The Birmingham cluster genetic algorithm is a package that performs global optimisations for homo- and bimetallic clusters based on either first principles methods or empirical potentials. Here, we present a new parallel implementation of the code which employs a pool strategy in order to eliminate sequential steps and significantly improve performance. The new approach meets all requirements of an evolutionary algorithm and contains the main features of the previous implementation. The performance of the pool genetic algorithm is tested using the Gupta potential for the global optimisation of the Au10Pd10 cluster, which demonstrates the high efficiency of the method. The new implementation is also used for the global optimisation of the Au10 and Au20 clusters directly at the density functional theory level.

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The transition from 2D to 3D structures in small gold clusters occurs around 10 atoms. Density functional theory predicts a planar D2h structure for Au10 in contrast to recent second-order Møller-Plesset perturbation theory calculations, which predict a 3D C2v arrangement. The validity of the use of single-reference second-order Møller-Plesset theory for near metallic systems remains, however, questionable. On the other hand, it is less than clear how well density functional approximations perform for such clusters. We, therefore, decided to carry out quantum chemical calculations for Au10 using a variety of different density functionals as well as wavefunction-based methods including coupled cluster theory to compare the different energetically low lying 2D and 3D cluster isomers. The results are perhaps not encouraging showing that most computational methods do not predict correctly the energetic sequence of isomers compared to coupled cluster theory. As perturbative triple corrections in the coupled cluster treatment change the order in cluster stability, the onset of 2D to 3D transition in these gold clusters remains elusive. As expected, second-order Møller-Plesset theory is not suitable for correctly describing such systems.

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Combining molecular beam electric deflection experiments and global optimization techniques has proven to be a powerful tool for resolving equilibrium structures of neutral metal and semiconductor clusters. Herein, we present electric molecular beam deflection experiments on PbN (N = 7-18) clusters. Promising structures are generated using the unbiased Birmingham Cluster Genetic Algorithm approach based on density functional theory. The structures are further relaxed within the framework of two-component density functional theory taking scalar relativistic and spin orbit effects into account. Quantum chemical results are used to model electric molecular beam deflection profiles based on molecular dynamics calculations. Comparison of measured and simulated beam profiles allows the assignment of equilibrium structures for the most cluster sizes in the examined range for the first time. Neutral lead clusters adopt mainly spherical geometries and resemble the structures of lead cluster cations apart from Pb10. Their growth pattern deviates strongly from the one observed for tin and germanium clusters.