Ab initio calculation of the line strengths in the electric dipole vibration-rotation spectrum of the HD molecule
Explore the source record for details and available documents.
Ford, A. L.↗
SEARCH · Search NASA
Results for “ab initio calculations”
Search indexed NASA NTRS and DOE OSTI research on propulsion, heat transfer, battery materials and energy systems. Follow report and document links to the original sources.
Quote a phrase for an exact phrase match. Source license links do not imply unrestricted reuse.
Ab initio calculation of the X 1 Sigma + and A 1 Sigma + states of CsH
Explore the source record for details and available documents.
Laskowski, B.↗
Ab initio calculation of the third-order susceptibility of H2
The third-order susceptibility of H2 has been calculated under coherent anti-Stokes Raman-scattering conditions, with use of wave functions specifically designed for accurate calculations of sum-rule properties. The theoretical values are in good agreement with experiment.
Huo, W. M.↗
Ab initio calculation of the X 1 Sigma + state of CsH
Stevens et al. (1981) considered CsH as a two electron problem, but simulated the Cs core electrons by an empirical pseudopotential following the work by Bardsley (1970). These potentials, since they are derived empirically, include the relativistic effects and the atomic core-valence interactions. However, molecular core-core interactions are not accounted for. Stevens et al. obtained an R(e) value which is too small. This result was attributed to uncorrected core (proton)-core interactions in the molecule. The present investigation is concerned with a resolution of the discrepancies between the results of earlier studies. The X 1 Sigma + ground state of CsH is recalculated. The calculation employs a nine valence electron relativistic effective core potential (RECP) for Cs. The bonding in CsH is found to involve a Cs(6s)-H(1s) bond but with a significant ionic (Cs+H-) component.
Laskowski, B. C.↗
Ab initio calculation of the two-photon absorption cross section of the X1Sigma g(+) - (EF)1Sigma g(+) in H2
Explore the source record for details and available documents.
Huo, W. M.↗
Ab initio calculations on the positive ions of the alkaline-earth oxides, fluorides, and hydroxides
Theoretical dissociation energies are presented for the alkaline-earth fluoride, hydroxide, and oxide positive ions that are considered to be accurate to 0.1-0.2 eV. The r(e) for the positive ions are found to be consistently shorter than the corresponding neutrals by 0.07 + or -0.02 A. The bonding in the ground states is demonstrated to be of predominantly M + 2 X - character. The a 3 Pi and A 1 Pi are found to lie considerably above the X 1 Sigma + ground states of the alkaline-earth fluoride and hydroxide positive ions. The overall agreement of the theoretical ionization potentials with the available experimental appearance potentials is satisfactory; these values should represent the most accurate and consistent set available.
Partridge, H.↗
Accurate ab initio calculations which demonstrate a 3Pi(u) ground state for Al2
The computational requirements for accurately describing the spectroscopic constants for all three candidates for the ground state of Al2 are determined. Full CI(FCI) calculations are used to calibrate approximate methods of including the electron correlation. CASSCF/MRCI calculations which accurately reproduce the FCI results in the valence DZ + 2d Gaussian basis are carried to chemical accuracy by using extensive one-particle basis sets. The effect of 2s and 2p correlation and relativistic effects are considered as well as the valence 3s and 3p correlation. Several excited states are also considered. It is shown that the computed vibrational frequency of the (2) 3Pi(g) state and the Franck-Condon factors for the (1) 3Pi(u) - (2) 3Pi(g) transition are consistent with the spectrum obtained by Douglas et al. (1983) and Abe and Kolb (1983). It is therefore concluded that the ground state is 3Pi(u).
Bauschlicher, Charles W., Jr.↗
Accurate ab initio calculations which demonstrate a 3 Pi u ground state for Al2
The spectroscopic parameters and separations between the three low-lying X 3 Pi u, A 3 Sigma g -, and a 1 Sigma g + states of Al2 are studied as a function of both the one-particle and n-particle basis set. Approximate correlation treatments are calibrated against full Cl calculations correlating the six valence electrons in a double-zeta plus two d-function basis set. Since the CASSCF/MRCI 3 Pi u to 3 Sigma g - separation is in excellent agreement wtih the FCI value, the MRCI calculations were carried out in an extended (20s13p6d4f)/(6s5p3d2f) gaussian basis. Including a small correction for relativistic effects, the best estimate is that 3 Sigma g - state lies 174/cm above the 3 Pi u ground state. The 1 Sigma g + state lies at least 2000/cm higher in energy. At the CPF level, inclusion of 2s and 2p correlation has little effect on D sub e, reduces T sub e by only 26/cm, and shortens the bond lengths by about 0.02 a sub o. Further strong support for a 3 Pi u ground state comes from the experimental absorption spectra, since both observed transitions can be convincingly assigned as 3 Pi u yields 3 Pi g. The (2) 3 Pi g state is observed to be sensitive to the level of correlation treatment, and to have its minimum shifted to shorter rho values, such that the strongest experimental absorption peak probably corresponds to the 0 yields 2 transition.
