Highly accurate HF dimer ab initio potential energy surface.

A highly accurate, (HF) potential energy surface (PES) is constructed based on ab initio calculations performed at the coupled-cluster single double triple level of theory with an aug-cc-pVQZ-F12 basis set at about 152 000 points. A higher correlation correction is computed at coupled-cluster single double triple quadruple level for 2000 points and is considered alongside other more minor corrections due to relativity, core-valence correlation, and Born-Oppenheimer failure. The analytical surface constructed uses 500 constants to reproduce the ab initio points with a standard deviation of 0.

Absolute C/C Isotope Amount Ratio for Vienna Pee Dee Belemnite from Infrared Absorption Spectroscopy.

Measurements of isotope ratios are predominantly made with reference to standard specimens that have been characterized in the past. In the 1950s, the carbon isotope ratio was referenced to a belemnite sample collected by Heinz Lowenstam and Harold Urey in South Carolina's Pee Dee region. Due to the exhaustion of the sample since then, reference materials that are traceable to the original artefact are used to define the Vienna Pee Dee Belemnite (VPDB) scale for stable carbon isotope analysis.

A highly accurate dipole moment surface for the ground electronic state of water vapour for spectra extending into the ultraviolet.

A new global and highly accurate dipole moment surface (DMS) for water vapour is presented. This DMS is based on a set of 17 628 multi-reference configuration interaction data points that were calculated with the aug-cc-pCV6Z basis set with the Douglas-Kroll-Hess Hamiltonian; tests are performed at several other levels of theory. This new "CKAPTEN" DMS improves agreement with recent experimental measurements compared with previous models that poorly predicted some bands in the infrared while also maintaining or improving on the agreement for all remaining strong lines.

High-accuracy water potential energy surface for the calculation of infrared spectra.

Transition intensities for small molecules such as water and CO can now be computed with such high accuracy that they are being used to systematically replace measurements in standard databases. These calculations use high-accuracy dipole moment surfaces and wave functions from spectroscopically determined potential energy surfaces (PESs). Here, an extra high-accuracy PES of the water molecule (HO) is produced starting from an PES which is then refined to empirical rovibrational energy levels.

High Accuracy ab Initio Calculations of Rotational-Vibrational Levels of the HCN/HNC System.

Highly accurate ab initio calculations of vibrational and rotational-vibrational energy levels of the HCN/HNC (hydrogen cyanide/hydrogen isocyanide) isomerising system are presented for several isotopologues. All-electron multireference configuration interaction (MRCI) electronic structure calculations were performed using basis sets up to aug-cc-pCV6Z on a grid of 1541 geometries. The ab initio energies were used to produce an analytical potential energy surface (PES) describing the two minima simultaneously.

Saddle point localization of molecular wavefunctions.

The quantum mechanical description of isomerization is based on bound eigenstates of the molecular potential energy surface. For the near-minimum regions there is a textbook-based relationship between the potential and eigenenergies. Here we show how the saddle point region that connects the two minima is encoded in the eigenstates of the model quartic potential and in the energy levels of the [H, C, N] potential energy surface.

High-Accuracy CO(2) Line Intensities Determined from Theory and Experiment.

Atmospheric CO(2) concentrations are being closely monitored by remote sensing experiments which rely on knowing line intensities with an uncertainty of 0.5% or better. Most available laboratory measurements have uncertainties much larger than this.

Communication: Visible line intensities of the triatomic hydrogen ion from experiment and theory.

The visible spectrum of H3(+) is studied using high-sensitivity action spectroscopy in a cryogenic radiofrequency multipole trap. Advances are made to measure the weak ro-vibrational transitions from the lowest rotational states of H3(+) up to high excitation energies providing visible line intensities and, after normalisation to an infrared calibration line, the corresponding Einstein B coefficients. Ab initio predictions for the Einstein B coefficients are obtained from a highly precise dipole moment surface of H3(+) and found to be in excellent agreement, even in the region where states have been classified as chaotic.

Calculation of rotation-vibration energy levels of the water molecule with near-experimental accuracy based on an ab initio potential energy surface.

A recently computed, high-accuracy ab initio Born-Oppenheimer (BO) potential energy surface (PES) for the water molecule is combined with relativistic, adiabatic, quantum electrodynamics, and, crucially, nonadiabatic corrections. Calculations of ro-vibrational levels are presented for several water isotopologues and shown to have unprecedented accuracy. A purely ab initio calculation reproduces some 200 known band origins associated with seven isotopologues of water with a standard deviation (σ) of about 0.

Spectroscopy of H3+ based on a new high-accuracy global potential energy surface.

The molecular ion H(3)(+) is the simplest polyatomic and poly-electronic molecular system, and its spectrum constitutes an important benchmark for which precise answers can be obtained ab initio from the equations of quantum mechanics. Significant progress in the computation of the ro-vibrational spectrum of H(3)(+) is discussed. A new, global potential energy surface (PES) based on ab initio points computed with an average accuracy of 0.

Calibration-quality adiabatic potential energy surfaces for H3(+) and its isotopologues.

Calibration-quality ab initio adiabatic potential energy surfaces (PES) have been determined for all isotopologues of the molecular ion H(3)(+). The underlying Born-Oppenheimer electronic structure computations used optimized explicitly correlated shifted Gaussian functions. The surfaces include diagonal Born-Oppenheimer corrections computed from the accurate electronic wave functions.

