The HetPCM approach is a simple but ad hoc modification of the ASC-PCM formalism, which we intend to test against rigorous Poisson boundary conditions that can describe an anisotropic continuum environment in a general way, by specifying a permittivity function ε(r). This flexibility facilitates the use of a heterogeneous (anisotropic) dielectric environment. The function ε(r) represents a model that can be used, for example, to describe the air/water interface, or different regions of a protein 81-83 or other complex system. Given a model ε(r), the corresponding Poisson boundary conditions are implemented in an exact way, up to controllable discretization errors. Such methods have a long history in classical biomolecular electrostatics calculations, and a variety of numerical solvers have been developed, but implementations of Poisson boundary conditions for electronic structure calculations have also been reported. The finite-difference algorithms that are typically used to solve eq. 2 require that the permittivity function ε(r) to be smoothly varying, else the induced charge may vary wildly in space and the discretization problem becomes severe.