# Large periodic and nonperiodic systems

Modern electronic-structure theory is well developed for the accurate treatment of small molecular systems, often with an accuracy rivaling that of experiment. For large systems, containing hundreds or thousands of atoms, for periodic systems (idealized solid-state and liquid systems) and for systems containing heavy atoms, the computational methods are less developed and incapable of yielding the high accuracy characteristic of small molecular systems. It is therefore important to develop an approach for the uniform treatment of large and small, periodic and nonperiodic, relativistic and nonrelativistic systems.

In the first phase of this work, we shall concentrate on the Hartree--Fock model and on density-functional theory, converting existing technology for molecular systems to extended and periodic systems, with emphasis on the calculation of properties by response theory. In this manner, an immediate benefit to solid-state theory will come from introducing the many advances made in molecular theory during the last two decades, enabling us to study a large variety of linear and nonlinear static and dynamic properties---for example, electromagnetic and optical properties. Second, we shall be in a position to treat all systems at a uniform level of theory (using, for example, the same localized one-electron Gaussian basis funct ions). This latter point is essential for all applications where comparisons are made for example between sm all and large systems or between periodic and nonperiodic systems.

Following the development of a uniform code for large and small, periodic and nonperiodic systems at the Hartree--Fock and density-functional levels, it becomes essential to consider correlated wave functions, in particular coupled-cluster methods. These methods have made little or no impact on solid-state chemistry. Properly implemented, such methods would revolutionize solid-state chemistry, allowing calculations of unprecedented accuracy in this important field of chemistry.