H.L. Sellers

Consistent generalization of the Møller-Plesset partitioning to open-shell and multiconfigurational SCF reference states in many-body perturbation theory

Abstract The Moller-Plesset partitioning of the many-electron Hamiltonian is generalized for arbitrary open-shell and multiconfigurational SCF reference states. The method has been initially implemented at the second and third orders for two-configuration generalized valence bond reference wavefunctions in which only two electrons are correlated. Potential curves for the hydrogen molecule, lithium hydride and the helium dication agree excellently with full CI results.

Normal coordinate ab initio force relaxation

Abstract A procedure for optimization of molecular geometries is presented, combining ab initio calculations with vibrational molecular data from spectroscopy or empirical force fields. Theoretical cartesian forces are transformed to vibrational normal coordinate forces from which geometry increments are calculated. Test results indicate that the method saves considerable effort compared to other optimization schemes.

Ab initio studies of structural features not easily amenable to experiment: The molecular structures of two low-energy forms of unionized serine

Abstract The structures of two unionized conformations of serine were refined without geometrical constraints using standard single determinant ab initio procedures on the 4-21G level. The results represent the best now available estimate for the unobserved equilibrium geometry of this system.

Ab initio equilibrium structures of unionized amino acids: alanine

Abstract The structures of two unionized conformations of alanine were optimized by relaxing all of their geometrical parameters using standard single determinant MO theory with Pulays FORCE method a 4–21G basis. The results represent the best now available estimate for the unobserved geometry of this system.

Abinitio studies of structural features not easily amenable to experiment. II. The influence of bond delocalization effects on the molecular structures of some lithium fluoride clusters

The geometries of linear and cyclic forms of Li2F2 and Li3F3 were determined by unconstrained ab initio force relaxations using the 4‐31G basis set on F and 5‐21G on Li. Optimized Li–F bond distances depend strongly on their molecular environment. Linear clusters are more stable than bent chains. Planar, cyclic forms are more stable than open chains. Some of the refined structures deviate significantly from the equilibrium geometries calculated for these molecules on the basis of electrostatic bonding models. It is concluded that the ionic bonding concept may be intrinsically deficient in quantitatively rationalizing the molecular structures even of systems the potential energy minima of which are primarily determined by electrostatic interactions.

Ab initio studies of structural features not easily amenable to experiment: the molecular structures of some hydrogenfluoride clusters

Abstract The completely relaxed ab initio structures of some forms of H2F2 and H3F3 reflect the cooperative nature of hydrogen bonding and can be used to estimate the order of magnitude of the variations in local geometry which are neglected when interactive potentials for HF in clusters or in the liquid state are evaluated with constrained geometries.

The ring puckering potential of oxetane: local correlation results

Abstract The ring puckering potential of oxetane has been determined by the local correlation treatment of Saebo and Pulay and different many-body theories (Moller-Plesset theory and the Chiles-Dykstra ACCD method). The results show that dispersion forces between non-bonded atoms contribute significantly to the observed double-minimum potential; vibrational modulation effects are also significant. Our best results still underestimate the barrier height significantly.

Ab initio studies of structural features not easily amenable to experiment: Part 15. The influence of solvation on molecular structure in some formic acid and carbonic acid hydrates

Abstract The completely relaxed ab initio geometries (4—21G) of a trihydrate of carbonic acid and of the monohydrates of cis and trans formic acid are compared with the corresponding unhydrated structures. The maximum structural changes caused by hydration in the free acid structures are of the order of magnitude of 0.03 A and 3° for bond distances and bond angles, respectively. The corresponding changes in free and complexed water are 0.005 A and 5°, respectively. The results are significant for the general problem of the transferability of gas phase molecular structures to molecules in solution and for estimates of the uncertainties in theoretical hydration energy surfaces which are generated by using fixed, monomer geometries for water and solvate molecules. Compared with the free geometries, the sum total of the structural changes in some of the systems studied corresponds to energies of several kcal mol−1.

Normal coordinate ab initio calculations of cubic anharmonicity constants

Abstract A procedure is described to calculate cubic molecular anharmonicity constants by numerical differentiation of ab initio forces in vibrational normal coordinate space. The method is simpler than comparable procedures and is useful in determining normal coordinate anharmonicity of isotopic species. Results for H 2 O and its isotopic homologs are in excellent agreement with literature values.

The effect of inner-shell polarization on diagonal stretching force constants: Part III. ACCD results for HF, OH−, NH3, N2, F2 and H2CO

Abstract Results of quadratic stretching force-constant determinations at the TZ + P ACCD and TZ + 2P ACCD levels, in which the second set of polarization functions has been optimized to account for inner-shell polarization (ISP), are presented. Essentially perfect agreement with experiment was found in the cases of HF and NH3 when inner-shell polarization was taken into account. Although in other test cases errors in the force constants arise from other sources, improvement in the force constant is observed on inclusion of ISP functions in the basis.