Harry Partridge

The determination of an accurate isotope dependent potential energy surface for water from extensive ab initio calculations and experimental data

We report on the determination of a high quality ab initio potential energy surface (PES) and dipole moment function for water. This PES is empirically adjusted to improve the agreement between the computed line positions and those from the HITRAN 92 data base with J⩽5 for H216O. The changes in the PES are small, nonetheless including an estimate of core (oxygen 1s) electron correlation greatly improves the agreement with the experiment. Using this adjusted PES, we can match 30 092 of the 30 117 transitions in the HITRAN 96 data base for H216O with theoretical lines. The 10, 25, 50, 75, and 90 percentiles of the difference between the calculated and tabulated line positions are −0.11, −0.04, −0.01, 0.02, and 0.07 cm−1. Nonadiabatic effects are not explicitly included. About 3% of the tabulated line positions appear to be incorrect. Similar agreement using this adjusted PES is obtained for the 17O and 18O isotopes. For HD16O, the agreement is not as good, with a root-mean-square error of 0.25 cm−1 for line...

A modification of the Gaussian‐2 approach using density functional theory

The quadratic configuration interaction calculation in the Gaussian‐2 second‐order Mo/ller–Plesset perturbation theory approach, G2(MP2), is replaced by a coupled‐cluster (CC) singles and doubles calculation including a perturbational estimate of the triples excitations. In addition, the self‐consistent‐field (SCF) and MP2 geometry optimizations and SCF frequency calculation in the G2(MP2) approach are replaced by a density functional theory geometry optimization and frequency calculation [using the Becke three parameter hybrid functional with the Lee–Yang–Parr non‐local correlation functional (B3LYP)] in the proposed G2(B3LYP/MP2/CC) approach. This simplification does not affect the average absolute deviation from experiment, but decreases the maximum error compared with the G2(MP2) approach. The G2(B3LYP/MP2/CC) atomization energies are compared with those obtained using the B3LYP approach, and the G2(B3LYP/MP2/CC) model is found to be more reliable, even if the B3LYP calculations are performed using a ...

Chemical-Kinetic Parameters of Hyperbolic Earth Entry

Chemical-kinetic parameters governing the e ow in the shock layer over a heat shield of a blunt body entering Earth’ s atmosphere from a hyperbolic orbit are derived. By the use of the assumption that the heat shield is made of carbon phenolic and by allowing for an arbitrary rateof pyrolysis-gasinjection, chemical reactions occurring in the shock layer are postulated, and the collision integrals governing the transport properties, the rate coefe cients of the reactions, and the parameters needed for the bifurcation model and for the e nite-rate kinetic wall boundary conditions are determined using the best available techniques. Sample e owe eld calculations are performed using this set of parameters to show that the heating and surface removal rates are substantially smaller than calculated using theexisting setofsuch parameters and traditionalassumptionsof gas ‐surfaceequilibrium and quasi-steadystate ablation.

Near Hartree–Fock quality GTO basis sets for the first‐ and third‐row atoms

Energy‐optimized, near Hartree–Fock quality GTO basis sets are reported for the first‐row (Li to Ne) and third‐row (K to Kr) atoms. The most accurate basis sets reported for the first row are (18s13p) sets which are within 4 μEH of the numerical Hartree–Fock (NHF) results. For B to Ne basis sets with more than 15s functions are quadruple zeta in the valence space. For the first‐row transition metal atoms the (20s12p9d) basis sets are triple zeta in the valence space and are approximately equivalent to Clementi and Roetti’s accurate STO sets. Supplementing the (20s12p9d) basis sets optimized for the lowest state with the 4s23dn occupation with a diffuse d function gives SCF energy separations to the 4s13dn+1 and 3dn+2 states which are within 100 μEH of the NHF results. The most accurate basis sets for the transition metal atoms are within 30 μEH of the NHF results. In addition, energy optimized sets are reported for He(3P), Li(2P), and Be(3P).

