Regina F. Frey

Potential energy surfaces of CH+4

The potential energy surfaces of the three lowest electronic states of CH+4 are calculated. CI calculations based on average natural orbitals from preliminary multireference singles and doubles CI calculations are found to provide a balanced description of the three states. The calculated points are fit to the eigenvalues of a 3×3 matrix whose elements are a power‐series expansion in the normal‐mode displacements from the SCF tetrahedral geometry. The equilibrium structures for the C2v, D2d, and C3v Jahn–Teller distortions are located and the vibrational frequencies for these structures are calculated. In addition, the transition state for the pseudorotation between equivalent C2v minima is located and the nature of the pseudorotation is discussed in detail.

The case of glycine continued: some contradictory SCF results

Abstract The energies of five conformations of glycine were determined by ab initio HF/SCF gradient geometry optimization using 13 different basis sets. The conformations considered are the stretched form ( I ), a heavy-atom framework planar ( II ) and non-planar ( III ) cyclic form, and a planar ( IV ) and non-planar ( V ) bifurcated structure. Form I is the global energy minimum in all calculations, but contradictory results are obtained for the stabilities of the other forms. Of the cyclic forms, II is a local energy minimum at the 4-31G, 6-31G, 6-31+G, and (10s,5p/4s) levels, but a saddle point in calculations with both smaller and larger basis sets (including 6-31G ∗∗ , 6-31 + G ∗∗ , and 6-31 + + G ∗∗ ). Similarly contradictory results are obtained for IV . There is considerable scatter in calculated energy differences and in the optimized values of the non-planar N-C-CO torsions. Form I and the planar cyclic form II were previously found in the microwave spectrum of glycine. The significance of the new calculations for interpretations of the microwave data is discussed. It is concluded that assumption of slight non-planarity of the cyclic form, with a minimum at III and a low lying transition state at II is not necessarily in conflict with the available microwave data. In addition it is possible to conclude that a third conformer, IV or V , hitherto not observed, may exist in glycine vapors in amounts which are similar to those of the observed conformer II .

Energy partitioning of the self‐consistent field interaction energy of ScCO

The SCF interaction energy for ScCO was partitioned into the electrostatic, polarization, exchange, and charge‐transfer terms via the Morokuma analysis. This definition of polarization does not satisfy the Pauli exclusion principle and therefore, there is nothing to prevent the valence electrons of one fragment from collapsing into the core orbitals of the other fragment. To correct this problem, the polarization and charge‐transfer terms were calculated in the presence of exchange. The dominant interactions were exchange repulsion, electrostatic attraction, and distortion of the wave function by polarization and charge transfer. Pi backbonding was the dominant distortion for the 4 Σ− 4s1 3dπ2 state, while sigma distortions dominated the 4s2 3d 1 states.

An investigation into intramolecular hydrogen bonding: impact of basis set and electron correlation on the ab initio conformational analysis of 1,2-ethanediol and 1,2,3-propanetriol

Abstract Electron correlation effects are especially important in systems with strong nonbonded interactions. Despite this, very few ab initio studies of polyfunctional alcohols have included correlation effects in their geometry optimizations. In order to better understand intramolecular hydrogen bonding and to develop more reliable energy and geometric parameters for future molecular modeling, we optimized the geometries of 10 conformers of 1,2-ethanediol (ethylene glycol) and 11 conformers of 1,2,3-propanetriol (glycerol) at the HF/4-21G, HF/6-311G∗∗ and MP2/6-311G∗∗ levels of theory. All three computational methods are able to predict differences between internal coordinates optimized in different regions of conformational space, to within typical experimental accuracies. However, the inclusion of electron correlation has a major impact on the absolute values of these internal coordinates and on the depths of the associated energy minima. Compared with our HF/6-311G∗∗ results, the MP2-optimized structures have longer CO bonds by up to 0.020 A, OH bonds that are up to 0.023 A longer, OCC angles that are often 1 ° smaller, HOC angles in excess of 4 ° smaller, and torsional angles that may deviate from ideal trans or gauche values by an additional 5 ° for heavy-atom torsions and 8 ° for HOCC torsions. The net effect of all these conformational rearrangements is to greatly enhance intramolecular hydrogen bonding. Nonbonded O … H distances invariably decrease, by up to 0.19 A, and the alignments of hydrogens with hypothetical lone-pair orbitals on oxygen acceptors improve. Electron correlation selectively stabilizes those conformers with more intramolecular hydrogen bonds, but decreases the energy differences among conformers with the same number of hydrogen bonds. The errors associated with single-point MP2 energy calculations at HF-optimized geometries appear to increase with the size of the system, as do differences between MP2- and HF-optimized OCCO torsional angles. Thus, molecular mechanics parameters derived from MP2/6-311G∗∗ optimizations of prototypical small molecules such as ethylene glycol and glycerol are expected to result in significantly different macromolecular energies and structures than those based on HF/6-311G∗∗ optimizations.