Precision measurements and computations of transition energies in rotationally cold triatomic hydrogen ions up to the midvisible spectral range.

First-principles computations and experimental measurements of transition energies are carried out for vibrational overtone lines of the triatomic hydrogen ion H(3)(+) corresponding to floppy vibrations high above the barrier to linearity. Action spectroscopy is improved to detect extremely weak visible-light spectral lines on cold trapped H(3)(+) ions. A highly accurate potential surface is obtained from variational calculations using explicitly correlated Gaussian wave function expansions.

Collisionally assisted spectroscopy of water from 27,000 to 34,000 cm(-1).

We report here an experimental approach that enables measurement of weak transitions to a wide range of rovibrational levels of water in the energy region 27,000-34,200 cm(-1). We have previously demonstrated the use of laser double-resonance overtone excitation to access highly excited vibrational levels from single rovibrational states. Although this approach simplifies the assignment of the spectra, it strongly reduces the number of observed transitions and hence our ability to test theoretical predictions.

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.

Approaching the full set of energy levels of water.

We report here the measurements of rovibrational levels in the electronic ground state of water molecule at the previously inaccessible energies above 26,000 cm(-1). The use of laser double-resonance overtone excitation extends this limit to 34,200 cm(-1), which corresponds to 83% of the water dissociation energy. We use experimental data to generate a semiempirical potential energy surface that now allows prediction of water levels with sub-cm(-1) accuracy at any energy up to the new limit.

CVRQD ab initio ground-state adiabatic potential energy surfaces for the water molecule.

The high accuracy ab initio adiabatic potential energy surfaces (PESs) of the ground electronic state of the water molecule, determined originally by Polyansky et al. [Science 299, 539 (2003)] and called CVRQD, are extended and carefully characterized and analyzed. The CVRQD potential energy surfaces are obtained from extrapolation to the complete basis set of nearly full configuration interaction valence-only electronic structure computations, augmented by core, relativistic, quantum electrodynamics, and diagonal Born-Oppenheimer corrections.

Continuous-wave cavity ringdown spectroscopy of the 8nu polyad of water in the 25,195-25,340 cm(-1) range.

State-of-the-art experiments and calculations are used to record and assign the data obtained in the weakly absorbing blue energy region of the H2O spectrum. Continuous-wave cavity ringdown absorption spectroscopy with Doppler resolution is used to probe the range from 25,195 to 25,470 cm(-1) with an absorption sensitivity of approximately 1 parts per 10(9) (ppb)/cm. 62 lines of the polyad nu(OH)=8 are reported, of which 43 are assigned using variational nuclear calculations.

On equilibrium structures of the water molecule.

Equilibrium structures are fundamental entities in molecular sciences. They can be inferred from experimental data by complicated inverse procedures which often rely on several assumptions, including the Born-Oppenheimer approximation. Theory provides a direct route to equilibrium geometries.

A 3000 K laboratory emission spectrum of water.

An emission spectrum of hot water with a temperature of about 3000 K is obtained using an oxy-acetylene torch. This spectrum contains a very large number of transitions. The spectrum, along with previous cooler laboratory emission spectra and an absorption spectrum recorded from a sunspot, is analyzed in the 500-2000 cm(-1) region.

Analysis of hot D2O emission using spectroscopically determined potentials.

Fourier transform emission spectra of D2O vapor were recorded at a temperature of 1500 degrees C in the wavenumber range 380-1880 cm(-1). 15 346 lines were measured, of which the majority were identified as belonging to D2O. The spectrum was analyzed using variational nuclear motion calculations based on spectroscopically determined potential-energy surfaces.

High-accuracy ab initio rotation-vibration transitions for water.

The spectrum of water vapor is of fundamental importance for a variety of processes, including the absorption and retention of sunlight in Earth's atmosphere. Therefore, there has long been an urgent need for a robust and accurate predictive model for this spectrum. In our work on the high-resolution spectrum of water, we report first-principles calculations that approach experimental accuracy.

Ab initio rotation-vibration energy levels of triatomics to spectroscopic accuracy.

The factors that need to be taken into account to achieve spectroscopic accuracy for triatomic molecules are considered focusing on H3+ and water as examples. The magnitude of the adiabatic and non-adiabatic corrections to the Born-Oppenheimer approximation is illustrated for both molecules, and methods of including them ab initio are discussed. Electronic relativistic effects are not important for H3+, but are for water for which the magnitude of the various effects is discussed.

The Water Vapor Spectrum in the Region 8600-15 000 cm(-1): Experimental and Theoretical Studies for a New Spectral Line Database.

The new laboratory measurements of R. Schermaul et al. (J.

The Water Vapor Spectrum in the Region 8600-15 000 cm(-1): Experimental and Theoretical Studies for a New Spectral Line Database.

New laboratory measurements are presented for the near-infrared and visible spectrum (8600-15 000 cm(-1)) of water vapor. Spectral line parameters, principally intensities and air-broadening coefficients, are derived from Fourier transform spectroscopic measurements at high resolution (0.03 cm(-1)), a range of optical path lengths (5-513 m), and temperatures of both 252 and 296 K.