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.

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MSDGW_F is the constant energy ratio $F$ 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 $\Delta E_i$ and replaced by other energies $E_i$, such that $F=\Delta E_i/E_i$. The original orbitals are replaced by MSDGW_NXI randomly linear combined orbitals. Larger values of MSDGW_F increase the compression level at 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 virtual states required for the calculation of the screened interaction in GW calculations. It has been demonstrated thay one can reduce the virtual manifold by more than 50 per cent and speed up the GW step by a factor of 2 or more with an resulting error of only 50 meV or less on the quasi particle band gap. Comparable results can be achieved for Constrained Random Phase Approximation (CRPA) calculations.

Mind: Avaliable as of VASP version > 6.5.1.

Use cases

• Recommended for ALGO=EVG0W[R|RK]|CRPA[R|RK]:

The compression can be used for all type of GW 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 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

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