Bauschlicher, Charles W., Jr.↗
Accurate ab initio calculations for the ground states of N2, O2 and F2
Spectroscopic constants and dissociation energies for the ground states of N2, O2, and F2 determined at the CAS SCF MRCI correlation level are in excellent agreement with experiment when very large primitive valence and polarization one-particle Gaussian basis sets are employed. The dissociation energy (De) for N2 is larger than experiment unless the 2s electrons are correlated. The basis set requirement for an accurate determination of De is found to increase with the degree of multiple-bond character in the molecule.
Langhoff, Stephen R.↗
Ab Initio Calculations of Anharmonic Vibrational Spectroscopy for Hydrogen Fluoride (HF)n (n=3,4) and Mixed Hydrogen Fluoride/Water (HF)n(H20)n (n=1,2,4) Clusters
Anharmonic vibrational frequencies and intensities are computed for hydrogen fluoride clusters (HF)n with n=3,4 and mixed clusters of hydrogen fluoride with water (HF)n(H2O)n where n=1,2. For the (HF)4(H2O)4 complex, the vibrational spectra are calculated at the harmonic level, and anharmonic effects are estimated. Potential energy surfaces for these systems are obtained at the MP2/TZP level of electronic structure theory. Vibrational states are calculated from the potential surface points using the correlation-corrected vibrational self-consistent field (CC-VSCF) method. The method accounts for the anharmonicities and couplings between all vibrational modes and provides fairly accurate anharmonic vibrational spectra that can be directly compared with experimental results without a need for empirical scaling. For (HF)n, good agreement is found with experimental data. This agreement shows that the MP2 potential surfaces for these systems are reasonably reliable. The accuracy is best for the stiff intramolecular modes, which indicates the validity of MP2 in describing coupling between intramolecular and intermolecular degrees of freedom. For (HF)n(H2O)n experimental results are unavailable. The computed intramolecular frequencies show a strong dependence on cluster size. Intensity features are predicted for future experiments.
Chaban, Galina M.↗
Ab initio calculations of anharmonic vibrational spectroscopy for hydrogen fluoride (HF)n (n = 3, 4) and mixed hydrogen fluoride/water (HF)n(H2O)n (n = 1, 2, 4) clusters
Anharmonic vibrational frequencies and intensities are computed for hydrogen fluoride clusters (HF)n, with n = 3, 4 and mixed clusters of hydrogen fluoride with water (HF)n(H2O)n where n = 1, 2. For the (HF)4(H2O)4 complex, the vibrational spectra are calculated at the harmonic level, and anharmonic effects are estimated. Potential energy surfaces for these systems are obtained at the MP2/TZP level of electronic structure theory. Vibrational states are calculated from the potential surface points using the correlation-corrected vibrational self-consistent field method. The method accounts for the anharmonicities and couplings between all vibrational modes and provides fairly accurate anharmonic vibrational spectra that can be directly compared with experimental results without a need for empirical scaling. For (HF)n, good agreement is found with experimental data. This agreement shows that the Moller-Plesset (MP2) potential surfaces for these systems are reasonably reliable. The accuracy is best for the stiff intramolecular modes, which indicates the validity of MP2 in describing coupling between intramolecular and intermolecular degrees of freedom. For (HF)n(H2O)n experimental results are unavailable. The computed intramolecular frequencies show a strong dependence on cluster size. Intensity features are predicted for future experiments.
Water/chemistry↗
Towards Accurate Ab Initio Predictions of the Spectrum of Methane
We have carried out extensive ab initio calculations of the electronic structure of methane, and these results are used to compute vibrational energy levels. We include basis set extrapolations, core-valence correlation, relativistic effects, and Born- Oppenheimer breakdown terms in our calculations. Our ab initio predictions of the lowest lying levels are superb.
Schwenke, David W.↗
Towards accurate ab initio predictions of the vibrational spectrum of methane
We have carried out extensive ab initio calculations of the electronic structure of methane, and these results are used to compute vibrational energy levels. We include basis set extrapolations, core-valence correlation, relativistic effects, and Born-Oppenheimer breakdown terms in our calculations. Our ab initio predictions of the lowest lying levels are superb.
