Explicitly correlated atomic orbital basis second order Møller-Plesset theory.

The scope of problems treatable by ab initio wavefunction methods has expanded greatly through the application of local approximations. In particular, atomic orbital (AO) based wavefunction methods have emerged as powerful techniques for exploiting sparsity and have been applied to biomolecules as large as 1707 atoms [S. A.

Analytic gradients for density cumulant functional theory: the DCFT-06 model.

Density cumulant functional theory (DCFT) is one of a number of nascent electron correlation methods that are derived from reduced density matrices and cumulants thereof, instead of the wavefunction. Deriving properties from the density cumulant naturally yields methods that are size extensive and size consistent. In this work, we derive expressions for the analytic gradient, with respect to an external perturbation, for the DCFT-06 variant of density cumulant functional theory.

1-Germavinylidene (Ge═CH2), germyne (HGeCH), and 2-germavinylidene (H2Ge═C) molecules and isomerization reactions among them: anharmonic rovibrational analyses.

Theoretical investigations of three equilibrium structures and two associated isomerization reactions of the GeCH(2) - HGeCH - H(2)GeC system have been systematically carried out. This research employed ab initio self-consistent-field (SCF), coupled cluster (CC) with single and double excitations (CCSD), and CCSD with perturbative triple excitations [CCSD(T)] wave functions and a wide variety of correlation-consistent polarized valence cc-pVXZ and cc-pVXZ-DK (where X = D, T, Q) basis sets. For each structure, the total energy, geometry, dipole moment, harmonic vibrational frequencies, and infrared intensities are predicted.

Spin-Restriction in Explicitly Correlated Coupled Cluster Theory: The Z-Averaged CCSD(2)R12 Approach.

R12 methods have now been established to improve both the efficiency and accuracy of wave function-based theories. While closed-shell and spin-orbital methodologies for coupled cluster theory are well-studied, R12 corrections based on an open-shell, spin-restricted formalism have not been well developed. We present an efficient spin-restricted R12 method based on the symmetric exchange or Z-averaged approach that reduces the number of variational parameters.

Aluminum foils: the contrasting characters of hyperconjugation and steric repulsion in aluminum dimetallocenes.

The novel sandwich complex Cp2*Al2I2, which was recently synthesized by Minasian and Arnold, has been characterized using ab initio and density functional methods. A large family of related compounds was also investigated. Although a few Al(II)–Al(II) bonds are known, this is the first such bond to be supported by Cp-type ligands.

Anharmonic rovibrational analysis for disilacyclopropenylidene (Si2CH2).

The global minimum on the Si(2)CH(2) electronic singlet potential energy surface has been theoretically predicted to be a peculiar hydrogen bridged (Si···H···Si) disilacyclopropenylidene structure (Si(2)CH(2)). An accurate quartic force field for Si(2)CH(2) has been determined employing ab initio coupled-cluster theory with single and double excitations and a perturbative treatment for triple excitations [CCSD(T)], in combination with the correlation consistent core-valence quadruple zeta (cc-pCVQZ) basis set. The vibration-rotation coupling constants, equilibrium and zero-point vibration corrected rotational constants, centrifugal distortion constants, and harmonic and fundamental vibrational frequencies for six isotopologues of Si(2)CH(2) are predicted using vibrational second-order perturbation theory (VPT2).

The Beryllium tetramer: profiling an elusive molecule.

The structure and energetics of Be(4) are investigated using state-of-the-art coupled-cluster methods. We compute the optimized bond length, dissociation energy, and anharmonic vibrational frequencies. A composite approach is employed, starting from coupled-cluster theory with single, double, and perturbative triple excitations extrapolated to the complete basis set (CBS) limit using Dunning's correlation consistent cc-pCVQZ and cc-pCV5Z basis sets.

Density cumulant functional theory: first implementation and benchmark results for the DCFT-06 model.

Density cumulant functional theory [W. Kutzelnigg, J. Chem.

Anharmonic Vibrational Analysis for the Propadienylidene Molecule (H2C═C═C:).

Maier et al. found that photolysis of singlet cyclopropenylidene (1S) in a matrix yields triplet propargylene (2T), which upon further irradiation is converted to singlet propadienylidene (vinylidenecarbene, 3S). Their discovery was followed by interstellar identification of 3S by Cernicharo et al.

The subtleties of explicitly correlated Z-averaged perturbation theory: choosing an R12 method for high-spin open-shell molecules.

