Three-dimensional potential energy surfaces for the electronic ground state as well as the lowest three 1A″ states have been computed using highly correlated CASSCF-MRCI wave functions and a large basis set. An approximate diabatization scheme has been employed to generate quasidiabatic potential energy surfaces for the lowest two 1A″ states. The diabatization is based on the condition that both the orbitals as well as the configuration coefficients of the diabatic wave functions change as little as possible as function of geometry. The diabatic potential energy surfaces are used in time-dependent simulations of the absorption spectrum as well as the vibrational and rotational product distributions. Excellent agreement between the computed and experimental absorption spectra and product distributions is obtained, indicating that the ab initio potentials as well as the diabatization scheme are accurate.

A modified deterministic/stochastic genetic algorithm (DS‐GA) method is proposed for the determination of the global minimum of atomic clusters described by pairwise analytic interaction potentials. Our modification of the standard GA method involves a coarse local minimization of each member of the population at every generation, as well as including the gradient into the fitness function. For Lennard‐Jones (Ar)_n clusters with n<30, the DS‐GA converges far more quickly to the global minimum than either conventional GA methods or random search procedures. An application of this DS‐GA is made to heterogeneous clusters of B(²P) with multiple Ar atoms. The interaction potential is given by the lowest state of a 3×3 electronic Hamiltonian. The Ar–Ar potential and the lower energy (Π state) B–Ar potential are very similar. In contrast, the higher energy (Σ state) B–Ar interaction is essentially repulsive. Consequently, the B atom is nearly always found to substitute for one of the atoms in the corresponding (Ar)_n+1 cluster with the fewest number of nearest neighbors.

In practical applications, global cluster structure optimisation has so far been limited largely to homogeneous clusters of atoms or small molecules, with little or no choice in the calculation of inter-particle forces. We eliminate these limitations by presenting a new program suite OGOLEM that is universal by design, both in cluster composition (including arbitrarily heterogeneous clusters of complicated molecules) and in its interfaces to force calculation backends. This is demonstrated by exemplary applications in two novel fields: strongly heterogeneous Lennard–Jones clusters (ternary, quaternary, quinary) and mixed clusters of the aminoglycoside Kanamycin A with sodium cations.

Size selected water clusters are generated by photoionizing sodium doped clusters close to the ionization threshold. This procedure is free of fragmentation. Upon infrared excitation, size- and isomer-specific OH-stretch spectra are obtained over a large range of cluster sizes. In one application of this method the infrared spectra of single water cluster sizes are investigated. A comparison with calculations, based on structures optimized by genetic algorithms, has been made to tentatively derive cluster structures which reproduce the experimental spectra. We identified a single all-surface structure for n = 25 and mixtures with one or two interior molecules for n = 24 and 32. In another application the sizes are determined at which the crystallization sets in. Surprisingly, this process strongly depends on the cluster temperature. The crystallization starts at sizes below n = 200 at higher temperatures and the onset is shifted to sizes above n = 400 at lower temperatures.