ALGO= CHI | GW0 | GW | scGW | scGW0 | QPGW | QPGW0
ALGO=CHI calculates the frequency dependent response functions in the independent particle approximation and in the RPA (or DFT). The computational effort is fairly small for bulk systems, but possibly huge for large supercells with significant vacuum. Usually one wants to determine the response function at the point only (optically allowed transitions with momentum transfer ), and this can be achieved by setting NKRED to the number of k-points in all three direction. For a k-point grid, NKRED=4 will restrict the calculation of the frequency dependent response function to the important transitions with momentum transfer as measured by optical experiments. For a grid, one needs to set NKREDX=4, NKREDY=6 and NKREDZ=8 (compare Sec. 6.73.9). Furthermore, we note that ENCUTGW can be usually set to fairly small values (see Sec. 6.73.7). Often one third or one quarter of ENCUT (or less) will suffice. Note that the independent particle response function is independent of ENCUTGW, and the RPA converges reasonably fast with ENCUTGW. Reducing ENCUTGW, reduces the storage requirements and compute time significantly compared to the defaults.
For ALGO=GW and ALGO=GW0 the orbitals of the previous groundstate calculations are maintained, and single shot GW calculations are performed. If NELM is set as well, several iterations are performed, and the eigenvalues are updated in the calculation of G (ALGO=GW0) or W and G (ALGO=GW). A full update of the orbitals can be performed by specifying ALGO=SCGW and ALGO=SCGW0 (QPGW and QPGW0 are synonymous to these setting, and available in VASP.5.2.13). In the former case, the orbitals and eigenvalues are updated in G and W, whereas in the latter case the orbitals and eigenvalues are only updated in G. Convergence of the eingevalues with respect to ENCUTGW (see Sec. 6.73.7 and the number of unoccupied bands NBANDS is usually fairly slow and should be checked carefully.
We strongly recommend to read the following literature before performing calculations using VASP [111,112,113,114].