The Poisson's ratios and Young's moduli of 2D molecular networks having conventional and re-entrant honeycomb forms have been modelled using molecular modelling. Three principle deformation mechanisms were observed : bond hinging, flexure and stretching. Analytical models have also been developed that can be used to describe each of these modes of deformation acting either independently or concurrently. A parametric fit of the force constants in the concurrent analytical model calculations to the molecular model calculations yields good agreement in the mechanical properties for all the structures studied. Specific trends in the force constants required to fit the data are observed. Consequently, a force constants library can be compiled and has been used to predict accurately the properties of more complex variants of the networks. This semi-analytical sub-unit approach enables a more efficient use of computer-intensive molecular modelling programs.