Implementation in the Pyvib2 program of the localized mode method and application to a helicene

In this paper, after reviewing key elements for simulating and interpreting IR, Raman, VCD, and ROA spectra, as well as after describing the localized mode procedure, we present a graphical user interface to carry out the normal mode localizations and we illustrate its application on the ROA spectra of the [19]helicene molecule. The overall procedure consists of four steps, and therefore, a specific interface has been designed for each of them. The first and most important part of the procedure is the selection of the mode ensemble under which the localization procedure is performed. Then, during our stepby- step guided tour of the localized mode procedure in Pyvib2, we highlight the importance of the ordering of the localized modes and the importance to set correctly the phase factor between the localized modes. Finally, the vibrational coupling matrix (Ω̃), the intensity coupling matrix (Ĩ), and the unitary transformation matrix (U) can be analyzed from their representation in the different panels. The ROA spectrum of the [19]helicene molecule is dominated by the positive peaks associated with two normal modes. From the localized mode procedure, we have identified the atomic displacements of these modes as a few-node combination of localized modes characterized by atomic displacements that look like the motion of a "claw".