Spectrophotometry, Infrared/methods↗
Experimental and Ab Initio Studies of the HDO Absorption Spectrum in the 13165-13500 1/cm Spectral Region
The HDO absorption spectrum has been recorded in the 13165 - 13500 cm(exp-1) spectral region by Intracavity Laser Absorption Spectroscopy. The spectrum (615 lines), dominated by the 2n2 + 3n3 and n1+3n3 bands was assigned and modeled leading to the derivation of 196 accurate energy levels of the (103) and (023) vibrational states. Finally, 150 of these levels have been reproduced by an effective Hamiltonian involving two vibrational dark states interacting with the (023) and ( 103) bright states. The rms deviation achieved by variation of 28 parameters is 0.05-1 cm, compared to an averaged experimental uncertainty of 0.007-1 cm, indicating the limit of validity of the effective Hamiltonian approach for HDO at high vibrational excitation. The predictions of previous ab initio calculations of the HDO spectrum were extensively used in the assignment process. The particular spectral region under consideration has been used to test and discuss the improvements of new ab initio calculations recently performed on the basis of the same potential energy surface but with an improved dipole moment surface. The improvements concern both the energy levels and the line intensities. In particular, the strong hybrid character of the n1+3n3 band is very well accounted for by the the new ab initio calculations.
Schwenke, David↗
Conformational Characteristics of Poly(tetrafluoroethylene) (PTFE) Based Upon Ab Initio Electronic Structure Calculations on Model Molecules
Conformational energy contours of perfluoroalkanes, determined from ab initio calculations, confirm the well-known spitting of trans states into two minima at plus or minus 17 degrees but also show that the gauche states split as well, with minima at plus or minus 124 degrees and plus or minus 84 in order to relieve steric crowding. The directions of such split distortions from the perfectly staggered states are strongly coupled for adjacent pairs of bonds in a manner identical to the intradyad pair for poly (isobutylene) chains. These conformational characteristics are fully represented by a six-state rotational isomeric state (RIS) model for PTFE comprised of t(+), t(-), g(sup +)+, g(sup +)-, g(sup -) + and g(sup -)-states, located at the split energy minima. The resultant 6 x 6 statistical weight matrix is described by first-order interaction parameters for the g+(+) (ca. 0.6 kcal/mol) and g+- (ca. 2.0 kcal/mol) states, and second order parameters for the g(sup +)+g(sup +)+ (ca 0.6 kcal/mol) and g(sup +)+g(sup -)+ (ca. 1.0 kcal/mol) states. This six-state RIS model, without adjustment of the geometric or energy parameters as determined from the ab initio calculations, predicts the unperturbed chain dimensions and the fraction of gauche bonds as a function of temperature for PTFE in good agreement with available experimental values.
Smith, Grant D.↗
Zeeman effect of P II.
An investigation was conducted of the Zeeman effect of PII because it provides unique J-value assignments together with a sensitive check on the accuracy of wave functions obtained from least-squares fitting of energy levels. About forty new lines were added to the spectrum, and g values were obtained for 76 levels of PII. By incorporating this new experimental material in a systematic program of least-squares calculations, supplemented by ab initio calculations, it was possible to complete the description of the 3p5p and 3p5d configurations.
Li, H.↗
A coupled channel study of HN2 unimolecular decay based on a global ab initio potential surface
The unimolecular decay lifetimes of several vibrational states of HN2 are determined on the basis of an accurate coupled channel dynamics study using a global analytical potential surface. The surface reproduces the ab initio points with an rms error of 0.08 kcal/mol for energies below 20 kcal/mol. Modifications to the potential that describe the effect of improving the basis set in the ab initio calculations are provided. Converged coupled channel calculations are performed for the ground rotational state of HN2 to determine the lifetimes of the lowest ten vibrational states. Only the ground vibrational state (000) and first excited bend (001) are found to have lifetimes longer than 1 ps. The lifetimes of these states are estimated at 3 x 10 to the -9th and 2 x 10 to the -10th s, respectively. Variation of these results with quality of the ab initio calculations is not more than a factor of 5.
Koizumi, Hiroyasu↗
The rotational spectra of HOCO/plus/, HOCN, HN3, and HNCO from quantum mechanical calculations
Ab initio molecular orbital theory has been used to determine the equilibrium geometries, rotational constants, and rotational spectra of four isoelectronic molecules. Two of these, HOCO(plus) and HOCN, are candidate interstellar molecules. The other two, HNCO and HN3, have known rotational constants. Theoretical rotational constants and spectra for the two unknown species were corrected with the mean experimental to theoretical ratios from the two known species. This procedure resulted in predicted frequencies of 83.75 plus or minus 0.2 GHz for the 4(04) to 3(03) transition in HOCN and 85.08 plus or minus 0.2 GHz for the same transition in HOCO(plus). These are the central lines of triplets whose other members are the 4(14) to 3(13) and 4(13) to 3(12) transitions. The triplet splittings were predicted to be 0.36 plus or minus 0.01 GHz for HOCN and 0.33 plus or minus 0.01 GHz for HOCO(plus). These results indicate that HOCO(plus) is a better candidate for the source of a series of lines reported by Thaddeus, Guelin, and Linke than is HOCN.
Defrees, D. J.↗