Thom. H. Dunning

Gaussian Basis Functions for Use in Molecular Calculations. IV. The Representation of Polarization Functions for the First Row Atoms and Hydrogen

The representation of polarization functions for the first row atoms and hydrogen is investigated by means of Hartree—Fock calculations on the water and nitrogen molecules. It is found that a two‐term Gaussian expansion is required to adequately represent the 3d polarization functions of the first row atoms, whereas for the 2p polarization functions of hydrogen, a single Gaussian function is sufficient. The interaction of oxygen and hydrogen polarization functions in the water molecule is briefly discussed.

Near Hartree‐Fock Calculations on the Ground State of the Water Molecule: Energies, Ionization Potentials, Geometry, Force Constants, and One‐Electron Properties

Near Hartree‐Fock wavefunctions have been calculated for the ground state of the water molecule using both Slater and contracted Gaussian basis sets. Total energies of −76.063 hartree were obtained with a (5s4p1d/3s1p) Slater basis and a [6s5p2d/3s1p] contracted Gaussian basis derived from an (11s7p2d/5s1p) primitive set; these energies are estimated to be within 0.003±0.002 hartree of the Hartree‐Fock limit. The Hartree‐Fock wavefunctions account for ∼70% of the dissociation energy of water. The Hartree‐Fock vertical ionization potentials (in electron volts), 11.1(2B1), 13.3(2A1), and 17.6(2B2), are too low by 1–1.5 eV as expected. With the Gaussian basis set a potential surface was computed and the equilibrium geometry and harmonic force constants were calculated. The calculated bond length, 0.941 A, and bond angle 106.6°, are in good agreement with the experimental values, 0.957 A and 104.52°. In spite of the rather good agreement for the geometry, the force constants are in error by 15%–20%. This is a...

Ab initio study of the π‐electron states of trans‐butadiene

Extensive ab initio configuration interaction calculations were carried out on the π‐electron states of trans‐1,3‐butadiene. A double‐ζ contracted Gaussian basis set, augmented with two diffuse 2p π functions on each carbon atom, was used in the calculations, which were based on a frozen σ core obtained from a ground‐state SCF calculation. All excitations fro and to π orbitals were included in the CI treatment. Natural orbitals were obtained for many of the wavefunctions,and their spatial extenty was determined. Only five of the calculated excited states were found to have a valencelike charge distribution (computed vertical excitation energies in eV are given in parentheses): 1 3Bu (3.45), 13Ag(5.04), 21Ag(6.77), 33Bu (8.08), and 15Ag (9.61). These states all correlate with the valence N and T states of ethylene and can be readily described in terms of the ’’two‐vinyl model’’ as either N T (the first two) or TT (doubly excited, the last three). Except for the doubly excited 21Ag, all low‐lying singlet ex...

Comparison of slater and contracted gaussian basis sets in SCF and CI calculations on H2O

Abstract A series of SCF and CI calculations for the elctronic ground state of H 2 O have been carried out with two different 14-function basis sets; one a Clementi-type double zeta STO basis and the other a contracted GTO set. The results obtained with the two bases are compared and analyzed in terms of inner and outer shell correlation and of the contributions of different levels of excitation.

Ab Initio and Semiempirical Calculations of the Static Potential for Electron Scattering off the Nitrogen Molecule

Six calculations, three ab initio and three semiempirical, are presented for the static potential (i.e., the interaction potential energy of an electron with the unperturbed charge distribution) of a nitrogen molecule. The ab initio calculations are Hartree‐Fock calculations using contracted Gaussian basis sets capable of yielding wavefunctions (1) near the Hartree‐Fock limit, (2) near the sp‐limit, and (3) at the minimum basis set level. The semiempirical calculations are all within the INDO formalism. We find that the inclusion of d functions in the ab initio calculations is necessary for an accurate description of the anisotropy of the potential. We also find that the INDO calculations give the correct form of the potential at small electron‐molecule separation distances but are poor at large distances.

Formaldehyde Molecule in a Gaussian Basis: Electronic Density

Plots of the total molecular density and of the orbital densities of the six valence (canonical) molecular orbitals of formaldehyde in its ground state are presented and discussed. The relative spatial extent of the σ and π orbitals is examined in some detail and it is concluded that there is not a pronounced difference in the σ and π charge distributions.

The electronic states of carbon monofluoride: Low-lying valence states

Abstract Multiconfiguration Hartree-Fock calculations are reported on the low-lying valence states, the X2Π, 4Σ−, B2Δ, and 2Σ± states, of carbon monofluoride. The wavefunctions describe dissociation of the molecule to the correct atomic limits and take account of the atomic 2s-2p near-degeneracy effect. For the ground state the calculations give (with experimental values in parentheses): a bond length of 1.286 A (1.2667 A), a fundamental frequency of 1292 cm−1 (1308 cm1), and a dissociation energy of 3.93 eV (5.5 ± 0.1 eV). A 4Σ− state arising from the C ( 3 P) + F ( 2 P) manifold is calculated to lie just 2.66 eV above the ground state. The B2Δ state, calculated adiabatic excitation energy 6.59 eV (6.12 eV), is found to dissociate to C ( 1 D) + F ( 2 P) via a potential maximum. Calculations are also reported on a repulsive 2Δ state arising from ground state atoms.

Accelerating the convergence of matrix Hartree-Fock calculations

Abstract An extrapolation method based on the e k -transformation is applied to matrix Hartree-Fock calculations on the water and formaldehyde molecules. For water the two-electron energy is presented at each iteration for calculations with and without extrapolation. Results are also given to demonstrate the effectiveness of the method for different choices of starting vectors. Experience has shown that each extrapolation usually saves from four to seven iterations.

Chemical Fate of Contaminants in the Environment: Chlorinated Hydrocarbons in the Groundwater

Chlorinated hydrocarbons (CHCs) are the most common contaminant found at hazardous waste sites and are the most prevalent contaminants on (Department of Energy) DOE weapons production sites. Many of the chlorinated hydrocarbons are either known or suspected carcinogens and thus pose health risks to the public and/or site workers. Chlorinated hydrocarbons, unlike simple hydrocarbons, are resistant to biodegradation, but can degrade by abiotic processes such as hydrolysis, nucleophilic substitution, and dehydrochlorination. Unfortunately, few studies of the reactions of chlorinated hydrocarbons have been reported in the literature, and disagreement still exist about the mechanisms and rates of many of the key reactions.

Computational Thermochemistry and Benchmarking of Reliable Methods

During the first and second years of the Computational Thermochemistry and Benchmarking of Reliable Methods project, we completed several studies using the parallel computing capabilities of the NWChem software and Molecular Science Computing Facility (MSCF), including large-scale density functional theory (DFT), second-order Moeller-Plesset (MP2) perturbation theory, and CCSD(T) calculations. During the third year, we continued to pursue the computational thermodynamic and benchmarking studies outlined in our proposal. With the issues affecting the robustness of the coupled cluster part of NWChem resolved, we pursued studies of the heats-of-formation of compounds containing 5 to 7 first- and/or second-row elements and approximately 10 to 14 hydrogens. The size of these systems, when combined with the large basis sets (cc-pVQZ and aug-cc-pVQZ) that are necessary for extrapolating to the complete basis set limit, creates a formidable computational challenge, for which NWChem on NWMPP1 is well suited.