Bernard C. Laskowski

A proposal for the proper use of pseudopotentials in molecular orbital cluster model studies of chemisorption

The interaction of CO with Cu5, Ni5, and Al4 are treated as model systems for molecular adsorption on metal surfaces. The effect of the use of pseudopotentials for the metal atoms is studied by considering three types of clusters. In the first case, all of the metal electrons are explicitly included in the wave function; an all electron (AE) treatment. In the second case, the metal atom which directly interacts with the CO is described by as AE but the remaining metal atoms include a pseudopotential for their core electrons. Finally, in the third case, all of the metal atoms in the cluster have a pseudopotential treatment for the core electrons. The AE cluster results are taken as reference values for the two pseudopotential treatments. The mixed cluster results are in excellent agreement with those of the all AE clusters; however, the results for the all pseudopotential cluster of Ni5CO or of Cu5CO are qualitatively different. The pseudopotential treatment for all of the metal atoms often leads to result...

Calculated ground state potential surface and excitation energies for the copper trimer

The results of an SCF/SDCI treatment are presented for selected portions of the ground state potential energy surface for the Cu3 molecule. For equilateral triangle geometries (D3h) the lowest state is 2E’ arising from 4sa’214se’1. The 2E’ state exhibits strong Jahn–Teller distortion, leading to 2A1 (acute angle) and 2B2 (obtuse angle) minima in C2v symmetry. Here the 2B2 minimum is a true minimum on the surface while the 2A1 minimum is a saddle point or very shallow secondary minimum connecting adjacent 2B2 minima. This strong Jahn–Teller distortion is consistent with the observed ground state vibrational levels in the gas phase spectroscopic studies of Morse, Hopkins, Langridge‐Smith, and Smalley and in the matrix Raman studies of Moskovits. The 2B2 minimum is also consistent with the observed ESR spectrum of Cu3 in a matrix, which has been interpreted as an obtuse angle structure with most of the spin density on the end Cu atoms. A linear 2Σ+u state is found to be 0.26 eV higher. Two possible candidate...

Molecular arrangements and conformations of liquid n‐tridecane chains confined between two hard walls

Liquid n‐tridecane (C13H28) chains confined between two parallel hard walls separated by 5.0 nm have been studied by Monte Carlo simulations, employing united CH2 atoms linked with fixed bond lengths and angles and continuously varying torsional angles subject to appropriate interaction energies. Molecules located in the central region are in remarkable agreement with ideal unperturbed chains. Significant influences of walls persist over a distance of ∼1.5 nm, exhibiting progressively less‐dense and less‐pronounced segmental layers of ∼0.4 nm thickness. Segmental orientations preferentially aligned along the surface are found only in the first layer, accompanied by little perturbations in the fraction of trans‐conformations. Furthermore, nearly all the chain units in the first segmental layer adjacent to the walls belong to two‐dimensional chains, and exhibit considerable orientational correlations between neighboring segments of different chains. The terminal portions of some of the molecules in the succ...

Theoretical calculation of low-lying states of NaAr and NaXe

The potential curves X 2Σ+, A 2Π, B 2Σ+, C 2Σ+, (4) 2Σ+, (2) 2Π, and (1) 2Δ were calculated for NaAr and NaXe using a self‐consistent field plus configuration‐interaction procedure. To reduce the computational effort, the core electrons were replaced by an ab initio effective core potential. The molecular wave functions were constructed from a basis of atomic s, p, and d Gaussian‐type orbitals. Although higher angular momentum basis functions would be required to precisely determine the weak van der Waals binding at large separations, we feel that these wave functions provide an adequate description of the repulsive region of the potential curves. Our potential curves are considerably less repulsive than the semiempirical ones of Pascale and Vandeplanque and agree well with those deduced from high‐energy scattering data by Malerich and Cross. Dipole and transition moments were also computed; these moments should be a considerable improvements over those from the one‐electron semiempirical calculation of P...

