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Applying quantum chemistry to solids and liquids

Electronic structure studies of crystalline solids are dominated by density functional theory (DFT), which has proved capable of providing many powerful insights. Nevertheless, using conventional local, gradient corrected or hybrid functionals, these have a number of short-comings. Perhaps most seriously there is no clear route for systematic improvement of accuracy. A number of wavefunction-based methods have been developed to model crystalline solids, including quantum Monte Carlo and techniques that extend quantum chemical electronic structure methods to include periodic boundary conditions, with recent focus on periodic MP2 implementations. This talk concentrates on alternative methods which seek to address the electron correlation problem in solids using molecular electronic structure calculations on fragments. We outline the incremental scheme (or method of increments) and a hierarchial method. The chief advantage of such schemes over full periodic implementations is their simplicity, and the straightforward extension to more advanced electronic structure methods. We describe some recent applications including solid LiH, LiF, HF and crystalline neon. Liquids present further challenges for any theoretical method. We describe a many-body expansion technique aimed directly at the simulation of molecular liquids. We employ the many-body expansion to reduce the problem to a set of calculations on dimers, trimers etc., and we treat higher-order effects through a polarizable model which uses ab initio properties. MP2-level radial distribution functions for liquid water are presented ​
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