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High-level ab initio electronic structure and variational nuclear motion computations are combined to simulate the spectrum of the water molecule at and above its first dissociation limit. Results of these computations are compared with the related state-selective multi-photon measurements of Grechko et al. [J. Chem. Phys. 138 (2010) 081 103]. Both measured and computed spectra show pronounced structures due to quasi-bound (resonance) states. Traditional resonance features associated with trapping of vibrational or rotational energy of the system are identified and assigned. A strong and broad feature observed slightly above dissociation is found to be associated with direct photodissociation into the continuum.

Fully variational energies and structures are obtained for the few-electron prototypical atomic and molecular systems H þ ,H 2, HHe + , H þ , Be, and LiH at the Hartree–Fock basis-set limit (HFL). The HFL computations are made possible by a global optimization technique employing analytic derivatives of the energy with respect to nuclear centers as well as to positions and exponents of Gaussian-type basis functions (GTF). The efficiency of the procedure presented means that the HFL structure and energy of few-electron systems can be obtained even with a few-dozen distributed s-type GTFs. � 2007 Elsevier B.V. All rights reserved.

Relativistic corrections beyond the simple one-electron mass–velocity–Darwin (MVD1) approximation to the ground-state electronic energy of H2S are determined at over 250 geometries. The corrections considered include the two-electron Darwin, the Gaunt and Breit corrections, and the one-electron Lamb shift. Fitted correction surfaces are constructed and used with an accurate ab initio nonrelativistic Born–Oppenheimer potential, determined previously (J. Chem. Phys. 115 (2001) 1229), to calculate vibrational and rotational levels for H232S. The calculations suggest that one- and two-electron relativistic corrections have a noticable influence on the levels of H2S. As for water, the effects considered have markedly different characteristics for the stretching and bending states.

Two-electron relativistic corrections to the ground-state electronic energy of water are determined as a function of geometry at over 300 points. The corrections include the two-electron Darwin term (D2) of the Coulomb–Pauli Hamiltonian, obtained at the cc-pVQZ CCSD(T) level of theory, as well as the Gaunt and Breit corrections, calculated perturbationally using four-component fully variational Dirac–Hartree–Fock (DHF) wavefunctions and two different basis sets. Based on the calculated energy points, fitted relativistic correction surfaces are constructed. These surfaces are used with a high-accuracy ab initio nonrelativistic Born–Oppenheimer (BO) potential energy hypersurface to calculate vibrational band origins and rotational term values for H 2 16 O . The calculations suggest that these two-electron relativistic corrections, which go beyond the usual kinetic relativistic effects and which have so far been neglected in rovibrational calculations on light many-electron molecular systems, have a substantial influence on the rotation–vibration levels of water. The three effects considered have markedly different characteristics for the stretching and bending levels, which often leads to fortuitous cancellation of errors. The effect of the Breit interaction on the rovibrational levels is intermediate between the effect of the kinetic relativistic correction and that of the one-electron Lamb-shift effect.

High-quality ab initio quantum chemical methods, including higher-order coupled cluster (CC) and many-body perturbation (MP) theory, as well as full configuration interaction (FCI) benchmarks, with basis sets ranging from [S/H] [4s3p1d/2s1p] to [9s8p7d5f4g3h2i/7s6p5d4f3g2h] have been employed to obtain the best technically possible value for the barrier to linearity of hydrogen sulphide. Following careful extrapolations of MP2, CCSD and CCSD(T) energies to the complete basis set (CBS) limit and inclusion of small corrections due to scalar relativistic terms, core polarization and core correlation effects, and the diagonal Born–Oppenheimer correction (DBOC), the final electronic (vibrationless) extrapolated barrier height of this study is 24 423±75 cm −1 .

The relativistic correction to the electronic energy of the water molecule is calculated as a function of geometry using . CCSD T wavefunctions and first-order perturbation theory applied to the one-electron mass-velocity and Darwin terms. Based on the calculated 324 energy points, a fitted relativistic correction surface is constructed. This surface is used with a high-accuracy ab initio non-relativistic Born-Oppenheimer potential energy surface to calculate the vibrational band origins and rotational term values for H 16 O. These calculations suggest that the relativistic correction, has a stronger influence on 2 the vibration-rotation levels of water than the Born-Oppenheimer diagonal correction. The effect is particularly marked for vibrational levels with bending excitation or rotational states with high K. q 1998 Elsevier Science B.V. All rights a reserved.

The two lowest electronic states of NaO 2 have been studied using ab initio methods, including RHF SCF, CISD, Davidson corrected CISD, UHF SCF, UMP2-UMP4, CASSCF, and CISD on CASSCF based natural orbitals, with basis sets ranging in quality from Na(11s7p/7s5p), O(9s5pld/5s3pld) to Na(13s10p2dlf/7s6p2dlf), O(11s7p2dlf/6s4p2dlf), i.e. from TZ+d to TZ2P+f+R. Total and relative energies, geometries, vibrational frequencies, and dipole moments of stationary points on the X 2 A 2 and A 2 B 2 surfaces have been determined. A C 2v minimum is proposed for X 2 A 2 NaO 2 with r e (OO)=1.335 and r e (NaO)=2.10 A. The analogous structure for the A 2 B 2 state is predicted to have r e (OO)=1.34 and r e (NAO)=2.13 A with an adiabatic excitation energy of T e =8.5 kcal mol −1 . Linear, 2 ΠNaOO, which correlates to the 2 A 2 and 2 B 2 minima via C s transition states, is predicted to be a shallow minimum lying 17 kcal mol −1 above the X 2 A 2 structure but with a barrier to rearrangement of less than 1 kcal mol −1 . The dissociation energy of the ground state of NaO 2 is deduced to be D 0 =38 kcal mol −1 , a value substantially lower than recent experiment estimates.

Optimized geometries and complete, scaled quadratic force fields of HCOCN and HCSCN have been determined at different theoretical levels (HF/4-21, HF/6-3lG**; geometries also at MP2/6-3lG** and MP2/6-31 lG**). Frequencies calculated from the force fields confirm, with one exception, the assignment of the vibrational spectrum of ACOCN. The vibrational fundamentals calculated for HCSCN (accurate within about 50 cm-‘) could direct a spectroscopy study aimed at determining them. Calculated rotational and quark centrifugal distortion (QCD) constants are in good agreement with the experimental data for both molecules, but QCD constants only after scaling of the force fields.