Explicitly correlated MP2-R12 and coupled cluster R12 methods have proven to be effective in achieving the basis set limit of correlated wave function methods. However, correlated methods for high-spin open-shell states are typically based on semicanonical orbitals, leading to an unrestricted formalism, which for double excitations requires three independent sets of amplitudes. In contrast, Z-averaged perturbation theory redefines the Hamiltonian with a symmetric exchange operator, thereby allowing a spin-restricted formulation with equivalent alpha and beta subspaces.

A laboratory and theoretical study of protonated carbon disulfide, HSCS+.

The rotational spectrum of protonated carbon disulfide, HSCS(+), has been detected in the centimeter-wave band in a molecular beam by Fourier transform microwave spectroscopy. Rotational and centrifugal distortion constants have been determined from ten transitions in the K(a)=0 ladder of the normal isotopic species, HS(13)CS(+), and DSCS(+). The present assignment agrees well with high-level coupled cluster calculations of the HSCS(+) structure, which, like earlier work, predict this isomer to be the ground state on the HCS(2) (+) potential energy surface; HCSS(+), an isomer with C(2v) symmetry, is predicted to lie more than 20 kcal/mol higher in energy.

Conformers of Gaseous Cysteine.

Structures, accurate relative energies, equilibrium and vibrationally averaged rotational constants, quartic and sextic centrifugal distortion constants, dipole moments, (14)N nuclear quadrupole coupling constants, anharmonic vibrational frequencies, and double-harmonic infrared intensities have been determined from ab initio electronic structure computations for conformers of the neutral form of the natural amino acid l-cysteine (Cys). A systematic scan located 71 unique conformers of Cys using the MP2(FC)/cc-pVTZ method. The large number of structurally diverse low-energy conformers of Cys necessitates the highest possible levels of electronic structure theory to determine their relative energies with some certainty.

Origin of the SN2 benzylic effect.

The S N2 identity exchange reactions of the fluoride ion with benzyl fluoride and 10 para-substituted derivatives (RC6H 4CH 2F, R = CH3, OH, OCH 3, NH2, F, Cl, CCH, CN, COF, and NO2) have been investigated by both rigorous ab initio methods and carefully calibrated density functional theory. Groundbreaking focal-point computations were executed for the C6H5CH 2F + F (-) and C 6H 5CH2Cl + Cl (-) SN2 reactions at the highest possible levels of electronic structure theory, employing complete basis set (CBS) extrapolations of aug-cc-pV XZ (X = 2-5) Hartree-Fock and MP2 energies, and including higher-order electron correlation via CCSD/aug-cc-pVQZ and CCSD(T)/aug-cc-pVTZ coupled cluster wave functions. Strong linear dependences are found between the computed electrostatic potential at the reaction-center carbon atom and the effective SN2 activation energies within the series of para-substituted benzyl fluorides.

Establishment of the C(2)H(5)+O(2) reaction mechanism: a combustion archetype.

The celebrated C(2)H(5)+O(2) reaction is an archetype for hydrocarbon combustion, and the critical step in the process is the concerted elimination of HO(2) from the ethylperoxy intermediate (C(2)H(5)O(2)). Master equation kinetic models fitted to measured reaction rates place the concerted elimination barrier 3.0 kcal mol(-1) below the C(2)H(5)+O(2) reactants, whereas the best previous electronic structure computations yield a barrier more than 2.

Resonance bonding patterns of peroxide chemistry: cyclic three-center hyperbonding in "phosphadioxirane" intermediates.

We investigate the nature of valency and bonding in the highly unusual O(2)PR(3) "phosphadioxirane" species recently isolated as an intermediate in the reaction of (1)O(2) with organic phosphines PR(3). Commonly, this species is depicted as a Lewis structure with five bonds at the phosphorus center, suggesting hypervalent involvement of extra-valence d-orbitals in the hybridization. However, nonhypervalent bonding patterns, such as open zwitterionic peroxides or R(2)PO(2)(+)R(-) ion pairs, could also achieve the observed hypercoordination.

Conformational distinguishability of medium cycloalkanes in crystals via inelastic neutron scattering.

This paper begins an exploration of the use of the combination of DFT computations with experimental inelastic neutron scattering (INS) spectra as a method for establishing what conformation is present in a molecular crystal at low temperature. Presented here are INS spectra of a series of medium-sized cycloalkanes: C6H12, C7H14, C8H16, C10H20, C12H24, and C14H28. Optimized geometries and normal mode calculations were performed at B3LYP/6-311G(d,p) on the lowest energy conformations (i.