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Environ is a computational library aimed at introducing environment effects into atomistic first-principles simulations, in particular for applications in surface science and materials design.
A hierarchical, multiscale, strategy is at the base of the different methods implemented: while the atomistic and electronic details of the system of interest are fully preserved, the degrees of freedom of the surrounding environment (being it a liquid solution of a more complex embedding) are treated using simplified approaches. By reducing the number of degrees of freedom and by exploiting intrinsic or faster statistical averaging, the implemented methods allow the systematic un-expensive study of large systems.
Environ features the following approaches:
One common feature of the above approaches is the fact that the interface between the first-principles and the environment parts of the system are defined self-consistently in terms of the electronic density of the embedded system. An optimally smooth connection (Andreussi 2012) is used to ensure convergence of the calculations.
Each contribution can be used per se, to characterise individual environment effects corresponding to specific conditions: acluster under the influence of a well defined external pressure, the effect of dielectric screeneing on the electronic properties of a substrate, etc. Moreover, the combination of different contributions can be used to give a more complete characterization of a complex environment, such as a liquid solution. This strategy is at the core of the Self-Consistent Continuum Solvation (SCCS) approach recently proposed to model solvation effects in aqueous solutions, by including a combination of electrostatic (continuum dielectric) and non-electrostatic (dispersion-repulsion) effects by means of a parametrized combination of the different approaches of Environ. Results on solvation free energies of netrual (Andreussi 2012) and charged (Dupont 2014) organic molecules showed a remarkable agreement with experiments, in particular for all those compounds that do not form stable coordination compounds with water molecules and, therefore, for which a continuum description of the environment is reasonable.
SCCS can be used to run geometry optimizations of isolated, partially periodic and periodic systems in continuum embedding, but also to run Born-Oppeneimer molecular dynamics simulations, searches of reaction paths (through the NEB approach), and for the calculation of optical spectra (exploiting the TDDFpT approach, Timrov 2015). Extensions to Car-Parrinello molecular dynamics are in progress.