Cluster-Weighting in Bulk Phase Vibrational Circular Dichroism

We present a cluster-weighting approach for calculating the vibrational circular dichroism (VCD) spectra of bulk phase systems. Based on the quantum cluster equilibrium theory, cluster populations are received via self-consistent field calculations that allow a mixing of differently sized oligomers at the same time. The thereby obtained cluster weights are employed for the weighting of individual cluster VCD spectra in order to calculate an overall gas or bulk phase VCD spectrum. As the mixing of different oligomer sizes is possible, different structural motifs and interactions can be included, and explicit solvation, typically necessary for describing hydrogen bonds, is intrinsically taken care of, however, without neglecting monomeric structures and different conformers. We test the sensitivity of Boltzmann and cluster weights with respect to the level of theory employed and show that cluster weights are less sensitive. By a constant increase of the oligomer sizes included in our cluster sets, combined with a continuous truncation of low populated clusters, we are able to improve the agreement between theory and experiment until we reach an overlap which allows a certain assignment of the absolute configuration based on an experimental bulk phase spectrum. Combined with a computationally cost-efficient level of theory, this approach provides a valuable tool for the fast calculation of VCD spectra for bulk phase systems.