Rudolf Zahradník

The superposition error problem: The (HF)2 and (H2O)2 complexes at the SCF and MP2 levels

Abstract The title calculations were performed with the aim of providing data for the critical examination of the utility of the basis set superposition error (BSSE) correction. The main results obtained are as follows. The SCF interaction energies corrected for the BSSE and evaluated with the basis sets of split-valence and DZ origin are similar. With the stabilization energy, both the basis set superposition error and the intersystem correlation energy are important. MP2 stabilization energies are only slightly dependent on basis set for basis sets of DZ + P or better quality. The basis set superposition error at both the SCF and MP2 levels remains almost unchanged when passing from the 6–31G* to the 6–311G(2 d , 2 p ) basis set. At the SCF level sufficiently accurate geometries were obtained with the standard 6–31G* basis set. Optimization at the MP2 level with this and larger basis sets brings about only small changes with respect to optimum SCF geometries.

Conjugated radicals : Theoretical study of redox equilibria

Abstract Possibilities and limitations of the Coulomb repulsion integrals to estimate semiquinone formation constants of radicals are discussed. Elucidation of general dependency of semiquinone formation constants for violenes on the size of molecules and the aza-substitution is given.

Formation of hydrogen-bonded van der waals molecules: applicability of 3–21G intermolecular interaction energies

Abstract The 3–21G basis set was applied to the following H-bonded complexes (HP) 2 , (H 2 O) 2 , (HCl) 2 , H 2 O ∣dotHF, OCO ∣dot HF, HCN∣dotHF, HCl∣dotHF, HF∣dotHCl. The optimized geometries agree well with the 4–31G ones. The respective 3–21G interaction energies (Δ E ) are larger than the 4–31G ones, but the differences in Δ E are smaller when the basis set superposition error is taken into account.

Semiempirical all-valence-electron MO calculations on the electronic spectra of linear radicals with degenerate ground states

Abstract INDO-CI calculations are presented for CH, CF, NO, O 2 + , CCN, CNC, C 2 + , F 2 + , HF + , OH, CO 2 + , BO 2 , N 3 , N 2 O + , NCO, and diacetylene cation radicals. The results give a good account of transition energies for valence electronic states. The limits and merits of the method are briefly discussed.

Applicability of the supermolecule MP2 approach to intermolecular interactions: He2 and Ne2

Abstract Arguments are given for applying the full counterpoise correction to eliminate properly the basis set superposition error in the correlation energy. Comparison is made with the intermolecular perturbation theory and more complete treatments (MP4, CEPA). The results for the dimers studied suggest that MP2 is a poor approximation. Even if very extended basis sets are used, hardly more than 65% of the stabilization energy is recovered.

MP4 Interaction energies and basis set superposition errors for the (H2)2dimer

Abstract The energy of formation of the T-shaped (H 2 ) 2 dimer is calculated using different basis sets and Moller-Plesset perturbation theory up to fourth order. The roles of the second-, third- and fourth-order contributions as well as that of the different excitations at the fourth-order level were investigated. The results suggest that a correction for basis set superposition error should be included at both the SCF and post-SCF levels.

Stacking interactions: ab initio SCF and MP2 study on (H2O)2, (H2S)2, (HCN)2, (CH2O)2 and (C2H4)2

Abstract The stacking complexes (H2O)2, (H2S)2, (HCN)2, (CH2O)2 and (C2H4)2 were studied at the SCF and MP2 levels with different basis sets. The SCF interaction energies are only slightly basis set dependent, with the exception of STO-3G, for which they are underestimated. MP2 interaction energies are, on the other hand, strongly basis set dependent. Minimal and split-valence basis sets give small values of this energy. A modified 6–31 G* basis set (diffuse polarization functions) provides reasonable values of SCF interaction energies, correlation interaction energies and total interaction energies of stacking complexes. The dipole—dipole electrostatic energy is comparable with the SCF interaction energy at large distances only. The dispersion energy agrees satisfactorily with the correlation interaction energy obtained with a basis set containing diffuse polarization functions.

On the formation of C2H8+ in the reaction CH4+ + CH4 → CH3 + CH5+: A theoretical study

Abstract On the potential surface for the title process, eleven stationary points (UHF, 3-21G) have been located: six minima, three saddle points and two stationary points of higher order. The most stable C 2 H 8 + system has a linear CHHC bond.

Strained unsaturated molecules. Theoretical study of acyclic and cyclic cumulenes and acetylenes

Abstract There is a high probability for the photochemical formation of highly strained isomers of benzene, hydrocarbons 31–34 , at very low temperatures in inert matrices. This is also true for various derivatives and analogues of o -benzyne ( 22 ), benzologues of three isomers of benzyne ( 52–54 ), and six-membered cycles cycles having two or three triple CC bonds in the ring ( 25–27 ). These systems as well as the radical ions of o -benzyne and cyclic C n systems ( 46–51 ) have been studied theoretically. The stability and structure estimates were based on non-empirical and semi-empirical quantum chemical calculations. The reliability of these calculations has been partly tested by comparing the calculated and observed heats of formation for a representative ensemble of unsaturated and strained hydrocarbons. Some of the C n systems (acyclic and cyclic) can, in principle, exist in two forms which differ in their electron distribution and therefore also in their structure.

Metal ion interactions with water and ammonia

Abstract The interactions of the alkaline-metal ions (Li+, Na+, K+, Rb+), alkaline-earth metal ions (Be2+, Mg2+, Ca2+, Sr2+), trivalent-metal ions (Al3+, Ga3+) and some related metal ions (Cu+, Zn2+, Sc3+) with water and ammonia have been studied quantum chemically at the Hartree-Fock level. Huzinagas MINI-1 basis set was used together with corrections for the basis-set superposition error. In some cases, modified MIDI and medium-sized basis sets were employed. The geometry and stabilization energy characteristics of the complexes are discussed in connection with the influence of the ion charge, ion radius and the core-charge shielding effects of the d electrons. The geometry and stability relationships with the analogous water and ammonia complexes are explained in terms of the decomposition of the interaction energy based on the Kitaura-Morokuma scheme.