Extended solids, computed vibrational

Another way to make better use of the neutrons that are currently available is to extract more information from the spectra. This approach is totally reliant on increasing computing power that enables more complex calculations. Ab-initio calculations of isolated molecules or small clusters are now routine. Packages that can calculate vibrational frequencies and atomic displacements in the modes for extended solids are now available. In addition to the analysis of new systems, this capability also enables the mining of older data to provide new insights. 

Although the preferred method of calculating a spectrum is to perform an ab initio calculation on an extended solid, extracting frequencies and displacements across the Brillouin zone, on a fine A-grid, this approach can be computationally very expensive. In plane wave codes like CASTEP [18], CPMD [19], TWSCF [20], VASP [21], ABINIT [22], and some others, the number of plane waves that are taken into consideration, the selected correlation fimctional and the choice of pseudopotential will all have an impact on the quality of the calculations. Some codes (e.g. ABINIT) alleviate the problem by permitting frozen phonon calculations at the symmetry zone boundary, i.e. (0,0,0), (l/2,0,0), (l/2,l/2,0) and (l/2,l/2,l/2) and so determine the dynamical matrix at these points. The code then interpolates values of the d3mamical matrix for all the points within the Brillouin zone and uses these to calculate the solution to the vibrational problem inside the zone. 

An alternative interpretation may be that the elecnonic interaction of the stacked fluorene groups in the polymer side chain extends inherently up to ca. five units even for a long, completely uniform frozen jt-stacked polymer. Absorption measurements at an extremely low temperature or in a solid-state matrix that may significantly retard molecular vibration might provide an answer. Computational studies on the electronic states of poly(DBF) may also be necessary. 

---