New analytical model for the ozone electronic ground state potential surface and accurate ab initio vibrational predictions at high energy rangeWe present an ab initio potential energy surface for the ground electronic state of ozone. It is global, i.e., it covers the three identical C 2v ͑open͒ minima, the D 3h ͑ring͒ minimum, as well as the O( 3 P)ϩO 2 ( 3 ⌺ g Ϫ ) dissociation threshold. The electronic structure calculations are performed at the multireference configuration interaction level with complete active space self-consistent-field reference functions and correlation consistent polarized quadruple zeta atomic basis functions. Two of the O-O bond distances, R 1 and R 2 , and the O-O-O bending angle are varied on a regular grid ͑ca. 5000 points with R 1 уR 2 ). An analytical representation is obtained by a three-dimensional cubic spline. The calculated potential energy surface has a tiny dissociation barrier and a shallow van der Waals minimum in the exit channel. The ring minimum is separated from the three open minima by a high potential barrier and therefore presumably does not influence the low-temperature kinetics. The dissociation energy is reproduced up to 90% of the experimental value. All bound states of nonrotating ozone up to more than 99% of the dissociation energy are calculated using the filter diagonalization technique and employing Jacobi coordinates. The three lowest transition energies for 16 O 3 are 1101.9 cm Ϫ1 ͑1103.14 cm Ϫ1 ͒, 698.5 cm Ϫ1 ͑700.93 cm Ϫ1 ͒, and 1043.9 cm Ϫ1 ͑1042.14 cm Ϫ1 ͒ for the symmetric stretch, the bending, and the antisymmetric stretch modes, respectively; the numbers in parentheses are the experimental values. The root-mean-square error for all measured transition energies for 16 O 3 is only 5 cm Ϫ1 . The comparison is equally favorable for all other isotopomers, for which experimental frequencies are available. The assignment is made in terms of normal modes, despite the observation that with increasing energy an increasing number of states acquires local-mode character. At energies close to the threshold a large fraction of states is still unambiguously assignable, particularly those of the overtone progressions. This is in accord with the existence of stable classical periodic orbits up to very high energies.

In this overview we discuss the vibrational spectrum of phosphaethyne, HCP, in its electronic ground state, as revealed by complementary experimental and theoretical examinations. The main focus is the evolution of specific spectral patterns from the bottom of the potential well up to excitation energies of approximately 25,000 cm(-1), where large-amplitude, isomerization-type motion from H-CP to CP-H is prominent. Distinct structural and dynamical changes, caused by an abrupt transformation from essentially HC bonding to mainly PH bonding, set in around 13,000 cm(-1). They reflect saddle-node bifurcations in the classical phase space--a phenomenon well known in the nonlinear dynamics literature--and result in characteristic patterns in the spectrum and the quantum-number dependence of the vibrational fine-structure constants. Two polar opposites are employed to elucidate the spectral patterns: the exact solution of the Schrödinger equation, using an accurate potential energy surface and an effective or resonance Hamiltonian (expressed in a harmonic oscillator basis set and block diagonalized into polyads), which is defined by parameters adjusted to fit either the measured or the calculated vibrational energies. The combination of both approaches--together with classical mechanics and semiclassical analyses--provides a detailed spectroscopic picture of the breaking of one bond and the formation of a new one.