The valence orbital momentum distributions and binding energy spectra of methane by electron momentum spectroscopy : quantitative comparisons using near Hartree-Fock limit and correlated wavefunctions

Abstract The binding energies and momentum distributions of the two valence orbitals of SiH 4 have been measured by electron momentum spectroscopy. The measured binding energy spectrum is compared with ADC(3) and ADC(4) many-body Green function calculations. The momentum distributions are compared on a quantitative basis with SCF wavefunctions near the Hartree-Fock limit and also with full ion-neutral overlap calculations carried out using correlated wavefunctions by the method of configuration interaction. Quite good agreement with theory is found at the Hartree-Fock limit and inclusion of electron correlation has minimal effect on the calculated momentum distributions. A study of the angular dependence of the satellite structure in the inner valence region shows that these processes arise predominantly from final state correlations associated with the production of the 3a 1 −1 hole state.

The electronic structure of alkali trimer anions and cations

Vibrational frequencies for the alkali trimer cations have been calculated. The frequencies of the trimer cations provide good estimates for the frequencies of the neutral trimers in the absence of affects due to the conical intersection characteristic of the trimer potentials. The frequencies decrease with increasing atomic number, this variation resulting from both mass effects and a decrease in the force constant. The electron affinities for the alkali trimers have been estimated from Koopman’s theorem based upon comparisons with SCF–CI calculations. The trimer electron affinities decrease with increasing atomic number. They are found to be much larger than the known dimer electron affinities exhibiting a trend opposite to that of the dimer and trimer ionization potentials.

Electron correlation effects in aliphatic non-bonded interactions: comparison of n-alkane MP2 and HF geometries

The geometries of several n-alkanes were determined by HF/6-311G∗∗ and MP2/6-311G∗∗ gradient optimization. The results make it possible to study the effects of electron correlation on non-bonded aliphatic interactions by comparing structures devoid of dispersion forces (HF/6-31 IG∗∗) with those in which the dispersion forces are switched on (MP2/6-311 G∗∗). Conformations with trans bonds T (C-C-C-C torsions 180°), and gauche bonds G (C-C-C-C torsions 60°) were investigated, including T and G n-butane; TT, TG, and GG n-pentane; and TTT, GTT, TGG, GTG, and GGG n-hexane. When geometries are optimized at the MP2/6-311G∗∗ level, rotamer energies do not increase with the number of individual G bonds as expected, because GGG O.3 kcal mol−1) is larger in the MP2-optimized geometries than previously allowed (0.16 kcal mol−1). In general, in all rotamers with G torsions, small contractions in torsional angles (< 5°) cause non-bonded distances in the attractive region of the van der Waals potential to be shorter in MP2 structures than HF geometries, in contrast to 1,4 interactions, which are practically invariant. Specifically, average 1,5 non-bonded distances in GG pentane, and TGG and GGG hexane shrink by 0.22 to 0.27 A; by 0.06 to 0.08 A in TG pentane, and GTT and GTG hexane; and they are essentially unchanged in TT sequences. The results emphasize the importance of accurate geometries in conformational analyses: when the bond lengths and angles of a molecular model are wrong, calculated energies are also wrong, because non-bonded interactions are incorrectly evaluated.

Density functional calculations for Mgn+ clusters

Calculations using an extended basis set and the Becke 3-parameter exchange functional and Perdew 86 correlation functional are used to predict the geometry and isotropic hyperfine coupling constants for Mgn+ clusters (n⩽6). The Mg3+ results agree with our previous MRSDCI results and disagree with another recent DFT calculation. For n=4 and 5, our results agree with previous work. For n=6, we obtain a trapezoid-based bipyramid that was not considered in any previous papers. Based on these calculations, we are able to rationalize the experimental hyperfine couplings.

The valence orbital momentum distributions and binding energy spectra of H2CO: A comparison of electron momentum spectroscopy and quantum chemical calculations using near-Hartree-Fock quality and correlated wavefunctions

Abstract The high momentum resolution experimental momentum profiles (XMPs) of the valence orbitals of H2CO have been measured by electron momentum spectroscopy (EMS). Good quantitative agreement is obtained between the measured XMPs and the momentum distributions calculated from a near-Hartree-Fock wavefunction except for the outermost 2b2 orbital. A configuration interaction study suggests that the difference between theory and experiment observed for the outermost 2b2 orbital cannot be accounted for by inclusion of electron correlation and electronic relaxation effects. The measured 5a1 and 1b2 XMPs confirm earlier orbital assignments made by Hood et al. Extensive many-body structures are observed in the inner valence binding energy region and these are assigned predominantly to the (3a1)−1 process.

The Jahn–Teller distortion in SiH+4

The three states that correspond to the triply degenerate 2T2 state of SiH+4 at the tetrahedral configuration are studied using configuration‐interaction (CI) calculations. The potential‐energy surfaces for the three states of interest are modeled using a functional form of the potential that is based on normal‐mode distortions from the tetrahedral configuration. It is shown that this form of the potential, which includes quadratic terms in the normal‐mode displacements, is not able to correctly describe the region containing the van der Waals Cs minimum structure. However, using single‐root CI calculations, the ground and excited states of the Cs, C2v, and SiH+2 +H2 structures are then studied in detail.