and the internal coordinate motions has been obtained; for the anharmonic valley in which we use two- and three-body terms in the surface (s, Q 1, Q 2), we also obtain complete convergence. Our final computed stretching fundamentals are deficient because our potential energy surface is truncated at quartic terms in the normal coordinates, but our lower fundamentals are good.

A very high accuracy potential energy surface for H3

An exact quantum Monte Carlo (EQMC) method was used to calculate the potential energy surface (PES) for the ground electronic state of H3 over a grid of about 76000 nuclear geometries. The absolute abinitio statistical or sampling error of the calculation was ±0.01 kcal mol-1 for energies (V) smaller than 3 eV. This PES was fitted by a three-dimensional cubic spline method and the fitting accuracy was determined from a set of 3684 randomly selected nuclear geometries not used in the fitting. For the range V⩽3 eV the rms fitting error was ±0.010 kcal mol-1, and the absolute value of the corresponding maximum error was 0.018 kcal mol-1. This fitted EQMC PES is an order of magnitude more accurate than the best PES previously obtained for this system. Detailed comparisons are made with previous PESs, for the more dynamically important nuclear configurations.