As opposed to the more commonly used atomistic description of solvents in chemistry simulations, Environ implements a set of continuum models, with the goal of providing a method of describing systems in environments with the primary goal of maximizing the focus on the quantum mechanical system. The result is a relatively succinct description of the environment unsuitable for capturing certain effects within this regime, yet the advantages of reduced computational cost make this approach invaluable for many applications.
These models belong to a family of hierarchical approaches that separate the treatment of the quantum mechanical system from the environment. Varying choices for representing this environment result in varying groups of methods, each with their own ideal applications. For example Quantum Mechanics has been coupled with Molecular Mechanics (QM/MM) thus retaining atomistic details of the embedding system while still treating it with a less computationally intensive scheme. The drawback of such an approach is perhaps not going far enough in terms of cost reduction, since a statistical sampling is still required for the environment configurations. Additionally the accuracy of such an approach is strictly dependent on the quality of the classical parameterization used to describe the components within the environment.
Continuum models go one step further, now removing atomistic details in the environment under the assumption that a statistical average of the environment results in a homogenous continuum. This can be justified for many solvent systems (with the exception of perhaps solvent atoms close to the QM system, in certain scenarios). One popular continuum model within the chemistry community is the Polarizable Continuum Model (PCM), which has been progressively extended and optimized for simulation stability and speed over the years. The breakthrough in the field of condensed matter was the Fattebert and Gygi (FG) model, which kickstarted substantial interest and many publications in the following years. The continuum models implemented in Environ are in some ways derived from the ideas presented in the PCM and the FG model. These have been successfully applied to neutral liquids and electrolyte solutions, with periodic crystal structures and small isolated molecules.
More recent developments have been made to improve these methods, especially towards better transferability, and there has been work in proposed multi-scale approaches that combine multiple different embedding schemes, with the same goals of effectively distributing computational time towards more important areas.