### 7.1 Functionalities of RIPER

riper is a module for the calculation of ground state energies for molecular and periodic systems
at the DFT level. In this program a combination of the Resolution of Identity (RI) approximation
and continuous fast multipole method (CFMM) is applied for efficient evaluation of the electronic
Coulomb term. For the exchange-correlation term an octree-based adaptive numerical integration
scheme is used. For the molecular systems a low-memory modification of the RI approximation is
implemented aimed specifically for calculations on very large molecules. More details are given in
Refs [97], [98] and [99].

Current functionalities and limitations of riper are:

- both closed–shell RHF and open–shell UHF calculations
- sequential and parallel runs (OpenMP parallelization, see Sec. 3.2.2)
- all pure GGA exchange-correlation functionals for periodic systems
- hybrid GGA functionals for molecules
- almost O(N) scaling with the system size
- as for now only C
_{1} symmetry point group for molecules and P_{1} space group for
periodic systems