Showing papers by "Pavel Hobza published in 1986"

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(2d, 2p) 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.

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.

The Remarkable Structure of Lithium Cyanide/Isocyanide

Abstract: Electron correlation corrections have a considerable influence on the relative stabilities of lithium isocyanide (1), lithium cyanide (2), and the bridged form, 3. While Hartree-Fock theory finds 1 to be most stable and 3 not to be a minimum, MP2/6-31G* optimization indicates 3 to be the global minimum. At higher levels employing full fourth-order Moller-Plesset theory and a quadruply split valence and polarized basis set (MP4STDQ/6-311+G*), 2 is only about 2 kcal/mol less stable than 1 and 3, which are indicated to have nearly the same energy. LiNC thus is similar to C(Na)N and C(K)N, both of which are known to prefer T-shaped (bridged) structures in the gas phase. However, to an even greater extent than formerly realized, rotation of the lithium cation around the cyanide anion nucleus should be practically free. ΔH (LiCN) = 32.8 kcal/mol is estimated from the calculated lithium cation affinity of 151.2 kcal/mol. In addition, we find at the MP4SDTQ/6-31+G*//MP2/6-31G* level that the bridged form of NaCN is favored by 2–3 kcal/mol over the corresponding linear forms, which have nearly the same energy.

Weak intermolecular interactions: Statics and dynamics

Abstract: This contribution consists of four parts: energy calculations, physical (mainly spectroscopic) characteristics, and static and dynamic aspects of interactions leading to the formation of van der Waals species. The main, first, part includes specific comments on computational procedures for systems of different size ([number of atoms, number of electrons]: small [4, 10], medium [dozens, hundreds], large [103, 104]) and colloid systems (considering also supermolecular structures). Rigorous and simplified methods of molecular quantum mechanics can be used with the first and second groups, respectively; for larger systems, only empirical potentials and the methods of the physics of a continuum are available. The transferability of empirical potential parameters is critically examined. The role of temperature, Gibbs energy, and entropy is mentioned together with the ensemble theory. The search for stationary points on potential energy surfaces (PES) and analytical fits to PES are reviewed briefly. The second part is an outline of what is expected from computational chemistry to meet the needs of spectroscopists. The third section deals with selection rules, equilibria and the rates of processes involving van der Waals species, and the role of these species in common chemical reactivity. The crucial role of entropy is mentioned in connection with hydrophobic phenomena, entropy-driven processes, and partitioning of substances between water and a nonpolar phase. In the final section, the role of computer experiments (molecular dynamics, Monte Carlo) is pointed out. Some shortcomings and promising features of these techniques are summarized.