New theoretical insight into the interactions and properties of formic acid: development of a quantum-based pair potential for formic acid

We performed ab initio quantum-chemical studies for the development of intra- and intermolecular interaction potentials for formic acid for use in molecular-dynamics simulations of formic acid molecular crystal. The formic acid structures considered in the ab initio studies include both the cis and trans monomers which are the conformers that have been postulated as part of chains constituting liquid and crystal phases under extreme conditions. Although the cis to trans transformation is not energetically favored, the trans isomer was found as a component of stable gas-phase species. Our decomposition scheme for the interaction energy indicates that the hydrogen-bonded complexes are dominated by the Hartree-Fock forces while parallel clusters are stabilized by the electron correlation energy. The calculated three-body and higher interactions are found to be negligible, thus rationalizing the development of an atom-atom pair potential for formic acid based on high-level ab initio calculations of small formic acid clusters. Here we present an atom-atom pair potential that includes both intra- and inter molecular degrees of freedom for formic acid. The newly developed pair potential is used to examine formic acid in the condensed phase via molecular-dynamics simulations. The isothermal compression under hydrostatic pressure obtained from molecular-dynamics simulations is in good agreement with experiment. Further, the calculated equilibrium melting temperature is found to be in good agreement with experiment.     

Determination of the correlation effects in molecules using the complete error potential

Summary The complete error potential is used for the study of the correlation of molecules. The correlation can be seen as a relaxation of the electrons and a relaxation of the nuclei. From the Fermi sea, the electronic relaxation gives rise to a partial excitation of each orbital which collectively generates a linear, a surface or a volume pressure; the resulting forces acting on the nuclei are defined in a general theory of the intramolecular forces. H₃, H ₃ ⁺ , the diatoms and the hydrid of the first nine elements are taken as examples.     

Effects of contraction and reduction of basis set size on the He2 interaction energy components

The effect of basis set contraction and elimination of primitive Gaussian orbitals on the He₂ interaction energy components have been studied within the SCF counterpoise corrected approach supplemented by a dispersion term calculated within the variation-perturbation scheme. Despite elimination of almost half of the primitive Gaussian functions from the saturated sp basis set and complete contraction of the remaining ones, the components of interaction energy in He₂ suffer a remarkably small loss of accuracy except for the short range charge transfer contribution.     

The effect of electron detachment on the structure and properties of the chlorine-acetonitrile anionic complex

The results of the theoretical study of ground state potential energy surfaces for the chlorine-acetonitrile anion and its photodetachment product are presented. The shallow potential surfaces allow for the nondefinitive position of the chlorine within the complex. The dissociation energy of the neutral complex, estimated through the thermodynamic cycle, indicates significant structural changes due to the photodetachment process. The excess negative charge is localized mostly on the chlorine atom, and the electron detachment proceeds as an electron is removed from chlorine. The process leads to drastic changes in the electrostatic interactions within the complex. The first electronic excited state corresponds to the excess electron transfer from chlorine to acetonitrile fragment. This state is a precursor of the observed charge-transfer-to-solvent state.     

Crystal and molecular structure of a 1:1 complex of a chiral α-d-glucosido-benzo-18-crown-6 and potassium thiocyanate

C₂₈H₃₆O₁₀. KSCN is monoclinic, space groupP2₁ withZ=2,a=10.390(3),b=8.959(7),c=16.377(7) Å, =92.49(5)°. FinalR=0.053 for 1437 reflections measured at room temperature. The K^– ion lies on the least-squares plane formed by the six oxygen atoms in the macrocyclic ring. The SCN^– ion was found on the same face of the macrocycle as the chiral glucopyranoside moiety.Methyl-4,6-O-benzylidene-2,3-O-(1,2-bis(ethoxyethoxy)benzenediyl)--d-glucopyranoside.     

Structures and electron attachment properties of halomethanes (CX(n)Y(m), X=H, F; Y=Cl, Br, I; n=0,4; m=4-n)

Theoretical studies of structures of neutral molecules and their anions as well as dissociative electron attachment properties are presented for the halomethanes of general formula CX(n) Y(m); X=H, F; Y=Cl, Br, I; n=0,4; m=4-n. The dissociative electron attachment seems to be the primary process resulting in toxicity of these species. The halomethane anions containing hydrogens are formed as radical-anion adducts. When H is replaced by F, these species become true sigma* radicals. The electron affinities are computed using a variety of computational techniques including the four-order M?ller-Plesset (MP4) technique that included 250 basis functions. It is challenging to compare the computed results with experiment due to dearth of experimental data and uncertainties in the existing experimental data. Thus in certain cases larger differences are found between the computed and experimental values.     

An ab initio method for approximation of the frozen molecular fragment

An ab initio method for calculation on many-electron molecular systems with the approximation of the inactive part of a molecule by frozen molecular fragment is presented. In the following method the SCF calculations are performed in two series. First the molecular orbitals resulting from the first SCF calculation (modest basis set) are localized. In the second SCF run, the basis set is extended for the active part of the molecule, while molecular orbitals of the inactive part, selected from the localized set, are kept frozen. The results are in good agreement with the extended basis set calculation.     

Formulation of the LCAS MS SCF method within the Gaussian basis set

Within the presented LCAS MS (linear combination of atomic spinors–molecular spinors) SCF formalism both large and small components of the spinor radial parts have been expanded within the Gaussian basis set. The respective expressions for matrix elements as well as for one- and two-electron integrals are given.