Theoretical studies of the first- and second-row transition-metal methyls and their positive ions

The metal–carbon bond dissociation energies (D0) and geometries for the first‐ and second‐row transition‐metal methyl neutrals and positive ions are determined. The computed D0 values for the positive ions compare favorably with experiment, except for RuCH+3, RhCH+3, and PdCH+3 where the experimental values are 10–15 kcal/mol larger. The computed D0 values for the hydride and methyl positive ions are similar for all metals in both transition rows except for Cu and Ag. However, for the neutral systems the D0 values for the methyls are smaller, especially on the right‐hand side of both transition rows where the differences approach 15 kcal/mol. In general, the dissociation energies do not follow simple trends, as the individual D0 values are significantly affected by the relative spacings between the atomic states of the metal. The study of all of the methyl neutral and ions of both transition rows presented here provides a consistent set of data for the dissociation energies, thereby allowing a critical as...

Convergence testing of the analytic representation of an ab initio dipole moment function for water: Improved fitting yields improved intensities

In general, when computing intensities for polyatomics, one has to interpolate the dipole moment function obtained from ab initio calculations. For some high overtones of the water molecule, the computed intensities can be very sensitive to the way in which the interpolation is done. Our previous analytic representation [H. Partridge and D. W. Schwenke, J. Chem. Phys. 106, 4618 (1997)] was not adequate. We show that stable results can be obtained, and these results are in much improved agreement with experiment. We also test the importance of core electron correlation on intensities, and find the effect to be negligible. Of the existing water dipole moment functions in the literature, the present one is the most accurate.

THE SENSITIVITY OF B3LYP ATOMIZATION ENERGIES TO THE BASIS SET AND A COMPARISON OF BASIS SET REQUIREMENTS FOR CCSD(T) AND B3LYP

The atomization energies of the 55 G2 molecules are computed using the B3LYP approach with a variety of basis sets. The 6–311 + G(3df) basis set is found to yield superior results to those obtained using the augumented-correlation-consistent valence-polarized triple-zeta set. The atomization energy of SO2 is found to be the most sensitive to basis set and is studied in detail. Including tight d functions is found to be important for obtaining good atomization energies. The results for SO2 are compared with those obtained using the coupled-cluster singles and doubles approach including a perturbational estimate of the triple excitations.

Near Hartree-Fock quality GTO basis sets for the second-row atoms

Energy optimized, near Hartree–Fock quality Gaussian basis sets ranging in size from (17s12p) to (20s15p) are reported for the ground states of the second‐row atoms and for Na (2P), Na+, Na−, Mg (3P), P−, S−, and Cl−. In addition, optimized supplementary functions are given for the ground state basis sets to describe the negative ions, and the excited Na (2P) and Mg (3P) atomic states. The ratios of successive orbital exponents describing the inner part of the 1s and 2p orbitals are found to be nearly independent of both nuclear charge and basis set size. This provides a method of obtaining good starting guesses for other basis set optimizations.

All-electron molecular Dirac-Hartree-Fock calculations - The group IV tetrahydrides CH4, SiH4, GeH4, SnH4, and PbH4

We describe a basis‐set‐expansion Dirac–Hartree–Fock program for molecules. Bond lengths and harmonic frequencies are presented for the ground states of the group IV tetrahydrides CH4, SiH4, GeH4, SnH4, and PbH4. The results are compared with relativistic effective core potential (RECP) calculations, first‐order perturbation theory (PT) calculations and with experimental data. The bond lengths are well predicted by first‐order perturbation theory for all molecules, but none of the sets of RECPs considered provides a consistent prediction. Perturbation theory overestimates the relativistic correction to the harmonic frequencies; the RECP calculations underestimate the correction.

A theoretical study of the positive and dipositive ions of M(NH3)n and M(H2O)n for M=Mg, Ca, or Sr

The structure and binding energies are determined for many of the M(H2O)+n and M(H2O)2+n species, for n=1–3 and M=Mg, Ca, or Sr. The trends are explained in terms of metal sp or sdσ hybridization and core polarization. The M(NH3)+n systems, with M=Mg or Sr, are also studied. For the positive ions, the low‐lying excited states are also studied and compared with experiment. The calculations suggest an alternative interpretation of the SrNH+3 spectrum.