Relaxed and local mode force constants: Linear ABC as a model system

[Display omitted] •Derived from the F matrix of molecular force constants, the utility of the Compliance Matrix, C, (defined as the inverse of the F matrix; C = F−1) is explored.•The links between the elements of the C matrix, “relaxed” force constants and “local mode” force constants is demonstrated.•A linear ABC molecule is shown to be a useful model. Following on from Wilson’s FG approach to molecular vibrations, there have been two major developments: the exploitation of compliance constants and the introduction of local modes. Both have been heavily augmented by the application of increasingly sophisticated DFT and high-level calculations. Using the linear ABC molecule as a model, we demonstrate the congruence of the two approaches.