We study the hydration of the actinyl cations, uranyl UO[2][2+] and plutonyl PuO[2][2+], by performing Kohn−Sham Density Functional Theory calculations using a new quantum chemistry code—MAGIC. The calculations have been performed on the separate uranyl and plutonyl species, and on the complexes AcO[2][2+]·nH[2]O (Ac = U, Pu and n = 4, 5, and 6), in the gas and aqueous phases. The liquid-state environmental effects are included via a simple cavity model and by using the self-consistent reaction field method. The calculations find that the solvent effects are crucial. By this, we mean that a simple cavity model, alone, will be incapable of giving insight into the chemical behavior of such molecules. The short-ranged interactions between the actinyls and their closest water molecules are very strong and involve an appreciable amount of charge transfer, an effect that cannot be included in cavity models. The actinyls form strongly bound complexes with the surrounding water molecules, with n = 5 being the most stable. Thus, the short-range solvent effects are important. The binding energies of the complexes are very [...] SPENCER, Steven, et al. Hydration of UO22+ and PuO22+. Journal of Physical Chemistry. A, 1999, vol. 103, no. 12, p. 1831-1837 DOI : 10.1021/jp983543s

MULTIMODE calculations of low-lying rovibrational energies are reported for C2H4 and C2D4 for total angular momentum J = 0 and 1. The calculations are compared with recent benchmark ones for C2H4 J = 0 [G. Avila and T. Carrington, Jr. J. Chem. Phys. 135, 064101 (2011)] and shown to be highly accurate and roughly two orders of magnitude faster. Calculations of rotation constants for the twelve fundamentals for both C2H4 and C2D4 are reported.

The purpose of this presentation is to show the importance of the Colle–Salvetti (Theor Chim Acta 37:329, 1975) paper in the development of modern computational density functional theory. To do this we cover the following topics (1) the Bright Wilson understanding (2) the Kohn–Sham equations (3) local density exchange (4) the exchange-hole (5) generalised gradient approximation for exchange (Becke and Cohen) (6) left–right correlation and dynamic correlation (7) the development of the Lee–Yang–Parr dynamic correlation functional from the Colle–Salvetti paper (8) the early success of GGA DFT. Finally we observe that the the BLYP and OLYP exchange-correlation functionals are not semi-empirical; this may explain their great success.

We have introduced the theory of the Reaction Path Hamiltonian into the variational scheme MULTIMODE, for the calculation of vibrational energy levels of polyatomic molecules which have a single large amplitude motion ( for which the classic example is hydrogen peroxide). As with all MULTIMODE calculations, the greatest difficulty is the size of the CI matrix. The algorithm is now enhanced to include user-defined contraction schemes in order to probe high-energy regions of the potential energy surface. Furthermore an increased efficiency in matrix element evaluation is reported. High torsional levels of hydrogen peroxide and methanol are reported; those for hydrogen peroxide are consistent with an 'exact' variational procedure in valence coordinates, whilst those for methanol are predictions based on a recently-derived potential energy surface. The coupling between the torsional mode and the remaining normal modes is highlighted. These 'technical' advances open up the use of MULTIMODE for the study of vibrations in larger systems.

The nature of exchange, dynamic correlation (DC) and left-right correlation (LRC) is considered in density functional theory and wavefunction-based quantum chemistry. The presence of LRC in approximate exchange density functionals is highlighted and the separation of LRC and DC is considered. For H2, the Heitler-London approach is shown to include the essential elements of exchange and LRC. The arguments are illustrated by a comparison of Gaussian orbital s-optimised Heitler-London and OPTX potential energy curves. They agree well near equilibrium, but differ at large distances due to the inability of the OPTX form to describe the dissociation process. LRC and DC values determined using the two approaches are compared. The influence of higher angular momentum functions in the Heitler-London approach is then investigated (commonly called self-consistent valence bond); the agreement with OPTX degrades, leading to a larger value of LRC and a smaller value of DC at H-2 equilibrium. (C) 2009 Wiley Periodicals, Inc. Int J Quantum Chem 111: 563-569, 2011

In Density Functional Theory (DFT) the exchange-correlation functional is often written as a sum of exchange and correlation parts. We give reasons for thinking that OPTX is a very good exchange functional. We ask the question ‘what is the equivalent quantum chemistry calculation which gives energies close to those obtained from Kohn–Sham calculations with OPTX?’ OPTX was determined by refining its parameters so that atomic Hartree–Fock energies (of H–Ar) were reproduced. For H2 we find that OPTX values are very close to those obtained with the Heitler–London wavefunction with the 1s atomic orbitals on the protons having an energy-optimised exponent as first calculated by Wang. This holds at equilibrium and stretched geometries. The consequences of this investigation on the definition of dynamic correlation are discussed. OPTX wavefunctions for other molecules are suggested.

