We present a consistent analytic representation of the two lowest potential energy surfaces for H₃ and their nonadiabatic coupling. The surfaces are fits to ab initio calculations published previously by Liu and Siegbahn and also to new ab initio calculations reported here. The analytic representations are especially designed to be valid in the vicinity of the conical intersection of the two lowest surfaces, at geometries important for the H+H₂ reaction, and in the van der Waals regions.

A simple, yet reliable, scheme based on treating uniformly singlet-pair and triplet-pair interactions is suggested to extrapolate atomic and molecular electron correlation energies calculated at two basis-set levels of ab initio theory to the infinite one-electron basis-set limit. The novel dual-level method is first tested on extrapolating the full correlation in single-reference coupled-cluster singles and doubles energies for the closed-shell systems CH2(A11), H2O, HF, N2, CO, Ne, and F2 with correlation-consistent basis sets of the type cc-pVXZ (X=D,T,Q,5,6) reported by Klopper [Mol. Phys. 6, 481 (2001)] against his own benchmark calculations with large uncontracted basis sets obtained from explicit correlated singles and doubles coupled-cluster theory. Comparisons are also reported for the same data set but using both single-reference Møller-Plesset and coupled-cluster doubles methods. The results show a similar, often better, accordance with the target results than Klopper’s extrapolations where singlet-pair and triplet-pair energies are extrapolated separately using the popular X−3 and X−5 dual-level laws, respectively. Applications to the extrapolation of the dynamical correlation in multireference configuration interaction calculations carried out anew for He, H2, HeH+, He2++, H3+(1A′1), H3+(1A′3), BH, CH, NH, OH, FH, B2, C2, N2, O2, F2, BO, CO, NO, BN, CN, SH, H2O, and NH3 with standard augmented correlation-consistent basis sets of the type aug-cc-pVXZ (X=D,T,Q,5,6) are also reported. Despite lacking accurate theoretical or experimental data for comparison in the case of most diatomic systems, the new method also shows in this case a good performance when judged from the results obtained with the traditional schemes which extrapolate using the two largest affordable basis sets. For the Hartree-Fock and complete-active space self-consistent field energies, a simple pragmatic extrapolation rule is examined whose results are shown to compare well with the ones obtained from the best reported schemes.

A simple theoretically motivated model to extrapolate the correlation energy based on correlation-consistent polarized X-tuple basis sets is suggested. It has the form $E_X^{\mathrm{cor}}{\mathrm{=E}}_{\infty }^{\mathrm{cor}}{\text{[1+A}}_3{\mathrm{ X}}^{\text{−3}}{\text{(1+A}}_4{\mathrm{ X}}^{\text{−1}}\mathrm{)],}$ where $E_X^{\mathrm{cor}}$ is the energy for the X-tuple basis set, $E_{\infty }^{\mathrm{cor}}$ and $A_3$ are parameters to be determined from a set ${\mathrm{(X}}_{\min }{\mathrm{-X}}_{\max })$ of correlation consistent basis sets at a given level of theory, and $A_4$ is a function of $A_3.$ Even for the simple (2,3) extrapolation scheme, the method is shown to yield energies for 33 test data sets that are more accurate than those obtained from pure correlation consistent sextuple-zeta basis sets at a much lower computational cost. Other extrapolation schemes have also been investigated, including a simple one-parameter rule $E_X^{\mathrm{cor}}{\mathrm{=E}}_{\infty }^{\mathrm{cor}}{\text{(1–2.4X}}^{\text{−3}}\mathrm{).}$

The extended Hartree–Fock approximate correlation energy potential model recently reported for diatomics has been improved to account for the Coulombic behaviour at the collapsed diatomic limit for vanishingly small interatomic separations, R→ 0. Also discussed is how to impose the appropriate inverse exponential dependence on R into the extended Hartree–Fock energy contribution as R→∞. Test results are presented for the ground states of 13 chemically stable diatomics and the Ar₂ van der Waals molecule. For Ar₂, the potential-energy function is obtained from a multiproperty fit including spectroscopic, scattering, and second virial coefficient data. For the remaining diatomics, the calibration procedure uses only spectroscopic Rydberg–Klein–Rees (RKR) data. The new Ar₂ potential is then used to calculate the thermophysical properties of gaseous argon over the temperature range 10²⩽T/K⩽ 10⁵. In all cases, the potential-energy function of the present work has been found to be very accurate.

The excitation function of the H+O2 (v=0)→OH+O reaction has been determined from trajectory calculations using the HO2 DMBE IV potential energy surface. Reactive cross sections for thirteen translational energies, corresponding to a total of a quarter of a million trajectories, have been computed covering the range 65≤Etr/kJ mol−1≤550. Various schemes for analyzing the trajectories, three of which aim to correct approximately for the zero-point energy problem of classical dynamics, have been investigated. One of these schemes aims to correct also for known requirements on rotational distributions, e.g., for the fact that by Hund’s rules for the coupling of angular momentum the product OH (2Π) molecule always rotates. It has been found that zero-point energy effects and lowest-J constraints on rotational distributions may have a crucial role, especially close to the threshold energy of reaction. Agreement with recent measurements of absolute reactive cross sections is generally satisfactory but, unlike experiment, no sharp maximum is found on the excitation function in the vicinity of Etr=170 kJ mol−1. Possible reasons for this discrepancy are discussed. There is also good agreement with existing experimental data on the products rotational distribution.