Time-dependent density functional theory calculation of van der Waals coefficient of sodium clusters

In this paper we employ all-electron \textit{ab-initio} time-dependent density functional theory based method to calculate the long range dipole-dipole dispersion coefficient (van der Waals coefficient) $C_{6}$ of sodium atom clusters containing even number of atoms ranging from 2 to 20 atoms. The dispersion coefficients are obtained via Casimir-Polder relation. The calculations are carried out with two different exchange-correlation potentials: (i) the asymptotically correct statistical average of orbital potential (SAOP) and (ii) Vosko-Wilk-Nusair representation of exchange-correlation potential within local density approximation. A comparison with the other theoretical results has been performed. We also present the results for the static polarizabilities of sodium clusters and also compare them with other theoretical and experimental results. These comparisons reveal that the SAOP results for C_{6} and static polarizability are quite accurate and very close to the experimental results. We examine the relationship between volume of the cluster and van der Waals coefficient and find that to a very high degree of correlation C_{6} scales as square of the volume. We also present the results for van der Waals coefficient corresponding to cluster-Ar atom and cluster-N_{2} molecule interactions.Comment: 22 pages including 6 figures. To be published in Journal of Chemical Physic

Proton affinity and acidity of hypohalous acids: a density functional study

The acidities and proton affinities of hypohalous acids HOX and also hydrohalic acids HX for X = F, Cl, Br, and I are calculated through the Kohn-Sham version of spin-polarized density functional theory with several available local as well as nonlocal gradient-corrected exchange-correlation functionals. The calculated values are observed to be in good agreement with the available reported results. Unlike the proton affinity or acidity values, the calculated gross electron populations at the atomic sites are not monotonic on going from HOF to HOI and thus cannot explain the calculated proton affinity or acidity trend. However, the trends in acidity as well as proton affinity are rationalized, in general, in terms of the calculated values of atomic Fukui reactivity indices

Hardness and polarizability profiles for intramolecular proton transfer in water dimer radical cation

In view of the recently reported discrepancies in the prediction of minimum energy structure (proton-transferred vs hemibonded) for the water dimer on ionization, we have performed complete active-space self-consistent field calculations followed by total energy evaluation using multiconfigurational quasi-degenerate perturbation theory to obtain very accurate relative energies of different structures and predicted the proton-transferred structure to be the most stable ones. The variations of hardness, polarizability, chemical potential, and energy for the proton-transfer process in this weakly interacting system (ionized water dimer species) are investigated through calculations using Hartree-Fock theory. It is observed that the transition state corresponding to the proton-transfer process is associated with maximum polarizability at different O-O distances for the water dimer cation, although the hardness minimum does not exactly correspond to the transition state. However, the hardness profiles scaled suitably with chemical potential are found to have minima at the transition states

Hydrogen-bonding interactions in selected super-molecular systems: electron density point of view

Ab initio and density functional theoretical calculations have been performed to quantify the hydrogen-bonding interactions for selected supermolecular systems, experimental investigations on which have been reported very recently (Angew. Chem., Int. Ed. 2001, 40, 3240). An analysis and rationalization of the nature of pairwise interactions in different hydrogen bonds involved in these ternary supermolecular systems is presented that uses the frameworks of Morokuma energy decomposition as well as Bader's topological theory of atoms in molecules involving the electron density ρ (r), its Laplacian ∇2ρ (r), and also other related quantities at the bond critical points. The pKa values of the aromatic acids, which have been used earlier to rationalize the specific intermolecular interactions between aromatic acids (hydrogen-bond donor) and isonicotinamide (hydrogen-bond acceptor as well as donor), are, however, found not to show any regular trend with the calculated binary interaction energy values or the electron density-based bonding parameters using experimental geometries. The calculated quantities corresponding to the computationally optimized geometries of the molecular species, however, do show some regular trends with the corresponding pKa parameters

New scale of atomic orbital radii and its relationship with polarizability, electronegativity, other atomic properties, and bond energies of diatomic molecules

A new scale of orbital radii is defined as the distance corresponding to the classical turning point of the electron in an orbital and is calculated for atomic systems using the self-interaction corrected version of the Kohn-Sham density functional theory with local spin-density approximation for the exchange and correlation. These orbital radii and different density and density derived quantities are shown to correlate very well with polarizability and other atomic properties of interest. A simple scheme is also proposed for the bond energy of a diatomic molecule in terms of the valence orbital radii and the electron density (at the boundary corresponding to the radii) of the constituent atoms. The calculated bond energies for simple heteronuclear diatomic molecules are shown to agree very well with the experimental values

Ab initio CASSCF and DFT investigations of (H<SUB>2</SUB>O)<SUB>2</SUB><SUP>+</SUP> and (H<SUB>2</SUB>S)<SUB>2</SUB><SUP>+</SUP>: hemi-bonded vs proton-transferred structure

High level ab initio calculations using a complete active space self-consistent field (CASSCF) and multiconfigurational quasi-degenerate perturbation theory (MCQDPT2) methods as well as density functional theory (DFT)-based calculations with different exchange-correlation energy density functionals have been performed for predicting the relative stability of the proton-transferred vs hemi-bonded isomers of (H2O)2+ and (H2S)2+ species. For (H2O)2+, DFT calculation using conventional exchange-correlation functionals predicts the hemi-bonded structure to be the ground state while use of full or half Hartree-Fock exchange and local correlation predicts a higher stability of the proton-transferred structure in agreement with ab initio results. For the (H2S)2+ system, all of the methods lead to the prediction of lower energy for the hemi-bonded isomer. No regular trend of the exchange-correlation energy component with the total energy difference is however observed. Dynamical electron correlation effect incorporated through MCQDPT2 is found to be much stronger in (H2O)2+ as compared to (H2S)2+. An analysis of the nature of interactions involved in the (H2O)2+ and (H2S)2+ systems within the framework of Bader's topological theory of atoms in molecules is also presented through the plots of the Laplacian &#8711;2&#961; of the electron density &#961; (r) and also other related quantities at the bond critical points with the objective of rationalizing the relative stability of the two isomers in both (H2O)2+ and (H2S)2+