Contracted Basis Sets Research Articles

Gaussian wave functions for CH3 and NH

AbstractGround state single determinant LCAO‐MO‐SCF wave functions, using a large contracted Gaussian basis set (6s, 2p, 1d/3s, 1p), have been computed for the 9 electron molecular systems of CH3 and NH. The minimum energies obtained using Roothaan's open shell SCF procedure for the planar equilibrium geometries were −39.5703 Hartree for CH3 and −55.8945 Hartree for NH. Additional properties such as electron populations and multiple moments were calculated from the planar wave functions.

Configuration Interaction Wavefunctions and Computed Inversion Barriers for NH3 and CH3−

Ground state LCAO-MO-SCF wavefunctions, using a large contracted Gaussian basis set, have been constructed for NH3 and CH3−. These wavefunctions were determined for four points along the inversion coordinates of the respective molecules, and were then improved by the method of configuration interaction (CI). In these CI calculations, the computed energies of NH3 and CH3− were obtained to within 0.18 hartree of their respective nonrelativistic limits. The inversion barrier, as computed with the CI wavefunctions, was 6.9 kcal/mole for NH3 and 2.8 kcal/mole for CH3−.

Geometry and force constant determination from correlated wave functions for polyatomic molecules: Ground states of H2O and CH2

Ab initio calculations including electron correlation are reported for the water and methylene molecules as a function of geometry. A large contracted gaussian basis set is used and the multiconfiguration wave functions, optimized by the iterative natural orbital procedure, include 277 and 617 configurations for H2O and CH2 respectively. The method of selecting configurations, yielding “first-order” wave functions, is discussed in some detail. For H2O, the SCF geometry is r=0,942 A, θ=105,8°, the correlated result is r=0,968 A, θ=103,2°, and the experimental r=0,957 A, θ=104,5°. The water stretching force constants, in millidynes/A, are 8,72 (SCF), 8,75 (CI), and 8,4 (experiment). Bending force constants are 0,88 (SCF), 0,83 (CI), and 0,76 (experiment). For methylene the SCF geometry is r=1,072 A, θ=129,5°, while the result from first-order wave functions is r=1,088 A, θ=134°. The predicted CH2 force constants are 6,16 (SCF) and 6,13 (CI) for stretching and 0,44 (SCF) and 0,33 (CI) for bending.

Gaussian Basis Functions for Use in Molecular Calculations. III. Contraction of (10s6p) Atomic Basis Sets for the First-Row Atoms

Contracted [5s3p] and [5s4p] Gaussian basis sets for the first-row atoms are derived from the (10s6p) primitive basis sets of Huzinaga. Contracted [2s] and [3s] sets for the hydrogen atom obtained from primitive sets ranging in size from (4s) to (6s) are also examined. Calculations on the water and nitrogen molecules indicate that such basis sets when augmented with suitable polarization functions should yield wavefunctions near the Hartree–Fock limit.

A comparison of one-electron properties calculated from Gaussian SCF and CI wavefunctions

Ground state LCAO-MO-SCF single- and multiconfiguration wavefunctions of NH3 and CH 3 − , constructed from a large contracted Gaussian basis set, have been analyzed in terms of one-electron expectation values. Both sets of values agree fairly well with the experimental data in the case of NH3. It is expected therefore that the results obtained for CH 3 − are also reliable.

Gaussian basis functions for use in molecular calculations. Contraction of (12s9p) atomic basis sets for the second row atoms

The contraction of the (12s9p) primitive basis sets of Veillard for the second row atoms is investigated. Contracted [7s], [6s], [5p], and [4p] basis sets are proposed. The [6s4p] contracted set gives a Hartree-Fock energy of −459.1706 au for the ground state of the chlorine atom. This is a substantial improvement over results previously reported for contracted gussian basis sets, even those obtained with much larger primitive sets.

Gaussian Basis Functions for Use in Molecular Calculations. I. Contraction of (9s5p) Atomic Basis Sets for the First-Row Atoms

The contraction of Gaussian basis functions for use in molecular calculations is investigated by considering the effects of contraction on the energies and one-electron properties of the water and nitrogen molecules. The emphasis is on obtaining principles which can be used to predict optimal contraction schemes for other systems without the necessity of such exhaustive calculations. Using these principles, contractions are predicted for the first-row atoms.

Geometry and electronic states of CuF 2

The non-empirical SCF MO theory using a contracted gaussian basis set has been applied to CuF 2 in an attempt to resolve questions about its geometry and electronic states.