Conformational properties, torsional potential, and vibrational force field for methacryloyl fluoride: An ab initio investigation

We have examined the structure, torsional potentials, vibrational spectra, and harmonic force fields for the s‐cis and s‐trans isomers of methacryloyl fluoride with the aim of understanding some of the conformational properties of these molecules and their relationship to macroscopic polymer properties. The calculations have been performed at the SCF level with a split valence basis set. We find the structure to be in reasonably good agreement with experiment and as expected find the structural variations between the two isomers to be slight. However, we do suggest a reexamination of the carbonyl geometry in light of our studies. Our calculations show the energy difference between the s‐cis and the s‐trans isomers to be less than 1 kcal/mol at both the split valence and the split valence polarized levels, favoring the s‐trans form. An examination of the torsional potential reveals that a rigid rotor approximation provides an adequate description of the motion of either the methyl group, or of the COF moie...

Ab initio study of the ground state surface of Cu3

An ab initio study that includes relativistic effects via first‐order perturbation theory and correlation effects using the coupled pair functional formalism is presented for the ground state surface of Cu3. An analogous calculation is presented for the 1Σ+g ground state of Cu2 for calibration. The ground state of Cu3 is found to be a 2B2 state corresponding to a Jahn–Teller distortion (R=4.396 a0, θ=64.5°) of a 2E’ equilateral triangle geometry. This structure is found to lie 59 cm−1 below the 2A1C2v geometry and 280 cm−1 below the D3h equilateral geometry in good agreement with the pseudorotation barrier and Jahn–Teller stabilization energy deduced by Truhlar and Thompson from analysis of the fluorescence spectrum of Rohlfing and Valentini.

Ab initio calculation of the X 1Σ+ state of CsH

Relativistic effective core potential (RECP) and nonrelativistic all electron (NRAE) calculations are reported for the X 1Σ+ state of CsH. The calculations are based on GVB plus single and double excitation CI wave functions with the Cs(5s, 5p) as well as the two valence electrons correlated. The bonding in CsH is found to involve a Cs(6s)–H(1s) bond but with a significant ionic (Cs+H−) component. Correlating the Cs(5s, 5p) electrons leads to a significant bond shortening effect (∼0.15a0). This effect is attributed to a contraction of the Cs(6s) orbital and a shift toward higher energy of the Cs+H− asymptote upon correlating the 5s, 5p shell. Bond shortening due to relativistic contraction of the Cs(6s) is only ∼0.05a0 in agreement with the small amount of Cs(6s) character in the ionic bond orbital. The NRAE and RECP potential curves are very similar except for the shorter Re for the RECP case (due to relativistic bond contraction).

Theoretical Determination of the X1Σ+g potential of Cs2 using relativistic effective core potentials

Abstract Theoretical potential energy curves are computed for the X 1 Σ + g state of Cs 2 using relativistic effective core potential and a large valence gaussian basis set. Eighteen electrons are correlated by a four-reference MC SCF Cl(SD) procedure. Our best calculation (with experimental values in parenthesis) gave R e = 9.05 (8.78) bohr and D e = 3141 (3648 ± 8) cm−1.

The generator coordinate method for molecular wavefunctions: A moment method and a simple intrinsic function

An extension of the generator coordinate method to the description of the electronic structure of molecules is presented. An exact formal solution to the Hill–Wheeler equation is obtained for a certain class of intrinsic functions, namely, those whose Hamiltonian and overlap kernels are of degenerate form. Since the exact kernels are used in the Hill–Wheeler equation, the variational principle is retained. The formal solution is represented by the set of moments of the generator coordinate with respect to the weighting function. The features of the method are illustrated by application to the hydrogen molecule. A simple trial intrinsic function and a PPP Hamiltonian are used to describe the π‐electron structure of three linear conjugated polyenes (1,3‐butadiene; 1,3,5‐hexatriene; and 1,3,5,7‐octatetraene). A significant part of the apparent ground state correlation energy is recovered for each molecule and π‐electron excited state energies are also calculated. These results are compared with PPP CI calcul...

Effects due to proximity-induced electric dipole moments on Br + H2 collisions in an intense laser field☆

Abstract Further calculations are carried out for collinear Br( 2 P 3 2 , 1 2 ) + H 2 ( 1 Σ + g ) collisions occurring in an intense laser field. The model system, first employed to illustrate the use of a general quantum approach to such problems, is here improved by the inclusion of proximity-induced permanent and transition electric dipole moments that arise during the collision. Comparison is made between the present results and those obtained in the earlier work. Threshold effects are noted and discussed.