92 research outputs found
Performance of ab initio and density functional methods for conformational equilibria of CnH2n+2 alkane isomers (n=2-8)
Conformational energies of n-butane, n-pentane, and n-hexane have been
calculated at the CCSD(T) level and at or near the basis set limit.
Post-CCSD(T) contribution were considered and found to be unimportant. The data
thus obtained were used to assess the performance of a variety of density
functional methods. Double-hybrid functionals like B2GP-PLYP and B2K-PLYP,
especially with a small Grimme-type empirical dispersion correction, are
capable of rendering conformational energies of CCSD(T) quality. These were
then used as a `secondary standard' for a larger sample of alkanes, including
isopentane and the branched hexanes as well as key isomers of heptane and
octane. Popular DFT functionals like B3LYP, B3PW91, BLYP, PBE, and PBE0 tend to
overestimate conformer energies without dispersion correction, while the M06
family severely underestimates GG interaction energies. Grimme-type dispersion
corrections for these overcorrect and lead to qualitatively wrong conformer
orderings. All of these functionals also exhibit deficiencies in the conformer
geometries, particularly the backbone torsion angles. The PW6B95 and, to a
lesser extent, BMK functionals are relatively free of these deficiencies.
Performance of these methods is further investigated to derive conformer
ensemble corrections to the enthalpy function, , and the Gibbs
energy function, , of these alkanes. While
is only moderately sensitive to the level of theory, exhibits more pronounced sensitivity. Once again, double hybrids
acquit themselves very well.Comment: J. Phys. Chem. A, revised [Walter Thiel festschrift
Benchmark thermochemistry of the C_nH_{2n+2} alkane isomers (n=2--8) and performance of DFT and composite ab initio methods for dispersion-driven isomeric equilibria
The thermochemistry of linear and branched alkanes with up to eight carbons
has been reexamined by means of W4, W3.2lite and W1h theories. `Quasi-W4'
atomization energies have been obtained via isodesmic and hypohomodesmotic
reactions. Our best atomization energies at 0 K (in kcal/mol) are: 1220.04
n-butane, 1497.01 n-pentane, 1774.15 n-hexane, 2051.17 n-heptane, 2328.30
n-octane, 1221.73 isobutane, 1498.27 isopentane, 1501.01 neopentane, 1775.22
isohexane, 1774.61 3-methylpentane, 1775.67 diisopropyl, 1777.27 neohexane,
2052.43 isoheptane, 2054.41 neoheptane, 2330.67 isooctane, and 2330.81
hexamethylethane. Our best estimates for are: -30.00
n-butane, -34.84 n-pentane, -39.84 n-hexane, -44.74 n-heptane, -49.71 n-octane,
-32.01 isobutane, -36.49 isopentane, -39.69 neopentane, -41.42 isohexane,
-40.72 3-methylpentane, -42.08 diisopropyl, -43.77 neohexane, -46.43
isoheptane, -48.84 neoheptane, -53.29 isooctane, and -53.68 hexamethylethane.
These are in excellent agreement (typically better than 1 kJ/mol) with the
experimental heats of formation at 298 K obtained from the CCCBDB and/or NIST
Chemistry WebBook databases. However, at 0 K a large discrepancy between theory
and experiment (1.1 kcal/mol) is observed for only neopentane. This deviation
is mainly due to the erroneous heat content function for neopentane used in
calculating the 0 K CCCBDB value. The thermochemistry of these systems,
especially of the larger alkanes, is an extremely difficult test for density
functional methods. A posteriori corrections for dispersion are essential.
Particularly for the atomization energies, the B2GP-PLYP and B2K-PLYP
double-hybrids, and the PW6B95 hybrid-meta GGA clearly outperform other DFT
functionals.Comment: (J. Phys. Chem. A, in press
Target highlights in CASP13: experimental target structures through the eyes of their authors
The functional and biological significance of selected CASP13 targets are described by the authors of the structures. The structural biologists discuss the most interesting structural features of the target proteins and assess whether these features were correctly reproduced in the predictions submitted to the CASP13 experiment
Implementing Quantum Gates and Algorithms in Ultracold Polar Molecules
We numerically investigate the implementation of small quantum algorithms, an arithmetic adder and the Grover search algorithm, in registers of ultracold polar molecules trapped in a lattice by concatenating intramolecular and intermolecular gates. The molecular states are modulated by the exposition to static electric and magnetic fields different for each molecule. The examples are carried out in a two-molecule case. Qubits are encoded either in rovibrational or in hyperfine states, and intermolecular gates involve states of neighboring molecules. Here we use pi pulses (i.e. laser pulses such that the integral of the product of the transition dipole moment and their envelope is equal to pi, thus ensuring a total population inversion between two states) and pulses designed by optimal control theory adapted to a multi-target problem to drive unitary transformations between the qubit states.info:eu-repo/semantics/publishe
Quantum gates driven by microwave pulses in hyperfine levels of ultracold heteronuclear dimers
We theoretically investigated the implementation of universal quantum gates in hyperfine
levels of ultracold heteronuclear polar molecules in their lowest rotational manifolds.
Quantum bits are manipulated by microwave pulses, taking advantage of the strong state
mixing generated by the hyperfine interactions. Gate operations are either driven by a
sequence of Gaussian pulses or by a pulse shaped by optimal control theory. Alkaline
molecules of experimental interest are considered. We show that high fidelity gates can be
driven by microsecond pulses. The richness of the energy structure and the state mixing
offer promising perspectives for the manipulation of a large number of qubits
Structure, Energy, and Vibrational Frequencies of Oxygen Allotropes On (n ≤ 6) in the Covalently Bound and van der Waals Forms: Ab Initio Study at the CCSD(T) Level
Recent experiments on the UV and electron beam irradiation of solid O2 reveals a series of IR features near
the valence antisymmetric vibration band of O3 which are frequently interpreted as the formation of unusual On allotropes in the forms of weak complexes or covalently bound molecules. In order to elucidate the question of the nature of the irradiation products, the structure, relative energies, and vibrational
frequencies of various forms of On (n = 1−6) in the singlet, triplet, and, in some cases, quintet states were studied using the CCSD(T) method up to the CCSD(T,full)/cc-pCVTZ and CCSD(T,FC)/aug-cc-pVTZ levels. The results of calculations demonstrate the existence of stable highly symmetric structures O4(D3h), O4 (D2d), and O6 (D3d) as well as the intermolecular complexes O2·O2, O2·O3, and O3·O3 in different conformations. The
calculations show that the local minimum corresponding to the O3···O complex is quite shallow and cannot explain the ν3 band features close to 1040 cm−1, as was proposed previously. For the ozone dimer, a new conformer was found which is more stable than the structure known to date. The effect of the ozone dimer on the registered IR spectra is discussed
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