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SUMMARY:Global optimization studies of atomic clusters - Edwin Flikkema (A
 berystwyth University)
DTSTART:20230824T140000Z
DTEND:20230824T143000Z
UID:TALK204040@talks.cam.ac.uk
DESCRIPTION:Within the field of structure prediction of atomic clusters\, 
 optimising the interaction energy (and hence finding low-energy geometries
  of such clusters) is an important problem. This gives geometries that are
  thermodynamically likely to be formed. Finding minima is also the first s
 tep in an &ldquo\;energy landscape&rdquo\; approach\, where\, alongside mi
 nima\, &ldquo\;transition states&rdquo\; (i.e.\, certain saddle points) ar
 e also considered. This presentation focuses on atomic clusters made of si
 licon and oxygen atoms\, i.e.\, SiO2\, also called silica.Silica (SiO2) is
  a versatile material with many applications in such diverse fields as mic
 ro-electronics\, chemistry (catalysis) and photonics. Many bulk polymorphs
  exist such as the dense quartz phase or the more open zeolites. This pres
 entation is about trying to find the optimal geometries of various types o
 f silica (nano-)clusters. The methodology consists of a two-step approach\
 , where classical potentials are used first and promising candidates for t
 he energetic global minimum are subsequently refined using Density Functio
 nal Theory (DFT) with the B3LYP functional and a 6-31G** basis set.In an i
 nitial study (SiO2)N clusters consisting purely of silica were considered.
  The Basin Hopping global optimization algorithm was used together with a 
 specifically parameterized potential to produce candidate geometries for t
 he energetic global minimum\, followed by DFT refinement. The potential us
 ed is of the same form as the BKS and TTAM potentials known in the literat
 ure. The potential has been re-parameterized to give more accurate results
  for clusters (rather than the bulk material). Clusters of sizes up to 27 
 SiO2 units have been considered and global minima were proposed.A second s
 tudy focuses specifically on `fully-coordinated' silica clusters\, i.e. de
 fect-less clusters where every silicon atom is bonded to 4 oxygen atoms an
 d every oxygen atom is bonded to 2 silicon atoms. This fully-coordinated a
 rrangement of atoms is common in bulk silica\, whereas for clusters defect
 s tend to occur. Fully-coordinated clusters are expected to have special p
 roperties\, such as an improved chemical stability\, making them possible 
 building blocks for cluster-assembled materials. An algorithm for specific
 ally searching for (low energy) fully-coordinated clusters was developed. 
 This algorithm is based on performing Monte Carlo moves in the space of gr
 aphs (rather than in coordinate space)\, the graph being the network of ch
 emical bonds between atoms. This approach ensures that the clusters remain
  fully-coordinated during the search. Fully-coordinated clusters of sizes 
 up to 24 SiO2 units are considered. The main purpose of this investigation
  is to find out how the energetic difference between the lowest-energy ful
 ly-coordinated cluster and the lowest energy defective cluster diminishes 
 with cluster size.Another study focuses on hydroxylated silica clusters (S
 iO2)M(H2O)N. Such clusters are more likely to occur in nature and are rele
 vant to understanding the processes involved in the synthesis of zeolites.
  Here\, a simplified version of the potential introduced by Hassanali and 
 Singer is used in combination with the Basin Hopping global optimisation a
 lgorithm and DFT refinement. Structural and energetic trends with increasi
 ng level of hydroxylation are being studied. This has led to a re-interpre
 tation of an experiment on atomic mixing in hydroxylated silica clusters i
 n solution.If time permits\, work on two-dimensional foams (in collaborati
 on with Simon Cox) can be discussed as well.
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