The non-bonded components of a molecular mechanics force field aim to describe the quantum mechanical electrostatic, dispersion and exchange-repulsion interactions in a computationally efficient manner. The charge parameters are generally fit to the quantum mechanical electrostatic potential of small molecules. The Lennard-Jones parameters are then fit to reproduce experimental data, such as liquid densities and heats of vaporization. The aim of QUBE is to move away from the requirement for transferable force field parameters, and instead to derive bespoke parameters for molecules directly from QM calculations. First, a QM simulation of the molecule under study is performed. From the output of the QM calculation, the total electron density of the molecule is partitioned onto individual atoms using an atoms-in-molecule (AIM) weighting scheme. There is no unique method to perform this partitioning, but we favor the density derived electrostatic and chemical (DDEC) scheme, which is a weighted combination of the iterative stockholder atoms and iterative Hirshfeld approaches. With the electron density partitioned to individual atoms, the atom-centered charges can be simply found by integrating the electron density over all space (and adding the nuclear charge). We have implemented the DDEC approach in the ONETEP software package, which allows us to perform QM calculations of, and assign parameters to, systems comprising thousands of atoms. The derived charges have been shown to be suitable for use in flexible force field design in multiple works, because they are able to reproduce the underlying QM electrostatic dependence while exhibiting low conformation-dependence. The charges are specific to the system under study and, by performing the QM calculation in the presence of an implicit solvent model, polarization of the charges in the condensed phase can be included in the model. The dispersion and exchange-repulsion interactions are described using a Lennard-Jones potential with a form: