The chemical space describes the space of all possible drug-like molecules and is estimated to be around $10^{60}$(1). This vast amount of potential compounds is called chemical space. I like to think of it as a multidimensional-space, where each dimension represent a porperty or feature and each point a possible molecule. Source: 2 This means if we had the perfect set of molecular properties, we could define a desirable subspace within the vastness of chemical space. A practical way to do this is through a Multi-Parameter Optimization (MPO) approach. Instead of a single, perfect scoring function, we define this subspace as the collection of all molecules that simultaneously satisfy multiple property constraints. Using this MPO framework, we can formally define the subspace S as: ...