One-Electron Properties of Near-Hartree–Fock Wavefunctions. II. HCHO, CO

The results of Hartree–Fock level self-consistent-field molecular orbital calculations using contracted Gaussian basis sets are reported for the ground states of the formaldehyde and carbon monoxide molecules. The best computed wavefunctions are shown to have total energies, at most, 0.05 a.u. from their Hartree–Fock limits. A large number of one-electron properties were computed from the wavefunctions. Results for the carbon monoxide molecule are in excellent agreement with the Slater basis Hartree–Fock results of Huo [J. Chem. Phys. 43, 624 (1965)]. For formaldehyde the calculated molecular properties are in good agreement with experiment. Our best estimates for several of the properties in HCHO are: dipole moment, μz = 2.82 D; quadrupole moment, θaa = 0.0056 × 10−26 esu·cm2, θbb = 0.250 × 10−26esu·cm2, θcc = − 0.256 × 10−26esu·cm2; diamagnetic shielding at the proton, σaad(H) = 92.63 ppm, σbbd(H) = 94.68 ppm, σabd(H) = 52.80 ppm, σccd(H) = 147.13 ppm, and σAvd(H) = 111.48 ppm; diamagnetic shielding at 13C, σaad(C) = 288.50 ppm, σbbd(C) = 355.34 ppm, σccd(C) = 371.60 ppm, and σAvd(C) = 338.48 ppm, diamagnetic susceptibilities, χaad = − 27.99, χbbd = − 54.19, χccd = − 61.53, and χAvd = − 47.91 emu/mole; deuteron quadrupole coupling constants, (eqQ/h)AA = 172.4 kc/sec, (eqQ/h)BB = − 87.22 kc/sec, and (eqQ/h)CC = − 85.21 kc/sec; 17O quadrupole coupling constants, (eqQ/h)aa = − 2.28 Mc/sec, (eqQ/h)bb = 12.8 Mc/sec, and (eqQ/h)cc = −10.5 Mc/sec.

Ab initio Calculations on cytosine, thymine and adenine

An ab initio LCAO SCF calculation has been performed in a small contracted GTO basis set on three bases of ADN. A check of the representativity of the basis set is reported for formamide and pyrrole.

One-Electron Properties of Near-Hartree–Fock Wavefunctions. I. Water

Self-consistent-field calculations are reported for the ground state of the water molecule in a contracted and uncontracted Gaussian basis set. The uncontracted set is shown to be near the Hartree–Fock limit for water. One-electron properties were computed from both wavefunctions. Our best estimates for several of these quantities are: dipole moment, μz = 1.995 D; quadrupole moment, θzz = − 0.108 and θxx = − 2.422 in buckinghams; octupole moment, Ωxxz = − 1.337 and Ωzzz = − 0.960 in units of 10−34 esu·cm3; average diamagnetic shielding at the proton, σAvd = 102.9 ppm; quadrupole coupling constant at the deuteron, (eqQ / h)AA = 343.9 kc/sec, and at the oxygen, (eqQ / h)aa = − 8.34 Mc / sec. The effect of including d-type Gaussian functions in the basis is examined.

Study of the Electronic Structure of Molecules. II. Wavefunctions for the NH3+HCl→NH4Cl Reaction

Ab initio computations are presented for the reaction NH3+HCl→NH4Cl. The two reactants are studied at a large number of positions and for each point an SCF LCAO MO wavefunction and the corresponding total energy are obtained. These results are then collected in an energy surface diagram. All the electrons of the system are considered and a contracted Gaussian basis set has been used in this work. This study considers the two reactants as one system and both charge transfer as well as hydrogen-bonding characteristics are analyzed within the molecular-orbitals framework instead of the valence-bond approximation as customary in the past. The reaction has been analyzed for the case where the HCl approaches the NH3 molecule in a path normal to the plane of the three hydrogens of NH3; the H atom of HCl is between the N atom and the Cl atom. The first stage of the reaction (NH3 and HCl at large distances) indicates mutual polarization of the two molecules. In this region the SCF LCAO MO approximation is rather poor since it can not take into consideration dispersive forces. With HCl closer to the ammonia molecule, there is first hydrogen bonding formation and partial charge transfer. When NH4Cl is formed the molecule is stable with respect to the NH3 and HCl components by about 19 kcal/mole. No activation energy has been found in the reaction, contrary to previous assumptions. The computed geometry of the NH4Cl molecule at the equilibrium configuration is discussed in this work.