Ab initio calculations
Our recent investigations have been connected with (1) complete basis set calculations, with (2) unsaturated non-cyclic hydrocarbons, and with (3) clusters formed from IIa-VIa and IIIa-Va diatomics.
(1) The Hartree-Fock, DFT, MP2, MP4, and CCSD(T) calculations were performed for sets of atoms, molecules, ions, and radical ions with the Dunning correlation-consistent Gaussian basis sets of atomic orbitals ranging from valence double-zeta through quadruple-zeta (with small systems also with 5Z). Extrapolations to a complete basis set (BS) have been carried out by means of the simplest two-parameter linear relationship (R. Zahradník, L. ©roubková, Isr. J. Chem., 43, 243-265 (2003)). Energy and physical characteristics have been plotted against 1/ni, where ni is the total number of one-electron functions. Analogous study has been performed for van der Waals species (R. Zahradník, L. ©roubková, Int. J. Quantum Chem. 104, 52-63 (2005)).
(2) Polyynes, polyenes, cumulenes, and related systems have been theoretically studied as potential elements for molecular machines and precursors of carbon clusters. Remarkable deviations from linearity with derivatives of long polyynes and cumulenes have been ascribed to the lowest-energy deformation vibrations, which decrease rapidly when passing from short to long chains. Physical characteristics for cumulenes split into two distinct classes for coplanar species (D2h symmetry) and species with orthogonal terminal CH2 groups (D2d symmetry). (R. Zahradník, L. ©roubková, Helv. Chim. Acta 86, 979-1000 (2003)). Moreover, electronic spectra of these systems have been calculated by means of the time-dependent DFT theory and, for a group of selected molecules, also by symmetry-adapted cluster CI method. The TD DFT (with B3 LYP) is a reliable tool for calculation of band positions in the longest-wavelength region. Special attention has been paid to the influence of substituents of various types on the position of the first intensive bands (R. Zahradník, M. Srnec, Z. Havlas, Collect. Czech. Chem. Commun. 70, 559-578 (2005)).
(3) Diatomics formed by all combinations of atoms of IIa (Be, Mg, Ca, Sr, Ba) with atoms of VIa (O, S, Se, Te, Po) elements form an unsymmetrical matrix. Ground state and electronically excited states properties of the elements of this matrix (i.e. 25 diatomics) have been studied at various levels of sophistication. Moreover, geometry and other features of oligomers of those diatomics have been analyzed. Electronic spectra of several monomers exhibit in accordance with experiments the first electronic transitions in the near infrared region. The passage to oligomers is accompanied by a significant hypsochromic shift of the whole electronic spectrum (M. Srnec, R. Zahradník, Chem. Phys. Lett. 407, 283-288 (2005)).