Theoretical Determination of the Dissociation Energy of Molecular Hydrogen

Quantum confinement of the covalent bond beyond the Born-Oppenheimer approximation

Dirichlet boundary conditions with different symmetries, spherical and cylindrical impenetrable surfaces, are imposed on the covalent electron pair of a molecular bond. Accurate results for different observable like energy and interparticle distances are calculated using quantum Monte Carlo methods beyond the Born-Oppenheimer approximation. The spherical confinement induces a raise in the bond energy and shortens the internuclear distances even for a relatively soft confinement. When cylindrical symmetry is considered, similar qualitative behavior is observed though only the electrons are confined. A compression followed by a relaxation process of the confined bond is shown to induce a vibrationally excited state. Finally, a brief qualitative discussion based on a simplified picture of the role of compression/relaxation cycles in enzyme catalysis is given.

Fundamental vibration of molecular hydrogen

The fundamental ground tone vibration of H(2), HD, and D(2) is determined to an accuracy of 2×10(-4) cm(-1) from Doppler-free laser spectroscopy in the collisionless environment of a molecular beam. This rotationless vibrational splitting is derived from the combination difference between electronic excitation from the X(1)Σ(g)(+), v=0, and v=1 levels to a common EF(1)Σ(g)(+), v=0 level. Agreement within 1σ between the experimental result and a full ab initio calculation provides a stringent test of quantum electrodynamics in a chemically bound system.

Perspective: relativistic effects

This perspective article discusses some broadly-known and some less broadly-known consequences of Einstein's special relativity in quantum chemistry, and provides a brief outline of the theoretical methods currently in use, along with a discussion of recent developments and selected applications. The treatment of the electron correlation problem in relativistic quantum chemistry methods, and expanding the reach of the available relativistic methods to calculate all kinds of energy derivative properties, in particular spectroscopic and magnetic properties, requires on-going efforts.

Observable quasi-bound states of the H2 molecule

Characteristic parameters (energetic positions, widths, etc.) of the 23 spectroscopically observed (or potentially observable) quasi-bound energy levels for the ground state H(2) molecule are determined with high accuracy. The recent advances concerning nonadiabatic, relativistic, and quantum electrodynamical corrections have been taken into consideration. Theoretical approach is based on the careful investigation of the energy dependences of the real stationary scattering-state wave functions (their phase shifts and amplitudes). These energy dependences can be very accurately fitted to simple analytic formulas resulting from the quantum scattering theory. General criteria for the assignment of quasi-bound states are discussed and a new criterion is proposed, which excludes too broad and ambiguous resonances. To implement the proposed approach, a special high-precision computational method was used, which is described in full detail.

Benchmarking Experimental and Computational Thermochemical Data: A Case Study of the Butane Conformers

Due to its crucial importance, numerous studies have been conducted to determine the enthalpy difference between the conformers of butane. However, it is shown here that the most reliable experimental values are biased due to the statistical model utilized during the evaluation of the raw experimental data. In this study, using the appropriate statistical model, both the experimental expectation values and the associated uncertainties are revised. For the 133-196 and 223-297 K temperature ranges, 668 ± 20 and 653 ± 125 cal mol(-1), respectively, are recommended as reference values. Furthermore, to show that present-day quantum chemistry is a favorable alternative to experimental techniques in the determination of enthalpy differences of conformers, a focal-point analysis, based on coupled-cluster electronic structure computations, has been performed that included contributions of up to perturbative quadruple excitations as well as small correction terms beyond the Born-Oppenheimer and nonrelativistic approximations. For the 133-196 and 223-297 K temperature ranges, in exceptional agreement with the corresponding revised experimental data, our computations yielded 668 ± 3 and 650 ± 6 cal mol(-1), respectively. The most reliable enthalpy difference values for 0 and 298.15 K are also provided by the computational approach, 680.9 ± 2.5 and 647.4 ± 7.0 cal mol(-1), respectively.

Precision measurements and computations of transition energies in rotationally cold triatomic hydrogen ions up to the midvisible spectral range

First-principles computations and experimental measurements of transition energies are carried out for vibrational overtone lines of the triatomic hydrogen ion H(3)(+) corresponding to floppy vibrations high above the barrier to linearity. Action spectroscopy is improved to detect extremely weak visible-light spectral lines on cold trapped H(3)(+) ions. A highly accurate potential surface is obtained from variational calculations using explicitly correlated Gaussian wave function expansions. After nonadiabatic corrections, the floppy H(3)(+) vibrational spectrum is reproduced at the 0.1 cm(-1) level up to 16600 cm(-1).

QED effects in molecules: test on rotational quantum states of H2

Quantum electrodynamic effects have been systematically tested in the progression of rotational quantum states in the X 1Σ(g)(+), v=0 vibronic ground state of molecular hydrogen. High-precision Doppler-free spectroscopy of the EF 1Σ(g)(+)-X 1Σ(g)(+) (0,0) band was performed with 0.005  cm(-1) accuracy on rotationally hot H2 (with rotational quantum states J up to 16). QED and relativistic contributions to rotational level energies as high as 0.13  cm(-1) are extracted, and are in perfect agreement with recent calculations of QED and high-order relativistic effects for the H2 ground state.

Relativistic and quantum electrodynamics effects in the helium pair potential

The helium pair potential was computed including relativistic and quantum electrodynamics contributions as well as improved accuracy adiabatic ones. Accurate asymptotic expansions were used for large distances R. Error estimates show that the present potential is more accurate than any published to date. The computed dissociation energy and the average R for the (4)He(2) bound state are 1.62+/-0.03 mK and 47.1+/-0.5 A. These values can be compared with the measured ones: 1.1(-0.2)(+0.3) mK and 52+/-4 A [R. E. Grisenti, Phys. Rev. Lett. 85, 2284 (2000)].