What Electronic Structure Method Can Be Used in the Global Optimization of Nanoclusters?

The problem of obtaining the spatial structure of nanoclusters
is known to be very difficult due to the large number of local minima
associated with their potential energy surfaces (isomers). In global
optimization approaches, such as basin hopping and genetic algorithms,
the problem is normally tackled by first using a low-level and affordable
method to evaluate the energy. Afterward, the putative global minimum
(and often a few others) is refined with calculations using higher
level methods and larger basis sets. There is no guarantee, however,
that the structure obtained at the lower level method will be the
global minimum at the refined one. In this work, we have performed
benchmark coupled cluster calculations at the complete basis set limit
for a large number of different isomers of representative clusters
of third row elements. Such calculations are then employed to check
the hypothesis that lower level methods can be used in the global
optimization with reliable results. For this, we have developed a
methodology that allows us to compare a large number of minima obtained
at different calculation levels. The results indicate that, if the
global optimization is capable of reaching not only the global minimum
but also a reduced number of low lying structures, most of the tested
density functional theory (DFT) functionals are good choices, with
emphasis on TPSSh. Besides giving a more solid ground to this commonly
used approach, this work helps guiding such global optimizations.
The use of the MP2 method and several scaled variants is also assessed,
from where it is concluded that the scaled variants yield better results
than standard MP2 or DFT approaches, except for one system where a
large number of van der Waals structures exist.