The use of supercomputers for the variational calculation of ro-vibrationally excited states of floppy molecules

The advent of supercomputers has led to great advances in electronic structure calculations and to the ab initio calculation of molecular spectra. Recent theoretical developments have allowed us to develop a two-step variational algorithm for the calculation of rotationally highly excited states of floppy molecules. This algorithm allows highly accurate nuclear motion calculations to be performed on low-lying ro-vibrational states and greatly extends the range of states that can practicably be considered. The algorithm has been adapted to run efficiently on the Cray supercomputers. Analysis of the timings suggest that construction of the secular matrix is highly vectorised and that the special structure of secular matrix can be used to give rapid diagonalisation. The limiting factor on these calculations is the available fast storage, but analysis suggests that this bottleneck could be removed by use of a Solid State Device (SSD). Sample results are given for calculations involving a range of rotational excitation. An adaptation of the algorithm to a loop of parallel processors is also suggested.This paper was presented at the International Conference on The Impact of Supercomputers on Chemistry, held at the University of London, London, UK, 13–16 April 1987     

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.     

Resonant States of H3+ and D2H+

Vibrational resonances for H(3) (+) and D(2)H(+), as well as H(3) (+) at J=3, are calculated using a complex absorbing potential (CAP) method with an automated procedure to find stability points in the complex plane. Two different CAP functional forms and different CAP extents are used to analyze the consistency of the results. Calculations are performed using discrete variable representation continuum basis elements calculated to high levels of accuracy by diagonalizing large, dense, Hamiltonian matrices. For D(2)H(+), two energy regions are analyzed: the one where D(2)+H(+) is the only dissociation product and the one where HD+D(+) can also be formed. Branching ratios are obtained in the latter case by using different CAPs. It is shown that H(3) (+) and D(2)H(+) support some narrow Feshbach-type resonances but that higher angular momentum states must be studied to model the pre-dissociation spectrum recorded by Carrington and co-workers [J. Chem. Phys. 98, 1073 (1993)].     

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.     

Line lists for H218O and H217O based on empirical line positions and ab initio intensities

New line lists for isotopically substituted water are presented. Most line positions were calculated from experimentally determined energy levels, while all line intensities were computed using an ab initio dipole moment surface. Transitions for which experimental energy levels are unavailable use calculated line positions. These line lists cover the range 0.05–20 000 cm^?1 and are significantly more complete and potentially more accurate than the line lists available via standard databases. All lines with intensities (scaled by isotopologue abundance) greater than 10^?29 cm/molecule at 296 K are included, augmented by weaker lines originating from pure rotational transitions. The final line lists contain 39 918 lines for H₂¹⁸O and 27 546 for H₂¹⁷O and are presented in standard HITRAN format. The number of experimentally determined H₂¹⁸O and H₂¹⁷O line positions is, respectively, 32 970 (83% of the total) and 17 073 (62%) and in both cases the average estimated uncertainty is 2×10^?4 cm^?1. The number of ab initio line intensities with an estimated uncertainty of 1% is 16 621 (42%) for H₂¹⁸O and 13 159 (48%) for H₂¹⁷O.     

Shift of the centers of H<Subscript>2</Subscript>O absorption lines in the region of 1.06 μm

The shift coefficients for the lines of the ν₁ + ν₂ + ν₃ and ν₂ + 2ν₃ bands of H₂O in the region from 9403 to 9413 cm^?1 are measured and calculated. The measurements are performed using an intracavity laser spectrometer based on a neodymium laser with a determination error of the line center of 0.003–0.004 cm^?1. The Ar, Kr, and Xe noble gases, as well as nitrogen, oxygen, and hydrogen were used as buffer gases. The coefficients of shifts in eight H₂O absorption lines induced by oxygen, nitrogen, and atmospheric air pressures fall into the region from ?0.004 to ?0.069 cm^?1/bar. The calculations are performed by a semiempirical method using variational wave functions, which, in contrast to other studies, correctly takes into account intramolecular interactions. The calculated values agree satisfactorily with experimental data.     

Measurement of the growth of sub-microscopic fatigue cracks by ellipsometry

Aluminum (1100) samples have been monitored with ellipsometry during fatigue cycling. Large changes in the ellipsometric parameters Δ and ψ are believed to be primarily associated with the formation of a sub-microscopic network of cracks. The ellipsometric results are interpreted in terms of the Fenstermaker-McCrackin model for surface roughening modified to simulate the crack system.     

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. Use of a calculated variational linelist for water allows significant progress to be made on assigning transitions involving highly excited vibrational and rotational states. In particular emission from rotationally excited states up to J=42 and vibrational levels with up to eight quanta of bending motion are assigned.     

Can ortho-para transitions for water be observed?

The spectrum of water can be considered as the juxtaposition of the spectra of two molecules, with different total nuclear spin: ortho-H2O, and para-H2O. No transitions have ever been observed between the two different nuclear-spin isotopomers. The interconversion time is unknown and it is widely assumed that interconversion is forbidden without some other intervention. However, weak nuclear spin-rotation interaction occurs and can drive ortho to para transitions. Ab initio calculations show that the hyperfine nuclear spin-rotational coupling constants are about 30 kHz. These constants are used to explore the whole vibration-rotation spectrum with special emphasis on the coupling between nearby levels. Predictions are made for different spectral regions where the strongest transitions between ortho and para levels of water could be experimentally observed.     

Electron-molecule collisions at low and intermediate energies using the R-matrix method

We present the latest developments of the R-matrix method as applied to electron-molecule collisions. A variety of calculations for H₂O are presented including the study of rotational excitation and preliminary data for dissociative electron attachment. Results for the application of the recently developed molecular R-matrix with pseudostates (MRMPS) method to neutral and cationic targets are also included. This method is currently being applied to the study of collisions with anionic targets.