MSDGW F
MSDGW_F = [real]
Default: MSDGW_F = -1
Description: A positive value of MSDGW_F triggers the mixed stochastic-deterministic compression algorithm of Altman and co-workers.[1]
MSDGW_F is the constant energy ratio [math]\displaystyle{ F }[/math] of the compression algorithm. If set to a positive value, energies beyond the protected space defined by MSDGW_NP are subdivided into energy bins of width [math]\displaystyle{ \Delta E_i }[/math] and replaced by other energies [math]\displaystyle{ E_i }[/math], such that [math]\displaystyle{ F=\Delta E_i/E_i }[/math]. The original orbitals are replaced by MSDGW_NXI randomly linear combined orbitals. Larger values of MSDGW_F increase the compression level at the expense of accuracy. The same holds true for smaller values of MSDGW_NXI.
This compression algorithm has been developed to reduce the large number of unoccupied states required for the calculation of the screened interaction in [calculations]. It has been demonstrated that one can reduce the unoccupied manifold by more than 50 per cent and speed up the GW step by a factor of 2 or more with a resulting error of only 50 meV or less on the quasi-particle band gap.[1]
Mind: Available as of VASP.6.6.0.
Warning: Not recommended for LRPAFORCE=T.
Use cases
- Recommended for ALGO=EVG0W[R|RK]|CRPA[R|RK]:
The compression can be used for all type of [calculations] regardless if the calculation is performed in steps or in the all-in-one mode. Following lines in stdout and OUTCAR indicate that the compression has been performed:
=> avg. energy ratio F (%): 1.00 bands after compression: 240
To test different compression settings, it is recommended perform the GW/CRPA calculation in steps and to set MSDGW_F only in the actual GW step. This avoids repeating the expensive exact diagonalization of the Kohn-Sham Hamiltonian.
Caveats
Care must be taken for GW/CRPA calculations that are performed in steps and include the long-wave limit stored in WAVEDER. After band compression, this limit must be re-calculated by setting LOPTICS in combination with LPEAD.
Related tags and articles
MSDGW_NXI, MSDGW_SEED, MSDGW_NP