The second step consists in the proposal of a functional form, and based on our experience we propose a form based on the VB-MM framework, which has some advantages with respect to other strategies: i) it presents a lower computational cost with respect to other strategies, ii) the adjustable parameters are transferable from previous similar full-dimensional surfaces, which represents a substantial saving of time, iii) it describes the reactive process based on physically intuitive terms associated with familiar nuclear motions, for instance, stretching, bending or torsion motions, iv) if new ab initio data at selected points or new ab initio data of higher level are available, easy refit to finetune the PES is possible, and finally v) due to the flexibility of the functional form it can reproduce zones of the process that were not included in the fitting process. For the title reaction we take advantage of prior knowledge of the H + CH3OH reaction and the OH + CH3OH reaction , which also occur through two pathways. Thus, with the corresponding modifications from methanol to methanethiol we reused the parameters describing the polyatomic molecules in reactants and products, and from the latter, we reused the parameters describing OH stretching for the reactant and the stretching and bending modes in the H2O product. Thus, based on known pieces one can assemble a new building, which is reminiscent of the popular Tetris game.