The direct configuration interaction method, including all Hartree–Fock interacting single and double replacements of a single root function, has been generalized to a wide class of systems. Programs capable of handling closed-shell singlets and open-shell doublets, triplets, quartets, and singlets in a routine fashion are described. The open-shell singlet presents some particular difficulties which have been overcome, and thus it appears that the direct CI method can be a fairly general and quite automatic approach to the electron correlation problem. The versatility of these programs is illustrated by applications to the vertical electronic spectra of NO2 (doublet and quartet spin states) and of NO2− (closed-shell singlet, open-shell singlet, and triplet spin states).

The vibrational energy levels of diazocarbene (diazomethylene) in its electronic ground state, (X) over tilde (3) Sigma(-) CNN, have been predicted using the variational method. The potential energy surfaces of (X) over tilde (3) A" CNN were determined by employing ab initio single reference coupled cluster with single and double excitations (CCSD), CCSD with perturbative triple excitations [CCSD(T)], multi-reference complete active space self-consistent-field (CASSCF), and internally contracted multi-reference configuration interaction (ICMRCI) methods. The correlation-consistent polarised valence quadruple zeta (cc-pVQZ) basis set was used. Four sets of vibrational energy levels determined from the four distinct analytical potential functions have been compared with the experimental values from the laser-induced fluorescence measurements of Wurfel et al. obtained in 1992. The CCSD, CCSD(T), and CASSCF potentials have not provided satisfactory agreement with the experimental observations. In this light, the importance of both non-dynamic (static) and dynamic correlation effects in describing the ground state of CNN is emphasised. Our best theoretical fundamental frequencies at the cc-pVQZ ICMRCI level of theory, v(1) = 1230, v(2) = 394, and v(3) = 1420 cm(-1) are in excellent agreement with the experimental values of v(1) = 1235, v(2) = 396, and v(3) = 1419cm(-1) and the mean absolute deviation between the 23 calculated and experimental vibrational energy levels is only 7.4 cm(-1). It is shown that the previously suggested observation of the v(3) frequency at about 2847cm(-1) was in fact the first overtone 2v(3).

The spin-rovibronic levels for the X 3Σ−,A 3Π electronic system of C2S are calculated variationally, using ab initio potential energy surfaces and taking into account the non-adiabatic coupling between the two states. The energies of selected levels with Σ and Π vibronic symmetry, up to ∼16500cm−1, are reported and compared with available experimental data.

An accurate potential energy surface has been determined for methanol from ab initio potential data at the CCSD(T) level of theory with an aug-cc-pVTZ basis. The resulting potential function is valid over all twelve vibrational degrees of freedom for all near-equilibrium and torsional configurations. A torsional reaction path has been derived for this potential, from which the low-lying vibrational levels of methanol have been calculated by the reaction path version of MULTIMODE. Comparisons with experiment and other calculations are made.

The evaluation of in density functional theory (DFT) is considered. Wang et al. [J. Chem. Phys. 102, 3477 (1995)] have derived an approximate, local density expression for and in the present study their formula is evaluated using densities from unrestricted Hartree-Fock (UHF) and a range of DFT exchange-correlation functionals. The results are compared with those obtained by evaluating the conventional UHF expression using the Kohn-Sham orbitals, which is appropriate for the noninteracting system. A generalized gradient approximation for is then proposed and investigated.

The spin-rovibronic levels for the X-3 Sigma(-), A(3)Pi electronic system of C2S are calculated variationally, using ab initio potential energy surfaces and taking into account the non-adiabatic coupling between the two states. The energies of selected levels with Sigma and Pi vibronic symmetry, up to similar to 16500cm(-1), are reported and compared with available experimental data.