Significant progress in predicting the crystal structures of small organic molecules--a report on the fourth blind test.

We report on the organization and outcome of the fourth blind test of crystal structure prediction, an international collaborative project organized to evaluate the present state in computational methods of predicting the crystal structures of small organic molecules. There were 14 research groups which took part, using a variety of methods to generate and rank the most likely crystal structures for four target systems: three single-component crystal structures and a 1:1 cocrystal. Participants were challenged to predict the crystal structures of the four systems, given only their molecular diagrams, while the recently determined but as-yet unpublished crystal structures were withheld by an independent referee. Three predictions were allowed for each system. The results demonstrate a dramatic improvement in rates of success over previous blind tests; in total, there were 13 successful predictions and, for each of the four targets, at least two groups correctly predicted the observed crystal structure. The successes include one participating group who correctly predicted all four crystal structures as their first ranked choice, albeit at a considerable computational expense. The results reflect important improvements in modelling methods and suggest that, at least for the small and fairly rigid types of molecules included in this blind test, such calculations can be constructively applied to help understand crystallization and polymorphism of organic molecules.

Electrostatic guidelines and molecular tailoring for density functional investigation of structures and energetics of (Li)n clusters.

A molecular electrostatic potential (MESP)-guided method for building metal aggregates is proposed and tested on prototype lithium (Li)(n) clusters from n=4 to 58. The smaller clusters are subsequently subjected to direct density functional theory based geometry optimization, while the larger ones are optimized via molecular tailoring approach (MTA). The calculations are performed using PW91-PW91 as well as B3LYP functionals, and the trends in the interaction energies are found to be similar. The MESP-guided model for building metal clusters is validated by comparing the resulting cluster geometries with the ones reported in the literature up to n=20. A comparison of the ionization potential and polarizability (up to n=22) with their experimental counterparts shows a fairly good agreement. A new MTA-based scheme for calculating the ionization potential and polarizability values of large metal clusters is proposed and tested on Li(40) and Li(58) clusters. Further, the existence of "magic numbered clusters" up to n=22 is justified in terms of "maximum hardness principle" as well based on molecular electron density topography and distance descriptors.