Exciton: a code for excitations in materials

Abstract

Exciton is a gaussian orbital code for computing electronic structures of crystalline solids. In its current form it is capable of performing Hartree–Fock calculations on metals and insulators, including spin-polarised systems. Further development of the code will include methods for calculating excitations in materials by many-body techniques. The algorithms used for self-consistent field calculations and analyses of the calculations such as densities of states and optical excitations are described. Symmetry in real and reciprocal space is used extensively; the symmetry algorithms applied to generation of matrix elements are described. Example calculations on silicon and nickel oxide are used to demonstrate the current capabilities of the code.

Keywords:

• Hartree–Fock
• excited states
• GW approximation

Acknowledgements

Development of the current version of Exciton was begun during a sabbatical year which the author spent at QTP, hosted by Rodney Bartlett. The author is grateful to many members of QTP for a warm welcome during that year. Earlier versions of the code were developed by Conor Hogan and Svjetlana Galamić-Mulaomerović in collaboration with the author. The author is also grateful to Vic Saunders, Roberto Dovesi and Carla Roetti for assistance in developing an interface to Crystal. Development work on Exciton is currently supported by Alin Elena of the Irish Centre for High End Computing (ICHEC). Computer facilities were provided by the Trinity Centre for High Performance Computing and ICHEC. The project is currently funded by Science Foundation Ireland under grant number SFI/09/MTR2295.

Notes

Note

1. Strictly speaking, the term ‘Hartree–Fock exchange’ can only be applied when the exchange operator corresponds to that derived from a single-determinant wave function and is used without any weighting factor. For simplicity and because the term is in common use, we use the term ‘weighted Hartree–Fock exchange’ to refer to an approximation to the exchange operator which contains an exchange term of the same form as that in Hartree–Fock theory as well as one or more other exchange functionals with appropriate weighting factors.