Ab-initio Package Research Articles

Double-ionization energies of the chloroethane molecules CH 3CH 2Cl, CH 3CHCl 2, CH 3CCl 3, CH 2ClCH 2Cl, CH 2ClCHCl 2, CH 2ClCCl 3, CHCl 2CHCl 2, CHCl 2CCl 3 and CCl 3CCl 3

Double-ionization energies of the molecules listed in the title were measured by double-charge-transfer spectroscopy. Since spin is expected to be conserved in double-electron-capture reactions, and OH + was used as the projective ion, the lowest energies measured should correspond to ground triplet states of the dications. Double-ionization energies to these states were calculated using the GAUSSIAN 86 ab initio program package at the Hartree—Fock level with second-order Møller—Plesset perturbation theory. The measured double-ionization energy of CH 3CH 2Cl is 29.8 ± 0.5 eV but the corresponding energies for all the other molecules are lower, lying within ± 0.7 eV of 28.1 eV. A similar pattern is seen in the calculated data. The measured energies are in general higher than those calculated but in all cases by not more than 5%. Double-ionization energies for the fluorine analogues were also calculated and are compared with previously published measured data.

Double ionization energies to form the C 2H 2+6 ion in triplet ground and electronically excited states

Double ionization energies to various triplet electronic states of C 2H 2+ 6 have been measured by double charge transfer spectroscopy using OH + as the projectile ion. Four peaks were observed in the spectrum, corresponding to double ionization energies (in eV) of 32.1 ± 0.4, 33.5 ± 0.3, 35.2 ± 0.5 and 36.4 ± 0.5. The first of these is associated with the ground triplet state. The double ionization energy to this state was calculated using the GAUSSIAN 86 ab initio program package at the Hartree—Fock level with second order Møller—Plesset perturbation theory. Seven basis sets were tried; the predicted energy with each is within ± 0.2eV of 32.1eV. A previous theoretical investigation of C 2H 2+ 6, using the Green's function method, predicted the energy spacings between the triplet states. When energies lying so close together as to be unresolvable experimentally are grouped together, the spacings between the double ionization energies measured in the present investigation and those calculated are in good agreement.

Ab initio molecular orbital calculations of the infrared spectra of hydrogen bonded complexes of water, ammonia and hydroxylamine. I. Effect of choice of basis set on the predicted geometries and spectra of the monomers

The wavenumbers and intensities in the infrared spectra of water, ammonia and cis- and trans-hydroxylamine monomers have been calculated using the GAUSSIAN-76 ab initio molecular orbital computer package, and 4-31G, 4-31G * and 6-31G ** basis sets. The results obtained using all three methods have been compared with one another, and with published experimental data.

Total energies of molecules with the local density functional approximation and gaussian basis sets

Total energies of small molecules were calculated with a local density functional (LDF) approximation within the LCAO MO SCF scheme. The local spin density functional (LSD) of Gunnarsson and Lundqvist was used. The basis sets used are of contracted gaussian type which allow comparison of LSD with Hartree-Fock (HF) results. The program for calculation of the LSD term was incorporated into the standard ab initio package. The LSD binding energies were in better agreement with experiment than those from HF.

Quantum theory of the structure and bonding in proteins: Part 2. The simple dipeptide

The conformational energy map for the simplest possible dipeptide is obtained from the ab initio Gaussian-70 package using the STO-3G basis set. This map may be interpreted using standard chemical ideas to give clear evidence for the existence of a C 7 hydrogen bond and non-bonded repulsions, together with additional but less clear-cut evidence for torsional barriers and a possible induction effect. Quantitative estimates of these effects are reported. The present evidence casts doubt on the nature of the alleged C 5 hydrogen bond which stabilises the extended form of the protein.